The Chinle Formation
Distribution and general features
In the landscape the Chinle is relatively inconspicuous. Its attitude prevents the development of such magnificent escarpments as characterize the flat-lying Chinle in Virgin River valley and along the Vermilion Cliffs. Furthermore, in many places it is traversed by close-spaced faults and tiny folds, and its less resistant beds are buried by talus and by the debris of landslides. Nevertheless, in Spring, Kanarra, and Squaw Canyons its architecture and coloring are attractive, and even in Coal Canyon, where it is intensively squeezed and fractured, the formation retains many of its regionally characteristic features.
Measured sections and the examination of widely spaced outcrops show that in gross composition and arrangement of beds the Chinle of eastern Southwest Utah is substantially like that in the adjoining Zion Park region, where its exceptionally complete exposures have been studied in detail (1).
Generally, in southwestern Utah the Chinle formation comprises four subdivisions, characterized by diverse lithic features and topographic forms, but in most places without clearly definable boundaries:
1. Lower sandstones: gray and brown sandstone generally less than 100 feet thick, conformably overlying the Shinarump conglomerate. They consist chiefly of lumpy lenticular beds of coarse and fine subangular quartz grains tightly cemented with gypsum, iron, and lime, and include mica, magnetite, and ironstone concretions.
2. Petrified Forest member: brightly colored shales, limestones, volcanic ashes, and thin sandstones of various composition-"the variegated marls" of the pioneer geologists. Characteristic features are picturesque erosion forms and fossil wood.
3. Springdale sandstone member: a series of overlapping sheets and wedges of light red sandstone so combined as to form a resistant ledge expressed in the topography by a prominent cliff or a hogback. The member consists chiefly of subangular quartz, but includes iron oxides, mica, tourmaline, concretionary limestone, and clay balls.
4. Upper sandstones: orange red, brick red sandstone in thick and thin beds that extend upward to the base of the Wingate sandstone or, in its absence, the Navajo sandstone.
In eastern Southwest Utah the four subdivisions of the Chinle are demonstrable and are recorded in measured sections, because tracing shows them to be continuous with the corresponding units in adjoining regions. However, they are poor representatives of the members as typically displayed in Virgin, Kanab, and Short Creek valleys. In particular, the lower sandstones and the Springdale sandstone member are differentiated with difficulty and thus substantially the Chinle of eastern Southwest Utah consists of the upper sandstones and the Petrified Forest member, and even these beds are present in modified form.
The upper sandstones-the highest distinctive sequence of beds in the Chinle vary much in thickness, color, and persistency of lamination. Throughout their extent in eastern Southwest Utah, groups of brick-red, white-streaked, massive sandstones, 3 to 8 feet thick, alternate with groups of sandy shale, but single beds of shale appear within the thick sandstone and single beds of sandstone within considerable thickness of shale. Generally the shale-like sandstones are more abundant near the base, where they tend to weather as gentle slopes and make possible the devlopment of local valleys. The more massive beds above are ledge makers and combine to form steep slopes, in places almost vertical walls. For stretches of several hundred feet some of the fine-grained beds are evenly foliated in relatively hard, glistening layers less than a half-inch thick, many of them ripple marked. Some of the thicker sandstones are gnarly and cross bedded. Examination of hand specimens from various outcrops show that, in general, the rock consists of subangular grains of quartz embedded in a matrix of red clay and subordinate calcite. An analysis considered typical for the dense thin beds reveals tiny grains of iron-coated quartz (70+ percent), partly decomposed feldspar (8+ percent), ferruginous clay, and bits of tourmaline, zircon, and barite. About 50 percent of the grains are 0.15 to 0.08 mm. in diameter and 20 percent of larger size. In places distinctive features are thin, short beds of compact, delicately laminated or concretionary limestone, and chunky conglomerate masses of ironstone and clay. As compared with the type section in Arizona and with sections in south central and eastern Utah, where the upper sandstones constitute one-fifth or less of the Chinle, the corresponding group in Southwest Utah comprises about two-thirds of the entire formation. Another local feature is the lack of evidence that might serve to define the upper boundary of the Chinle; except for slight differences in color, less dominant cross bedding, and less general massiveness, the topmost strata assigned to the formation differ little from the basal beds of the overlying Navajo sandstone. Chemical tests show that the red color of the sandstone is produced by red ferruginous clay-doubtless detritus weathered from older rocks exposed in a warm, humid climate and transported and redeposited without much change in composition.
The Petrified Forest member-the most conspicuous part of the Chinle formation-occurs as a broad band of variously colored rocks that lies below red sandstones and extends downward to the lower sandstones of the typical Chinle or, in places, to the Shinarump conglomerate. Generally throughout the plateau country the member is remarkable for vividness of color, for picturesque erosion forms, and for abundance of fossil wood. In eastern Southwest Utah these major characteristic features are recognizable, but the member contains fewer definable units and lacks the amazing color patterns that give fame to exposures in Zion National Park, in Paria Valley, Chinle Valley, Moenkopi Valley, and the Petrified Forest National Monument. Furthermore, because its component strata are uptilted, the member forms no prominent cliffs; in many places it is covered by its own disintegration products and by talus from formations above. Where most of it is exposed to view the Petrified Forest member consists of two roughly separate parts. The upper part is a sequence of sandstone, shale, conglomerate, gypsum, and limestone of distinctive colors, in most places lenticular and overlapping in complex relationship. The lower part is more variable in color, bedding, texture, and composition. At outcrops examined it includes gypsiferous, arenaceous, calcareous, and argillaceous shales, gypsum, hard limestone, limestone conglomerate, soft sandstones of many types, and some volcanic ash. The prevailing colors are lavender, ash gray, and seemingly innumerable shades of red and brown. Petrified wood, locally elsewhere present in quantities sufficient to constitute "fossil forests," is represented by dispersed fragments. The only fossils obtained from the Chinle in Southwest Utah are wood, algae, fish scales, and fragments of saurian (?) bones. Though these forms have not been critically studied, they probably belong to species found elsewhere in the formation. Tracing along the edge of the Kolob Terrace, across the Zion National Monument into the Virgin and Kanab valleys shows that the formation in Southwest Utah is but the western extension of a sequence of fossiliferous beds known to be of Upper Triassic age.
Strata assigned to the Chinle formation in the San Juan country are exposed in San Juan Canyon at the mouths of Clay Gulch, Castle Wash (Spring Gulch), Piute Creek, and Wilson Creek. North. of the San Juan River they form part of the prominent cliff that extends to Armstrong Canyon and. down the south wall of White Canyon . and terminates in Castle Butte overlooking the Colorado. Bears Ears, Woodenshoe Buttes, Horse Mountain, and similar landmarks on the plateaulike Elk Ridge, also Jacobs Chair and other isolated buttes between White Canyon and Dark Canyon, consist of Chinle beds resting on Shinarump conglomerate and capped by remnants of Wingate sandstone. Partial. or complete sections of the formation are exposed in Comb Ridge and in canyon branches of Cottonwood Canyon. Here and there on Red Rock Plateau streams tributary to the Colorado have cut through the overlying sandstones and exposed Chinle beds.The position of the relatively soft Chinle between two cliff makers accounts for its preservation in a region where erosion is vigorous. Where combined with the Wingate in a single steep slope the formation displays its full thickness for miles, but with the removal of its protecting cover it weathers into a mass of fluffy-topped mounds and benches that resist neither rain wash nor streams. In many places, the lower part of the formation is covered by landslides. Like the Shinarump conglomerate, the Chinle formation is nearly coextensive with the plateau province and presents at all exposures much the same lithology and erosion forms. Its outstanding features are fossil wood, a peculiar limestone conglomerate, and series of richly colored variegated shales:gray, red, pink, lavender, yellow, green; that weather in the manner of, marls. Although these subdivisions are generally recognizable and the beds that compose them combine to give the Chinle an appearance unlike that of the formations above and below, the variation in composition, texture, color, and order of deposition. is extreme. In no two sections are the relative amounts of sandstone, limestone, and shale ale or the position of these strata in the series the same. Most beds are lenses that within short distances along the strike, are replaced by rock of other kinds. Near the head of Cottonwood Canyon the limestones and the variegated "marls" are concentrated near the base, and at Clay Hills Pass these beds rest directly on the Shinarump. At the head of Comb Wash a single bed of limestone conglomerate 4 feet thick caps the friable "marls." In Moki Canyon the top 164 feet of the Chinle is mostly red sandstone; near the mouth of Red Canyon shales and limestones immediately underlie the Wingate. In nine sections measured the limestone ranges, in round numbers, from 3 to 20 percent, the shale from 3 to 70 percent, and the sandstone from 10 to 20 percent. Fossil wood appears generally in the lower half of the formation but is not abundant. Measured sections of the Chinle range in thickness from 580 to 856 feet. It is thinnest at the mouths of Red Canyon and White Canyon and thickest along the San Juan River. As a whole the Chinle of south eastern Utah is thinner, is less brilliantly colored, and contains less petrified wood and. seemingly less red sandstone than at typical exposures in northeastern Arizona and southwestern Utah. No "fossil forests" were discovered , and the great sandstone ledges in the middle of the Chinte on the Virgin River are lacking. In detail, published descriptions of the lithology of the Chinle in the Navajo country and in the Kaiparowits region, apply equally well to exposures of this formation in the San Juan country and need not be repeated here. Correlation with the Chinle at its type locality and with part of the Dolores formation, of Colorado is made with assurance.
CHINLE-WINGATE CONTACTAt most places where the two formations are in contact the massive, strongly cross-bedded, vertically jointed Wingate seems to overlie conformably 30 to 60 feet of regularly bedded sandstone. In tracing the contact; however, evidences of a lithologic break and of a surface of erosion appear. Near the head of Clay Hills Pass the Wingate rests on a wavy surface that slightly truncates beds of brown sandstone and white limestone conglomerate, red shale, and blue-gray clay. The contact is a horizon for springs and seeps. In the walls of the Tables of the Sun the lowermost beds of Wingate are Tentacular and conglomeratic and fill depressions in the underlying Chinle. Along the north side of Red Canyon the contact is marked by discontinuous accumulations of limestone concretions, mud balls, and pellets of clay; at the head of Comb Wash, by ripple-marked and mud-cracked shale. Though in color, mass composition, and general texture the sandstones above and below the contact, are much alike, the differences suggest that the Wingate sands were derived in part from disintegrating Chinle, in part from the same source as the Chinle, and in part from other sources. The evidence is sufficient to show. that the topmost Chinle was exposed to erosion before the Wingate was laid down.
RICO FORMATIONStrata of the Rico formation are continuously exposed on the crests and flanks of the Raplee and Mitten Butte anticlines where the topmost thin limestone forms the surface over large areas. They constitute the rim rock of the San Juan and tributary canyons between Comb Wash and Slickhorn Gulch, except along Lime Creek where they are lost to view in the Mexican Hat syncline. The Rico is exposed in Dark Canyon, Beef Basin, and Fable Valley, and beds provisionally assigned to this formation appear in Mule, Arch, and Hammond Canyons. In the walls, of canyons and gulches the limestone and hard sandstones interbedded with shale and soft sandstones form flights of colored steps that lead from the bottom to the top. On steep structural, slopes bright-red sandstone is revealed by scores of meandering gullies that have cut through the cover blue-gray limestone. Viewed from the west, the Raplee anticline is an exceptionally attractive feature. The Rico is made up of sandstones, shales, and limestones. The sandstones are, prevailingly "red"-bright red, dark red, pink, purple but a few beds are gray, or white. They include massive beds 10 to 20 feet thick and series of thin, lenticular beds. They calcareous fine to medium-grained rocks composed Quartz calcite, some mica, and a little feldspar. The gray cross-bedded pitted sandstones that constitute the oil sands in the Rico near Goodridge consist of clear quartz grains cemented by lime, the removal of which has produced a porosity favorable for the accumulation of disseminated petroleum. The shales are sandy differ from the sandstones chiefly in thinness of bedding. They might be classed as thin, irregularly bedded, lenticular, in places cross-bedded sandstone units that lose their identity in short distances along strike. The limestones are blue-gray, resistant, fossiliferous rocks that form benches. They are continuous and impervious enough to form caps for water oil under pressure beneath. In sections measured at the Honaker trial four beds of limestone constitute about 11 percent of the rock exposed. The bottom of the Rico is the top of the Hermosa; the place in the Goodridge formation where a Pennsylvanian fauna changes to one characteristic of the Permian. The top of the Rico is arbitrarily defined as the highest bed of limestone in the walls of San Juan and Johns Canyons, which generally is the highest fossilifusus stratum in the region. As thus limited, the Rico in the walls of the San Juan Canyon is 300 to 325 feet thick. Beds assigned to the Rico in the San Juan country have the same relation to the underlying Hermosa as Rico, Colo. Permian fossils from the Honaker trail characteristic of the type Rico, but those from Dark on contain suggestions of the Manzano fauna. In lithology, considerable differences appear. As described by Cross the sandstones in the Rico of Colorado are mostly coarse or conglomeratic, always showing grains of fresh feldspar mixed with mica flakes and quartz. When conglomeratic the pebbles are chiefly of schists and quartzite. The shales, aside from the sandy varieties, are of two kind the fine-grained, unlaminated red many beds... and the equally fine-grained, laminated clay shales of a green color. In Utah the sandstone beds are finer-grained and less arkosic and contain oil; the shales show less variation bedding and composition. Except for the fossils and the larger numbers of persistent limestone beds in the Rico there is little to distinguish that formation from the overlying Cutler. Both are Permian red beds, both are dominantly calcareous, irregularly bedded, more or less arkosic sandstones with considerable range in texture. Were it not for established usage the Rico and the lowest Cutler (Halgaito member) might be combined in one formation that derived its sediments from Colorado high lands and deposited them westward in shallow sea water that intermittently withdrew. The periods in which the land provided red sediments seem to have increase progressively in length during the whole time represented by the Hermosa, the Rico, and the Halgaito.
1. Gregory, H. E.. The Zion Park region; U. S. Geol. Survey Prof. paper [awaiting publication].
-from Geology of Eastern Iron County, Utah by Herbert E. Gregory
DEFINITION AND AREAL EXTENT.The name Chinle formation is proposed for the -group of shales, "marls," thin, soft sandstones, and limestone conglomerates lying between the Shinarump conglomerate and the Wingate sandstone. The strata composing the formation are highly varied in structure and in composition, but as a whole they constitute a stratigraphic unit of unmistakable individuality. The Chinle is widely distributed. The entire formation is present in Chinle Valley, from which its name is derived. It may be continuously traced from Colorado River at Lees Ferry southward along the Echo Cliffs, across the Painted Desert, eastward past Monument Point, through the Hopi Buttes, thence northward along the Pueblo Colorado and Chinle valleys to the mouth of Lukachukai Creek. Another belt of the Chinle formation follows Black Creek Valley and the west base of the Chuska Mountains. In Monument Valley it forms a band at the base of Comb Ridge, and in Dutton Plateau it constitutes the pedestal on which rest the towering walls of Wingate sandstone. The upper part of the formation is exposed in San Juan, Piute, and Navajo canyons. Where the dip is steep, as in Black Creek Valley, the belt of Chinle strata is less than 1 mile wide; on the gently sloping west limb of the De Chelly up warp and on the slopes of Zuni Mountain and in Monument Valley its width is 3 to 10 miles. South of the Hopi Buttes, where the strata are essentially horizontal, the Chinle crops out at the surface as a band over 20 miles wide.
FIELD APPEARANCE.The Chinle is the most beautifully colored formation in the Navajo country. Where limestone conglomerate is prominent, the cliff faces are striated with bands of purple, pink, and gray; the friable marls and shales constitute a painted landscape with patches and bands of yellow, ash-gray, drab, lavender, rose, pink, slate, maroon, sienna, lilac, cream, and various shades of red and brown. Patches of blue, of white, and even of black are seen, and chocolate-colored shales predominate toward the base. Travelers on the Santa Fe Railway find in the Chinle beds at the base of Dutton Plateau and between Pinta and Adamana a sample of the brilliantly colored rock fabric that covers large parts of the Navajo country. The formation is displayed in all its beauty north of Fort Defiance, in the vicinity of Chinle, at Ganado, in the Painted Desert, and north of Winslow. Between Holbrook and Indian Wells the painted beds are in sight along the road for 20 miles, and a view of the landscape of vivid and changing color from one of the lava-topped buttes of this region is long to be remembered. Erosion has carved the less resistant parts of the Chinle formation into badland forms mounds and domes and short, low ridges, isolated or in groups, separated by trenchlike valleys of intricate pattern. Where sandstone beds are present the resulting erosion forms are mesas and buttes and towers, hats and inverted cups; where limestone is present flat-topped mesas or long lines of cliffs reached by a stairway of shale risers and limestone treads make up the landscape. The Chinle formation is a valley maker, in contrast to the overlying sandstones of the La Plata group which form a red frame for the varicolored pictures developed in the shales.
FEATURES OF OTHER SECTIONS.At the base of Dutton Plateau continuous exposures of the Chinle may be traced from San Antonio Springs to the hogback east of Gallup. In this area the red sandstones and shales of division A and the shales and marls separated by thin beds and lenses of limestone conglomerate (division B) differ in no essential from corresponding portions of the Chinle formation analyzed above. The same may be said of exposures in Navajo, Piute, and Copper canyons, and of the variegated shales exposed between Moenkopi Wash and Lees Ferry and in the petrified forests north and south of the Puerco at Adamana. At all these places sections were measured. In Black Creek Valley between Fort Defiance and Red Lake nearly 1,000 feet of the Chinle is exposed, about half of which is assigned to division C. The strata include an unusual amount of mica on bedding planes, and many of the sandstones of division D are tangentially cross-bedded and rippled. Massive limestone was not found at this locality and the numerous lenses of limestone conglomerate are thin and impure. The strata are so variable in extent and composition that in two sections 1 mile apart only the sandstones of division A could be correlated with confidence. In Todilto Park, where all of division. A is exposed, the regular alternation of red sandstone and shale is a noticeable feature, the couplet sandstone 4 feet and shale 4 feet being repeated 13 times. The lava caps of many of the Hopi Buttes preserve parts of the formation. In this area the strata are unusually friable; thin dikes of gypsum and of calcite are more resistant to erosion than the "marls" and "mortar beds" of concretionary limestone. The "marls" of division C on Newberry Mesa are almost without structure; they form long, even slopes uninterrupted except for protruding logs of petrified wood and pillows of concretionary limestone. East of Tanner Crossing Mr. Heald found the limestone conglomerate at the top of division D to contain over 50 per cent of quartz pebbles and to change abruptly along the strike into sandstone containing pellets of shale and nodules of chert.
GENERAL FEATURES OF STRATIGRAPHY AND LITHOLOGY.Vertebrate or invertebrate fossils were found in the limestone conglomerates of divisions B, C, and D, and petrified wood was obtained from all the subdivisions. In the beds overlying the Shinarump conglomerate wood is not common except immediately at the bottom; in division B it occurs irregularly and is wanting in many localities, and the only wood observed in division A was a few impressions of twigs. Division C is the principal wood zone, and in it occur the newly discovered fossil forests at Round Rock, Beautiful Valley, Willow Springs, and elsewhere, as well as the forests north of Adamana, first described by Whipple. The peculiar limestone conglomerate was found at all exposures where divisions B, C, and D were examined, and at a few localities small lenses were observed in division A. In the lower beds the conglomerate is exceedingly variable in thickness and extent of lenses and in size of constituent pebbles. At Chinle five overlapping beds with a combined thickness of 10 feet were measured by Mr. Heald, and 5 miles farther north seven lenses ranging in dimensions from 6 inches by 6 feet to 20 by 200 feet and consisting of loosely cemented pebbles one sixty-fourth inch to 3 inches in diameter were observed. At the Fort Defiance school 10 feet of conglomerate with pebbles varying in shape from rounded pellets the size of a pea to shaly limestone fragments 4 inches in length caps a hogback. In the Oljeto region lenses of conglomerate 300 to 400 feet long and 5 to 30 feet thick are interbedded with shales of division D. In the marls and friable shales and sandstone of division C the conglomerate occurs with extreme irregularity in thin bands, in fat, short lenses, and at many places in detached balls, pellets, and lozenges displayed without apparent relation to stratification. In the upper beds 2 miles southeast of Tunnel Springs, 100 feet below the Wingate sandstone, the limestone occurs as pebbles one sixty-fourth to 1 inch in diameter scattered through lenses of green-gray sandstone and associated with reddish fragments of chert reaching 4 inches in longest diameter. The limestone conglomerates of divisions C and D are commonly associated with gray, greenish-gray, and purple blotched lenticular cross-bedded sandstone. In division B limestone attains its greatest development and becomes a persistent strati-graphic member. As shown in the sections on pages 43-45, the strata range in thickness from 1 foot to 10 feet and occur as gray, drab, or pink lenses or as thin beds of wide extent. At no place were less than three beds noted ; five to seven beds are commonly present in the Hopi Buttes area; and 2 miles north of Tyende eight bands of limestone separated by shale retain their individuality for 5 miles, a mode of occurrence more or less typical for division B but quite unlike that of the conglomerate lenses in divisions C and D. (See Pl. XI, B.) Many of the limestone beds are compact and so resistant as to determine the topography; part of them are loosely cemented. Pebbles of lime constitute over 90 per cent of the conglomerate, and in certain places there is no other ingredient. Chert, however, is a common constituent; shale fragments and tiny well-rounded grains of quartz also occur in the limestone. The shales throughout the Chinle formation are arenaceous and calcareous. They have a noticeable argillaceous content only in division C, and even here fine shalt' sandstone and even coarse sandstone with pebbles of quartz and quartzite are found. In this division all the beds are highly irregular and change markedly in character within a distance of a few feet along the strike. In a branch of Pueblo Colorado Wash, 3 miles north of Tanner Spring, is a group of calcareous beds consisting of balls of limestone, grains of quartz, and slabs of brown sandy hardened mud, all arranged in laminae whose surfaces show mud cracks. Gypsum is widely disseminated in the shales of division D and particularly in division C, where stringers and lenses and balls of gypsum are of common occurrence. The cement of all the shales examined is calcareous. The materials called "marl" in the reports of Marcou, Newberry, Ward, and others are highly calcareous shales and sandstones with microscopic grains of quartz and lime, and practically without structure except for closely packed concretionary masses. The "marls" appear to be accumulations of lime silt that attain thicknesses of 5 to 60 feet. They are spotted and streaked with a variety of color in a capricious manner. On weathering they cover the surface with flakes and crumbs into which the foot sinks deeply. Climbing a slope of water-soaked "marl" is a precarious undertaking. Division A, though in general light or dark red, in many places takes on orange or even yellow tones. In many places along the base of the Chuska Mountains, in Pueblo Colorado Wash, near Bidahochi, along Chinle Wash, and elsewhere the thicker beds are beautifully banded with white in long parallel or divergent lines or short lenses or even dots. Near Zilditloi Mountain and at Indian Wells the material appears amorphous, like chunks of dark-red clay squeezed together. At these localities the beds weather into pillows and spools and bobbins of fantastic shape unlike typical exposures of the Chinle. Cross-bedding and ripple marks in sandstones, sun-baked surfaces and rare mud cracks, rain prints, and worm casts are features of many beds.
AGE AND CORRELATION.The age of the Chinle formation is determined by small collections of fossils from several places within and near the Navajo country. The fossils obtained from the "Leroux" (part of Chinle) at Tanner Crossing by Ward and Brown were described by Lucas, who determined the species Episcoposaurus sp. ; Heterodontosuchus ganei, Metoposaurus frassi, Placerias hesternus, Palaeoctonus sp. These forms were assigned by Lucas 1 to the Upper Triassic with the remark: " We have in these Triassic beds of Arizona ... the same combination of belodont and labyrinthodont as in the Keuper." saurus, probably kietoposaurus frassi,1 was obtained at Tucker Springs, and undescribed phytosaur and labyrinthodont bones were collected by Williston 2 on the flanks of Zuni Mountain. The Triassic crocodile Heterodontosuchus ganef was also found 10 miles north of San Juan River and is common in the Dolores formation of Colorado.3 Unio cristonensis, considered by Stanton4 as probably Triassic, was obtained from divisions B and. C, and the invertebrates collected by Woodruff near the top of the Chinle formation in San Juan Valley are considered also by Stanton as "probably Triassic." 5 Collections of vertebrate fragments from the Chinle beds (divisions C and D) at Copper Canyon and at Chinle have not been identified, but according to Prof. Lull 6 they have Triassic affinities. The petrified wood of the Chinle of the Navajo country has not been specifically determined. In the course of my field work I have become familiar with the Mesozoic formations studied by previous workers in the Plateau province south of Colorado River and to a lesser degree north of that river and north of the San Juan and have found little difficulty in recognizing the Chinle formation in Arizona, New Mexico, Utah, and Colorado. Its position between the Shinarump conglomerate and the Wingate sandstone and its assemblage of varied but persistent features warrants the making of lithologic correlations with confidence. The Chinle includes portions of Newberry's "Red Sandstone series" and "Variegated marls." Just what portions of these series are equivalent to the Chinle formation it is impossible to state, for the lower limit of the "Variegated marls" is set at 115 feet above the Shinarump conglomerate 7 and its upper part includes the La Plata and McElmo sandstones. The " Leroux member" of Ward's "Shinarump formation," consisting of 800 feet of " argillaceous and calcareous marls," sandstones, limestones, and "mortar beds," is apparently equivalent to division B and part of division C of the Chinle. Division A of the Chinle is represented in Ward's section by 100 feet of "red-orange sandstones," assigned to the "Painted Desert formation." Division D and part of division C of the Chinle are included by Ward in that remarkably heterogeneous assemblage of strata, the " Lithodendron member." The "upper Shinarump" "shales" and " clays" of Dutton, Gilbert, Howell, and Marvine are substantially the equivalent of the Chinle. Dutton makes no mention of limestone conglomerate in the strata exposed north of Wingate station, but Gilbert noted 6 feet of purplish limestone at this locality. Beds of limestone and limestone conglomerate are not described by the geologists of the Wheeler Survey, and no such strata are included in the "section of Jurassic and Triassic formations in the Kanab Valley". Huntington and Goldthwait found at Toquerville, Utah, beds corresponding in position to the Chinle, consisting of 250 feet of soft purple and red shale capped by 100 feet of "mauve sandstone." To these strata they applied the name "Painted Desert" in a sense unknown to Ward, the originator of the term. Many years of detailed study of the Mesozoic of southwestern Colorado by Cross has resulted in delimiting the Dolores formation, which occupies the stratigraphic position of the Chinle. The Dolores unconformably underlies the lowest La Plata sandstone (Wingate sandstone) and rests unconformably on the Cutler (Permian?), the Rico (Permian?), or the Hermosa (Pennsylvanian). About 400 feet of Dolores strata are exposed in the Rico quad? Ward, L. F., Status of the Mesozoic floras of the United States: U. S. Geol. Survey Mon. 48, p. 45, 1905. 9 Quoted by Cross, Whitman, and Howe, Ernest, Red Beds of southwestern Colorado and their correlation: Geol. Soc. America Bull., vol. 16, pp. 484-485, 1905. 10 Huntington, Ellsworth, and Goldthwait, J. W., The Hurricane fault in southwestern Utah: Jour. Geology, vol. 11, pp. 46-63, 1903; The Hurricane fault in the Toquerville district, Utah: Harvard Coll. Mus. Comp. Zool. Bull., vol. 42, pp. 201-259, 1904. The stratigraphic units proposed in these publications-Upper Verkin, Lower Verkin (changed in the second paper to Moencopie), Shinarump, Kanab, and Colob-are described briefly and without measured sections. It has been found unprofitable to attempt to correlate them with the subdivisions established by Dutton, Gilbert, Walcott, Ward, or Cross. 11 Cross, Whitman, U. S. Geol. Survey Geol. Atlas, La Plata folio (No. 60), 1899; Rico folio (No. 130), 1905. rangle and 1,700 feet in the La Plata quadrangle. The lower portion of the Dolores consists of "reddish sandstones, more or less shaly, with conglomerate consisting chiefly of very small limestone pebbles." 1 "The upper memember of the Dolores is commonly a very even, fine-grained reddish sandstone free from conglomerate but variably shaly."2 The upper part of the Dolores as described by Cross appears to be the equivalent of division A of the Chinle; the lower part may represent division B or D or even an abbreviated form of divisions B, C, and D combined. The variegated shales and "marls" of division C, with their wealth of fossil wood, have not been found in Colorado, and the 1,000 feet or more of "arkose sandstone and conglomerate" at the base of the Dolores in the La Plata quadrangle were not found in the Navajo country. From published descriptions supplemented by a small amount of field work along Dolores River I have gained the impression that the Dolores is to be correlated with the upper half (divisions A and B) of the Chinle. The statement of Cross 3 "that the Shinarump (as defined by Powell) may include important divisions not represented in the Dolores " is verified, and among those divisions is part, at least, of division C of the Chinle formation and the Shinarump conglomerate, which, as I have elsewhere shown, occurs at a lower horizon than the fossiliferous limestone conglomerates ("Saurian conglomerate") of the Chinle and the Dolores.
CHINLE-WIN GATE CONTACT.Wherever the contact between the red or orange-red shales of the upper Chinle and the massive Wingate sandstone was observed care was taken to search for evidence of unconformity. The strata are well displayed for such an examination, as along cliff bases and in canyon walls the contact may be traced without interruption for distances of 1 to 5 miles. No channeling or other conspicuous evidence of a break in sedimentation was observed, but at many places the contact is marked by a thin band of leached material and a disturbance of the otherwise regular stratification. In lower Piute Canyon 5 to 15 feet of irregularly curved, highly varied strata resting unevenly on shales and limestone conglomerates mark the base of the Wingate. East of Tanner Crossing Mr. Heald found at the top of the Chinle a 2-foot bed of "conglomerate" consisting of well-rounded pebbles of quartz with rare limestone, gray shale, and red quartzite fragments arranged as stringers, cross-bedded, and with ripple-marked and mud-cracked surfaces. A slight change in direction of strike and dip was also noted. Six miles southwest of Tuba a similar stratum forms the top of the Chinle formation. In Laguna Canyon, at Azansosi Mesa, and at the east base of Carrizo Mountain the Wingate sandstone rests on a slightly irregular knobby surface of calcareous shales on which are spread sheets of gray sandstone containing white o and black quartz and chunks of gray shale. In Piute and Copper canyons similar relations were noted. In Todilto Park an irregular bleached band at the base of the Wingate sandstone rests on wavy, knobby, imperfectly bedded layers of Chinle shale into which project strings of white sandstone like the filling of mud cracks. Although such features are proofs only of local unconformity, I am inclined to consider them of greater significance, in view of the fact that an erosion interval at this horizon has been noted beyond the borders of the Navajo Reservation. Thus Gilbert 5 states: In the region of the Virgin River and Kanab Creek the change from the variegated shales of the Upper Shinarump to the homogeneous sandstone of the Vermilion Cliff is gradual, * * * but in one locality, at least, there is direct evidence that the surface of the clay was exposed to the air before it was covered by the sand. On the northern flank of Mount Ellsworth are the vestiges of a system of mud cracks, such as form where wet clays are dried in the sun. Where the under surface of the Vermilion sandstone is exposed to view it is seen to be marked by a network of ridges, which once occupied the sun cracks of the Shinarump clay; and where the clay is seen in juxtaposition, tapering fillets of sand can be traced from the ridges downward 10 feet into the clay. The abundance of fossil wood both north and south of Colorado River is almost incredible, and its presence has made a profound impression on the native tribes. To the Navajos the logs are yeitsobitsin, the bones .of yeitso, and a monster who was destroyed by the sun and whose blood was congealed in lava flows. In the Piute mythology the broken trunks are the spent weapons of Shináray, the great Wolf God; the accumulated masses mark the sites of battlefields. In the Navajo country fossil wood constitutes a characteristic feature of the Triassic sedimentary beds; it is found wherever the Shinarump conglomerate or Chinle formation is exposed by erosion. On Lithodendron Creek, in Beautiful Valley, at Round Rock, and at Willow Springs petrified logs and chips are sufficiently abundant to justify the term fossil forests. At those localities solid logs exceeding 50 feet in length may be counted by the dozen, blocks 3 to 10 feet long occur in hundreds, and the scattered chips are innumerable. At other localities the wood is only slightly less abundant. In the North Forest, on Lithodendron Creek, where the trees are best displayed within an area of about 1,200 acres, a number of logs have lengths of 30 to 40 feet, and diameters of 3 to 4 feet; the largest seen is about 70 feet long and measures 6 1/2 feet at its flattened butt. The Beautiful Valley Forest, covering about 3 square miles,' contains 10 logs between 50 and 80 feet in length and averaging about 3 feet in diameter, in addition to hundreds of smaller dimensions. The floor of the valley in places is literally paved with blocks of fossil wood. In the Round Rock or Senakahn Forest the trees are as abundant as at any other place known. Trunks 30 to 60 feet in length, with diameters of 1 foot to 5 feet, were measured. The Willow Springs Forest, about 5 square miles in area, includes dozens of silicified trees in the midst of chips so abundant as to conceal the strata beneath. At this locality Mr. Pogue measured a trunk 4 feet in diameter at the butt and 150 feet long, the largest tree so far reported from the Navajo Reservation. At Lees Ferry logs 60 feet long are not uncommon. The tree trunks are very unevenly distributed. They usually occur in widely spread groups of unassorted large and small trees, all lying flat and trending in parallel or diverse directions, or overlying one another, like fallen timber in the path of a tornado. In Nokai Canyon a nicely laid pile of eight logs, 7 to 15 feet long and 3 to 4 feet in diameter, occupies an isolated position, and at certain localities only a single log is to be found within an area of several acres. No complete trees were seen; most of the logs terminate abruptly, with worn surfaces at both ends. A few trees are still attached to their upturned stumps, and at several places stumps with root bases attached were noted. There is a singular scarcity of small branches and twigs, and a somewhat careful search for cones and needles resulted in finding none. The logs are not confined to a single horizon. Their lowest stratigraphic position is the base of the Shinarump conglomerate, from which they protrude in places like projecting roof beams of houses. Some of the largest logs seen are lying directly on the old erosion surface separating the Permian (?) from the Triassic. Throughout the Shinarump conglomerate the wood occurs-in the coarser materials usually as angular pebbles, in the sandstone lenses as blocks and logs. Tree trunks are common in the uppermost part of the Shinarump conglomerate and in the immediately overlying Chinle shales-a horizon that includes the Willow Springs Forest and the largest logs at Lees Ferry, Nokai Canyon, and Copper Canyon and in Middle Moonlight Valley. To judge from the descriptions by Ward 1 and by Merrill,2 the trees in the Petrified Forest National Monument occur at this horizon. The Beautiful Valley Forest and the Senakahn (Round Rock) Forest are in division C of the Chinle formation, and the trees in the forest on Lithodendron Creek lie among the limestone conglomerates of division C or the lower part of division B. The conditions under which the large amount of fossil wood was accumulated in the Triassic sediments are not clearly understood. That the trees grew in the spots where they are now found is highly improbable. Of the standing trees reported one has its roots in the air, the others so far as I have observed are wedged among other logs in a manner common to driftwood. The few stumps noted are in proper position, and Ward3 is of the opinion that a group of stumps near Tanner Crossing is in place. Mr. Heald, who studied these stumps, considers this conclusion to be open to doubt. No roots extending downward have been found attached to stumps. It is believed that the tree trunks now turned to stone were carried by streams during floods. Many of them have worn ends and battered sides, and most of them are without bark. Trees of various sizes and ages are huddled together; the blunt end of one log abuts against the side of its neighbor; and collections of trunks are wedged tightly together with different angles of inclination. The sandstone in which most of them occur is cross-bedded and lenticular, is laterally unconformable, and has other features suggestive of fluviatile deposition. The accumulations of trunks in the fossil forests are closely similar to piles of driftwood now seen along Colorado, Little Colorado, and San Juan rivers-piles of trunks and branches, some much worn, some still retaining the bark, crowded together and overriding one another; stumps attached to trees or separate in various positions, some upright, some lying on the surface, others buried in alluvium or wind-blown sand. The logs now stranded on the surface of the lava at Black Falls are about equal in number to the fossil trees in Beautiful Valley. Most of the wood, particularly the logs, must have become silicified in its present location, the process being favored by rapid burial, a water table fluctuating through short periods, and the presence of alkaline solutions. As seen in the field, the logs appear to represent a number of different kinds of trees, and Knowlton 4 reports that several species are present in the fossil forests at Adamana. The species so far described, Araucarioxylon arizonicum and Woodworthia arizonica, were obtained in the valley of Lithodendron Wash, along the southern border of the Navajo country.
PREVIOUS DESCRIPTIONS.The presence of a "firmly cemented conglomerate containing many fragments of silicified wood" capping the cliffs composed of "variegated sandstones and marls" overlying the Carboniferous was noted by Powell 1873. The geologists of the Wheeler Survey repeatedly mention a conglomerate in the midst of "variegated clays" or "shales" or "saliferous and gypsiferous clays," and the sections published by Gilbert, Howell, and Marvine leave no doubt as to the position of the conglomerate in the Mesozoic formations. Mar-vine 2 found above Sunset Crossing (Winslow). 1 Powell, J. W., Some remarks on the geological structure of a district of country lying to the north of the Grand Canyon of the Colorado: Am. Jour. Sci., 3d ser., vol. 5, pp. 456-465, 1873. 2 Marvin, A. R., U. S. Geog. and Geol. Surveys W. 100th Mer. Rept., vol. 3, p. 215, 1875. "dark-red and chocolate-colored shaly sandstones, capped with a conglomerate of siliceous pebbles, the Shinarump Triassic conglomerate of Powell." Newberry 3 had previously noted below Winslow 20 feet of "conglomerate, a coarse light-brown sandstone with white, bluish, red, and black quartz pebbles, varying in size from that of a pea to an egg quite indistinguishable from much of the Carboniferous conglomerate in Ohio and Pennsylvania." The conglomerate at Fort Defiance was also recognized as the equivalent of that in the Little Colorado Valley.4 In 1876 Powell 5 proposed the term "Shinarump group" for all the strata between the "Upper Aubrey" (Kaibab) and the Vermilion Cliff sandstone and considered the entire group of which the conglomerate forms the middle part as Triassic in age. Dutton 6 and Gilbert 7 use "Shinarump group," in harmony with Powell's definition, Dutton ascribing the strata to Permian and Triassic and. Gilbert to the Jura-Trias. The middle member of the "Shin arump group" of the Henry Mountains is described by Gilbert as "gray conglomerate with silicified wood-the Shinarump conglomerate " ; and in the High Plateaus Dutton found "a singular conglomerate of, fragments of silicified wood embedded in a matrix of sand and gravel" in the midst of "transitional shales," leading "up to the base of the Vermilion Cliff sandstone." For the Grand Canyon district as a whole Dutton 8 remarks: Wherever we encounter a cliff which discloses the upper Permian bed we find at the summit of the escarpment a band of pale-brown sandstone of coarse texture, often becoming a conglomerate. Its thickness is usually from 40 to 75 feet. In a few places it is wanting from its proper horizon, and in some others its thickness becomes more than 100 feet. In contrast with the views of Powell, Dutton, Marvine, Howell, and Gilbert, who treated the conglomerate member of the "Shinarump group" as a bed usually less than 100 feet thick and of essential uniformity of character throughout a wide area, Ward 1 defines the Shinarump conglomerate, which he later called the "Lithodendron member" of his Shinarump formation, as a series of conglomerates, sandstones, argillaceous shales, and variegated marls 800 feet thick. Because of its value as a datum plane for stratigraphic work in the Plateau province, I have elsewhere 2 discussed the limits and character of the Shinarump conglomerate and propose to give to this unique stratum the rank of a formation.
AREAL EXTENT AND THICKNESS.The Shinarump conglomerate has been recognized in southern Utah, northern Arizona, and northwestern New Mexico at almost every place where the base of the Triassic is exposed. In Utah Dutton found this rock at Pine Valley Mountain, near St. George, " throughout the great circuit of cliffs south of the High Plateaus," within the High Plateaus, in the San Rafael Swell, and near the junction of Grand and Green rivers. Gilbert found the Shinarump conglomerate in the Henry Mountains, and I have observed it at Lees Ferry, Ariz., and at Kanab, Toquerville, Water Pocket Canyon, White Canyon, Elk Ridge, and other points in southern Utah. In Arizona the conglomerate has been found by Robinson 3 in Sycamore Canyon, 20 miles southwest of Flagstaff, and at other points south and west of the Little Colorado and Puerco valleys. In New Mexico it is exposed by the erosion of the Zuni upwarp. The distance from Fort Wingate, N. Mex., to Toquerville, Utah, is about 300 miles, and from Sycamore Canyon to San Rafael Swell, 290 miles, and within those limits the Shinarump conglomerate is to be found at nearly every locality where this portion of the Triassic is exposed. In Colorado this bed has not been recognized. (See p. 41.) 1 Ward. L. F., Status of the Mesozoic floras of the United States: U. S. Geol. Survey Mon. 48, pt. 1, p. 45, 1905. 2 Gregory, H. E., The Shinarump conglomerate: Am. Jour. Sci., 4th ser., vol. 35, pp. 424-438, 1913. 3 Robinson, H. H., The San Franciscan volcanic field: U. S. Geol. Survey Prof. Paper 76, p. 27, 1913. In the Navajo country the Shinarump conglomerate is displayed as a mammoth horseshoe with tips on Colorado River and Lukachukai Creek. Along the Echo Cliffs, up the Little Colorado and Puerco valleys, and across the length of Defiance Plateau the outcrops are nearly continuous. In Monument Valley many ridges and mesas owe their preservation to a cap of this resistant stratum. The thickness of the Shinarump conglomerate in the Grand Canyon district, according to Dutton, is usually from 40 to 75 feet, and in the High Plateaus it rarely exceeds 50 feet. Howell reports 50 feet at Last Bluff and a maximum thickness of 100 feet at St. George; and Gilbert records 30 feet in the Henry Mountain district. Robinson measured at Cedar ranch, north of San Francisco Mountain, 35 feet of "yellowish to white medium to coarse grained sandstone containing rounded pebbles up to 3 inches in diameter, many angular fragments of petrified wood," and at Anderson Mesa, south of Flagstaff, "5 feet of fine-grained red conglomerate."4 In the Navajo country the Shinarump conglomerate is 45 feet thick at Lees Ferry, 50 feet near Willow Springs, 30 to 60 feet at the mouth of Moenkopi Wash, 30 feet 4 miles southeast of Black Falls, 30 feet north of Tolchico, 40 to 50 feet at Tucker Springs, 30 feet at Hardy, 25 feet at Querino, 20 to 40 feet at Fort Defiance, 80 feet at Buell Park, 20 to 60 feet at Chinle, 40 to 60 feet at Sehili, and 45 feet at Olieto. TOPOGRAPHIC EXPRESSION. Because of the relative softness of the beds above and below it, the Shinarump conglomerate is a cliff maker and caps erosion remnants. Along the Little Colorado from Adamana westward to Winslow it has preserved the cliffs of the Moenkopi from destruction. The vertical walls of Canyon de Chelly, Canyon del Muerto, and Canyon Bonito are made possible by this resistant cap, which in certain places overhangs as an ornamental molding. In Monument Valley, as at Toquerville, the conglomerate forms domes or canoe-shaped ridges in harmony with the type of structure; where the dips are steep the upturned edge of the stratum is carved into series of pinnacles and teeth. Over the Defiance Plateau it forms the surface or rests as patches on both the steep eastern face and the gently inclined western face of the Defiance monocline. As reported by Mr. Pogue the conglomerate forms an almost continuous cover of the flat up warp west of Willow Springs. Pebbles from the Shinarump are widely distributed.
UPPER CONTACT.The plane of unconformity at the base of the Shinarump conglomerate is well exposed, but its contact with the overlying Chinle formation is in most places concealed by talus. Ward 1 says that "the lower marls ["Le Roux"] are often separated by beds of sandstone and true conglomerate, showing that it is all one great system of Shinarump sandstones," a conclusion in harmony with the views of Powell, Dutton, and Gilbert. Woodruff states: "It is probable that there is an unconformity at the top of the bed" ("Oljeto" includes Shinarump conglomerate). At Fort Defiance and Chinle the beds immediately overlying the Shinarump conglomerate were examined. In a branch of the Canyon de Chelly near Chinle 4 feet of purple-gray lumpy ripple-marked micaceous sandy shale rests on a wavy, eroded surface of the conglomerate. Weathered fragments of quartz and of wood are included in the shale. Two miles farther north Mr. Heald noted similar relations. On the back slope of the cuesta at Fort Defiance the upper wavy, scalloped surface of the conglomerate is immediately overlain by 2 feet of paper-thin drab, highly muscovitic shale, followed by irregularly foliated cross-bedded purple shale and sandstone. These typical exposures indicate that unconformities of at least local extent mark the upper limit of the Shinarump conglomerate, and in any case the abrupt change in lithologic character in passing into the Chinle formation is significant.
STRUCTURE, TEXTURE, AND COMPOSITION.The geologists of the early Government surveys were evidently impressed by the persistence and constancy of character shown by the Shinarump conglomerate in widely separated localities: Dutton, in speaking of the cliff exposures of the "Shinarump group," including the conglomerate, says: "As we pass from one of these localities to another not a line seems to have disappeared, not a color to have deepened or paled. The constancy, so far as known to me, is without a parallel in any other region." 2 "The Shinarump conglomerate keeps its aspect unaltered wherever it spreads." 3 After examining this formation at more than 40 localities, I recognize a considerable range of differences in structure and composition. The likenesses, however, are much more numerous than the differences, and outcrops separated by 200 miles resemble each other more closely than they resemble any parts of other Mesozoic formations. The Shinarump is every where lenticular; lenses of conglomerate overlap lenses of coarse or fine sand, and plasters of pebbles many square feet in area or long, narrow cobble pavements appear and disappear within the formation in a capricious manner. Cross-bedding is characteristic; short laminae meet each other at large angles, and longer beds form smaller angles with the horizon. Angles between 3° and 24° with a prevalent dip of N. 30° W. were measured on the Little Colorado. In many places pebbles are irregularly grouped like raisins in a pudding, and here and there lines of pebbles one-sixteenth inch to 2 inches in diameter simulate strings of beads. Again, a single cobble may be set in the midst of several cubic yards of finer-textured rock. At the mouth of Canyon de Chelly 1 foot of chocolate-colored shale, 1 foot of red-gray fine sandstone, and a thin bed of carbonaceous matter are included in 40 feet of conglomerate exposed in the wall of the canyon. At Tanner Crossing an otherwise massive bed includes several thin lenses of chocolate-colored shale. Near Winslow, at Oljeto, at St. Michaels, and elsewhere patches of gray clay shale are plastered on the under side of the lowest bed of conglomerate. At Tucker Spring the conglomerate is abnormally variable and sustains unusual relations with the underlying formations, as is shown in the following section: 2 Dutton C. E., Report on the geology of the High Plateaus of Utah, p. 144, U. S. Geog. and Geol. Survey Rocky Mtn. Region, 1880. 3 Dutton, C. E., Mount Taylor and the Zuñi Plateau: U. S. Geol. Survey Sixth Ann. Rept., p. 34, 1885. In color the Shinarump conglomerate is gray to white, rarely brown. In texture it varies from a conglomerate with pebbles as much as 3 inches in diameter to coarse grit or sandstone and in a few places to fine sandstone. The sandstone phase appears in all the outcrops observed, and conglomerate and sandstone do not appear to be geographically segregated. The outcrops along the Little Colorado between Holbrook and Leupp show about 50 percent of conglomerate with pebbles 1 to 3 inches in diameter. North of Tolchico 90 percent of the outcrop consists of clean-washed subangular quartz pebbles less than half an inch in diameter. East of Black Falls few of the pebbles exceed 1 inch in diameter, but below Tanner Crossing the outcrops again contain 30 to 50 percent of pebbles averaging 2 inches in diameter. Along the Puerco at Querino and Houck the conglomerate, arranged in irregular masses, lenses, and stringers, is composed of pebbles one sixty-fourth to 3 inches in diameter. It is also coarse in the lower part of Black Creek canyon, at Chinle, at Agathla, and at Oljeto. At Fort Defiance about 10 percent of the pebbles in the conglomerate lenses exceed 3 inches in length, and 50 percent are longer than 1 inch. The largest pebbles noted are elongated quartzites 5 to 7 inches in length. Probably more than 95 percent of the pebbles are sub-angular to round siliceous fragments of great variety; red, white, black, and topaz-colored quartz and black and gray quartzite are everywhere present; chert and chalcedony are occasionally found. Three miles north of Tolchico 5 percent of the rock consists of banded quartzite and 3 percent of limestone and chert fragments. Scattered pebbles of limestone were also found in the Shinarump at Warner Wash, near the Government bridge across the Little Colorado, and at a few other localities. Pellets of clay shale and of sandstone were observed at Oljeto and Sehili. In general matrix, pebbles, and cement are alike in composition and the rock is therefore firm and in places becomes a glistening hard quartzitic conglomerate. In the Little Colorado Valley above Grand Falls the cement is partly calcareous and the rock crumbles so readily that its presence is scarcely discernible. Iron oxide in several places forms part of the cement. Petrified wood is universally present, commonly as chunks several inches to several feet in length, but also as logs. The blocks of silicified wood are not water worn except some of the smallest particles, which exhibit rounded surfaces. Indeterminable fragments of bones and teeth were found at Warner Wash, at Oljeto, and in Buell Park.
CONDITIONS OF DEPOSITION.The geologists of the Wheeler Survey concluded that the Shinarump was a marine deposit. Dutton 2 states that the conditions under which the Shinarump conglomerate "accumulated would seem to have been remarkably uniform and may have been similar in some respects to those attending the formation of coal." Gilbert thought that an inland sea covered this region during Triassic time and speaks of "logs and leaves drifted from the shore." Huntington and Goldthwait 3 considered the origin of the Shinarump conglomerate an open question, but to these authors the weight of evidence seemed to indicate that "in part, at least, it is nonmarine" a conclusion based, however, on erroneous assumptions, including marine origin of the Moenkopi.
-from Geology of the Navajo Country, a Reconnaissance of Parts of Arizona, New Mexico and Utah by Herbert E. Gregory
The Chinle Formation consists of a relatively thick sequence of shales which display a variety of brilliant colors ranging from blue, purple, green, pink, gray, maroon, to brown. It weathers into a slope which is carved into an intricate network of gullies and forms typical badlands, West of Lee's Ferry most of the Chinle Formation is covered with erosional debris from the overlying cliffs, but southward toward Cameron, Arizona, large areas of the formation are exposed and the unit forms one of the most colorful and distinctive formations in the region. Low, dome-shaped hills commonly develop on the Chinle Formation and are almost devoid of vegetation, so that the multicolored nature of the rocks is exceptionally well displayed.
Southward, in Arizona, the formation forms the famous Painted Desert, and fossil wood and plant remains are abundant in Chinle beds in the Petrified Forest. The formation contains some of the best-preserved and most colorful fossil wood in the world. The Chinle is also noted for its important uranium deposits.
Geology Studies Vol. 15- Part 5-1968