Geology of Comb Ridge 

and Vicinity North of 

San Juan River 

San Juan County, Utah

GEOLOGICAL SURVEY BULLETIN 1021-E

A CONTRIBUTION TO GENERAL GEOLOGY

GEOLOGY OF COMB RIDGE AND VICINITY NORTH OF 

SAN JUAN RIVER, SAN JUAN COUNTY, UTAH

By JULIAN D. SEARS

ABSTRACT

The area described, in San Juan County, Utah, is approximately 7 miles wide 

and extends from the San Juan River northward for about 20 miles. It includes 

the northern part of Comb Ridge and a narrow belt on each side.

Altitudes range from 4,170 feet on San Juan River at the southwest to 6,000 

feet on Grand Gulch Plateau at the northwest, where its eastern rim forms a 

high escarpment. Comb Ridge, a conspicuous hogback of eastward-dipping rocks 

extending through the area and far to the southwest, is the most striking topographic feature and a nearly impassable barrier.

San Juan River is the major drainage course and the only perennial stream. 

It cuts across Comb Ridge, follows the strike in softer beds for 2 miles, and then 

flows in a deep, tortuous canyon through steeply dipping hard rocks. Comb and 

Butler Washes are the two principal tributaries, flowing in strike valleys. These 

streams and their numerous branches show close adjustment to the nature 

and structure of the formations over which they run.

This remote area is about 140 miles by road from the railroad to the north 

and about 215 miles from that to the south. No one is now living within its 

boundaries. The nearest communities are Blanding and Bluff to the east and 

Mexican Hat to the west. Only one main road, the newly improved State Highway 47, reaches the area, and it crosses the southern part. A new, similar rocksurfaced road extending from Blanding to the Natural Bridges comes within a 

few miles of the area's northern edge. The only other roads within the area 

are of dirt; after the abnormal rains and floods in the summer of 1953 they 

were nearly unusable.

The exposed sedimentary formations range in age from Pennsylvanian to 

Recent and aggregate 5,000 feet or more in thickness. The oldest rocks the 

upper part of the Hermosa and the lower part of the Rico are exposed in the 

deep canyon of San Juan River. Hard upper limestones of the Rico form the 

surface of a structural uplift and topographic upland near the river. Successively younger formations crop out toward the east, the youngest being the 

Bluff sandstone of Late Jurassic age, which forms a westward-facing escarpment along the east margin of the area. Most of the formations show approximately the same appearance and thickness that is characteristic of them over 

a wide region. A few show more variation or unusual features. The Cedar 

Mesa sandstone member of the Cutler formation, usually made up chiefly of 

a thick series of massive gray to buff sandstones, has a soft zone about 130 

feet thick at its base in the northern part of the area. Farther south, the entire

167
168 CONTRIBUTIONS TO GENERAL GEOLOGY

"member, 830 feet thick, grades laterally within a short distance into a soft facies 

 of very nonresistant sandstone, much light-colored clay, and many beds of 

:gypsum. The De Chelly sandstone member and the Hoskinnini tongue, both 

X)f the Cutler formation, together form only a thin zone in the southern part of 

the area and seem to wedge out northward in its central part. The formations 

of the San Bafael group, though maintaining a constant thickness, show marked 

lithologic changes from south to north in Butler Wash. In this group the Carmel 

formation changes from mostly soft reddish sandstones and mudstones to a 

unit composed chiefly of gray to buff sandstones with a soft reddish band in the 

middle. The upper part of the Entrada sandstone, which at the south is a zone 

of soft thin-bedded brick-red shaly sandstones, becomes northward a massive 

red sandstone. The Summerville formation is composed of grayish-brown or 

tan thin-bedded sandstones, sandy siltstones, and some minor shaly beds at the 

south; northward the upper part changes to a distinct reddish brown, becomes 

less thin bedded, and includes several thin grayish-white sandstones.

Alluvium, slope wash, windblown sand, and terrace gravels seen at many 

places within the area are omitted from the map.

Igneous rock was noted only in one small lamprophyre dike near San Juan 

River.

The area is in the eastern part of the broad regional Monument upwarp, and 

its principal structural feature is the great Comb monocline, which forms the 

eastern flank of that upwarp. In the northwest corner, on Grand Gulch Plateau, 

the area includes a relatively few square miles of the nearly horizontal beds 

that make up the broad top of the upwarp. The northeast end of one of the 

subordinate anticlines that complicate the Monument upwarp is exposed in 

the southwest corner, where the structure is clearly revealed by the shape of the 

surface eroded on the thin but very resistant uppermost limestone of the Rico 

formation.

A large fault, in part single, in part compound, is in the southern pai't of the 

area. With downthrow on the south, to a maximum of perhaps 200 feet, it 

begins in the RiCQ formation, extends nprtheast across the younger formations 

exposed in Comb Wash, and dies out before reaching Comb Ridge.

Two small faults on the northeast nose of the subordinate anticline mentioned

above show minor overthrusting toward the southwest and suggest that, after

uplift, the anticline may have subsided slightly and caused some lateral com- pression.

INTRODUCTION

LOCATION AND EXTENT OF THE AREA

The area (see fig. 40) described in this report is in the southern part 

of San Juan County, Utah. It is a northward-trending belt nearly

7 miles wide and 21 miles in maximum length and covers about 130 

square miles. On the east it is bounded by longitude 109°5T'30" W.

a.xx<l OI-L tlie, -west Toy lOO°-d=5' n/ncl extends rfix-om tt>G So-n .Txisxix !R.ive,i- oix

the south to latitude 37°30' E It thus covers about two and a quarter

7%-minute quadrangles; the Bluff 1ITW and Bluff 1 SW quadrangles

and the northern part of the Bluff 4 NW quadrangle.

PURPOSES OF THE INVESTIGATION

Though of considerable stratigraphic and structural interest, the 

area had previously been studied only by reconnaissance and mapped
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 169

38°

114° 112° 110°

FIGURE 40. Index map showing location of the area described in this report.

only on small scales. Added details and larger scale mapping were 

needed because of the marked variations in stratigraphy and the 

abrupt structural changes. Furthermore, the project was designed 

to supply a certain amount of instruction and practice in field investigation and in techniques of mapping stratigraphy and .structure by 

means of planetable and air photographs to selected groups of younger 

members of the U. S. Geological Survey well-trained geologists and 

paleontologists whose experience had been principally in other specialized lines.
170 CONTRIBUTIONS TO GENERAL GEOLOGY

PERSONNEL

The project was planned and carried out under immediate supervision of the writer assisted by R. Dewey Sample. For the first 8 weeks 

the staff members assigned to the party were Gerald W. Fuller, Mineral Deposits Branch; Chabot Kilburn, Fuels Branch; Sergius H. 

Mamay, Paleontology and Stratigraphy Branch; and Warren L. 

Peterson, Engineering Geology Branch. For the ensuing 6 weeks the 

members assigned were Mendell M. Bell, Mineral Deposits Branch; 

Max H. Bergendahl, General Geology Branch; Mario A. Conti, Military Geology Branch; and Ellis L. Yochelson, Paleontology and 

Stratigraphy Branch. Throughout the season, Robert A. Madden 

served as field assistant and aided in stadia traverses and other mapping as well as in miscellaneous camp duties.

FIELDWORK

The investigation on which this report is based was carried out in 

the field from early June to mid-September 1*953. For most of the 

season the party maintained a base camp in Comb Wash. Transportation by jeeps and light trucks to points where foot traverses started 

was impeded by an abnormal series of heavy rains, which caused the 

dirt roads to become virtually impassable and even brought temporary 

closing of the two main rock-surfaced roads.

Recent large-scale air photographs were used, by means of portable 

stereoscopes, in laying out the work and studying the geology; on them 

the f ormational boundaries were drawn and other geologic data were 

noted. Most of the actual mapping, however, was done by planetable 

and telescopic alidade. Significant points were located on both the

photographs and the planetable sheets, and boundaries between those

points were sketched from the photographs on the sheets. A triangulation network for horizontal control was established for the

whole area, Vertical control was set up by determining the elevation 

of triangulation points by vertical angles, starting from a bfincll

mark near the "Mormon Trail" road half a mile north of State Highway 47. Insofar as possible, instrument stations and other points 

were located by triangulation. However, in much of the area in

the deep canyons with, unscalable walls, in the lower part of Comb Wash, and on the rounded limestone upland west of the wash triangulation was not feasible, and it was necessary to resort to niOF6 

time-consuming stadia traverses. Even such traverses could not be

jnade in the deep canyons cutting Grand Gulch Plateau because of

the growing danger from recurring flash floods; for thOSG Canyons the 

geology that had been studied and recorded on the air photographs 

was transferred to a controlled photographic mosaic of the area 

and used in compiling the final map.
GEOLOGICAL SURVEY BULLETIN 1021 PLATE 18

A. LARGE NORMAL FALLT CROSSING COMB WASH

View up Comb Wash northward from San Juan River. Limestone at top of Rico formation in left foreground, 

ringed by successively younger formations. Comb Ridge at right. Photograph by J. R. Balsley.

B. HERMOSA AND RICO FORMATIONS IN SAN JUAN RIVER CANYON 

Rim of canyon and bench behind it are formed by limestone about 132 feet below top of Rico.
GEOLOGICAL SURVEY BULLETIN 1021 PLATE 10

A. THICK-BEDDED SANDSTONES REGIONALLY TYPICAL OE CEDAR MESA SANDSTONE MEMBER 

This member of the Cutler formation upholds nearly level top of Grand Gulch Plateau and is exposed in 

deep canyon cut into slope of eastward-dipping beds in Comb monocline. View westward from top of 

Comb Ridge near road from Blanding to the Natural Bridges. Elk Ridge in distance at right.

R. LOCAL KACIES Ol CKDAK MESA SANDSTONE MI.MIMK \\ITI1 11 UtD UPPER ['Mil \\\> SOFT

THIN-BEDDED LOWER PART

This facies is found in northern half of mapped area. Contact crosses middle of "haystack" at left center. 

About 20 feet below this, a thin bench-forming limestone causes the flat surface in foreground.
GEOLOGICAL SURVEY BULLETIN 1021 PLATE 20

A. NAVAJO SANDSTONE, KAYENTA FORMATION, WINGATE SANDSTONE, AND CHINLE FORMATION

ON WEST SIDE OF COMB RIDGE

View southward. Low road from Blanding to the Natural Bridges descends the dugway, crosses Comb 

Wash near the Perkins Ranch, and climbs the dip slope of Cedar Mesa sandstone member at extreme right.

h. SAGS IN SUMMI.KVILLK rOKMATION, OVERLAIN BY BLUFF SANDSTONE

On east side of Butler Wash, 4 miles north of State Highway 47. Locally, a thick sandstone at top of 

Summerville closely resembles the Bluff sandstone.
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 171

During the course of the fieldwork, about a score of section and 

quarter corner pipes established by the Bureau of Land Management 

were observed and located by triangulation. Most of the area, however, is unsurveyed or only partly surveyed. The corners found are 

mostly on a range-line survey along the west side of Tps. 38 and 39 S., 

R. 21 E.; two of them are within a partial interior survey in T. 38 

S., E. 21 E.; and a few are in the southern part of T. 40 S., R. 20 E.

The results of the season's fieldwork are brought together in a 

general geologic map of the field (pi. IT).

ACKNOWLEDGMENTS

The writer is grateful to numerous officials, merchants, and other 

residents in Blanding, Bluff, and Mexican Hat for the helpful information and other assistance they gave to the party during its fieldwork.

Lawrence C. Craig and Thomas E. Mullens, of the U. S. Geological 

Survey, visited the party during the season, shared their broad knowledge of the regional geology, and supplied an unpublished section they 

measured in the area in 1951.

William R. Hemphill, of the U. S. Geological Survey, contributed 

substantial assistance by making numerous measurements and calculations, from air photographs by means of a stereometer, of strikes and 

dips and of formation thicknesses on Comb Ridge and in Butler Wash.

GEOGRAPHY AND PHYSIOGRAPHY

GENERAL STATEMENT

The area of Comb Ridge and vicinity, north of the San Juan River, 

mapped in the summer of 1953, is but a small part of a region called by 

Gregory (1938) "the San Juan country." The reader is referred to 

his report, not only for the regional geologic and geographic setting 

for the smaller area but also for many details of geography and 

physiography that cannot be discussed in the present brief report and 

for an account of the human history of the region its prehistoric and 

later Indian dwellers and its exploration and settlement.

SURFACE FEATURES AND DRAINAGE

Total relief within the mapped area is approximately 1,800 feet, 

the altitude ranging from about 4,170 feet on San Juan River at the 

southwest corner to about 6,000 feet on Grand Gulch Plateau at the 

northwest corner.

Comb Ridge is the most striking topographic feature. This great 

hogback of eastward-dipping rocks begins southeast of Elk Ridge 

about 8 miles north of the mapped area and extends southward and 

southwestward more than 80 miles to a point beyond Kayenta in

352502 56 2
172 CONTRIBUTIONS TO GENERAL GEOLOGY

Arizona. Its eastern slope is a hummocky surface eroded on the 

Navajo sandstone in a belt from a third of a mile to a mile or more in 

width. Its western face is much more abrupt; above a steep slope of 

clays and sandstones of the Chinle formation, rises a well-nigh vertical cliff, at most places several hundred feet high, of massive Wingate 

sandstone and the lower sandstones of the Kayenta formation. Thus 

the west face of the ridge, towering to a constant height of about 800 

feet above the adjacent Comb Wash (see pi. l'&4), forms an almost 

impassable barrier. At only one place within the mapped area can it 

be crossed with vehicles where a dugway has been blasted out on the 

north side of a reentrant to carry State Highway 47. South of that

road, there are places where, because of jointing in the Wingate sandstone, the ridge can with difficulty be climbed on foot; but in the more 

than 16 miles between the highway and the north edge of the mapped 

area its westward-facing cliff probably is unscalable. Several miles 

still farther north, near the Perkins Ranch in Comb Wash, another reentrant on the west side of the ridge was used in the spring of 1953 to 

construct a dugway for a new road between Blanding and the Natural 

Bridges National Monument. (See pi. 20A.)

Another conspicuous, though somewhat less striking, topographic 

feature is the high escarpment that extends northeastward across the 

northwest corner of the area. This is the eastern margin of the wide 

upland known as Grand Gulch Plateau, which rises to an altitude 

several hundred feet greater than that of the northern part of Comb 

Ridge. The plateau is underlain by a thick series of nearly horizontal 

heavy sandstones, and the high escarpment and the walls of the deep 

canyons that are cut back into it are unscalable at most places. Where 

the escarpment crosses the west edge of the mapped area, however, the 

old "Mormon Trail" road zigzags to the top of the plateau.

The third prominent upland within the area is in the southwest 

corner, where by uplift and erosion the hard gray limestone at the 

top of the Rico formation is exposed as a domelike surface. After

passing Comb Wash, State Highway 47 climbs and crosses this upland 

on its way southwest into the valley of Lime Creek and to the bridge

over tlie river at Mexican Hat.

The San Juan Kiver, which forms the southern boundary of the 

mapped area, is the major drainage course and the only perennial

stream. Near and downstream from Bluff tne river occupies an open

valley cut in nearly horizontal rocks, but as the easterly dips of the 

beds become steeper the valley grows narrower. After passing in a 

fairly wide and low gap through Comb Ridge, the river turns south 

and follows the general strike of the softer beds in the Cutler formation 

for some 2 miles. At that point it swings to the northwest and cuts 

boldly into the steep dip of the hard Rico and Hermosa formations, in 

a deep, tortuous canyon the beginning of a series of spectacular
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 173

entrenched meanders, the best known and most visited of which are the 

"Goosenecks" downstream from Mexican Hat. Some 2 miles below 

its upper end the canyon has its right wall breached by the edge of a 

striking, deeply entrenched, and almost oval oxbow that marks a cutoff canyon loop occupied by the river when flowing at a higher level. 

The bottom of this oxbow is now marked by two small dry stream beds, 

which join near their lower ends and drain into the present canyon by 

a high "dry waterfall."

The two principal tributaries within the mapped area are Comb 

Wash and Butler Wash. Comb Wash and its upper branches head 

on the south flank of Elk Ridge; Butler Wash heads somewhat farther 

south. Both streams flow south and southwest, closely flanking Comb 

Ridge on its west and east sides, respectively, to their junction with 

the San Juan River. The locations and patterns of both streams 

reflect the relative hardness and structure of the underlying and 

adjacent formations. Both streams closely follow the strike; and 

as their valleys were being slowly lowered in the softer rocks, both 

streams probably migrated laterally eastward down the dip slopes 

of the underlying more resistant Cedar Mesa sandstone member and 

the Navajo sandstone respectively.

In its upper reaches Comb Wash flows south in a fairly wide shallow 

channel cut in earlier valley fill; its meanders are of moderate size, 

and its low alluvial banks are flanked by deposits of windblown sand 

and slope wash. Downstream, where the strike is more to the southwest and the dips are much steeper, the valley and the belt of alluvium 

are much narrower, and the exposures of bedrock are better. (See 

pi. 184.)

Butler Wash is similar in some ways and different in others. In its 

upper reaches it, too, flows in an alluvium-filled valley; but its 

meanders are much smaller and more twisted, and its channel is narrower and deeper. Downstream, beginning where the strike swings to 

the southwest (just as does the narrowing of Comb Wash valley), the 

alluvium abruptly ceases; and the wash flows in a narrow canyon 

cut in the Navajo sandstone.

The many side streams entering Comb and Butler Washes also 

clearly show the effect of the varied geology and topography of the 

area and present many interesting physiographic problems that deserve more detailed study than could be devoted to them in the course 

of the fieldwork. Those tributaries entering Comb Wash on the east 

are numerous; but being confined to the narrow slope of clay at the foot 

of the west side of Comb Ridge, they are short and shallow. Those on 

the west are much larger and more conspicuous. They are also much 

more varied, falling into three principal types. In the northern 

part of the area, where the thick Cedar Mesa sandstone member is 

very hard and resistant, the streams flow in deep, formidable canyons.
174 CONTRIBUTIONS TO GENERAL GEOLOGY

IFarther south, between the lower reaches of Road Canyon and Snake 

Gulch, the Cedar Mesa is much softer and less resistant, and the principal streams and their innumerable branches make an intricate leafvein pattern. Still farther south the streams have cut deeply into 

the southeastward-dipping hard limestones and sandstones of the Rico 

formation. Some of these streams show anomalies in their lower 

courses in that, where they cut across the steeply dipping hard layers 

in the Halgaito and Cedar Mesa, they flow northeast instead of southeast toward Comb Wash as would be expected. These anomalies 

are thought to be due to the softening effect of northeastward-trending 

fractures and incipient faults in those beds, parallel to the large fault 

shown on the map.

The side drainage of Butler Wash is of more usual types. The many 

roughly parallel streams flowing down the east slope of Comb Ridge 

are conspicuous because of the rounded trenches they have eroded in 

the dip slope of the Navajo sandstone; but they are of physiographic 

interest particularly because a number of them, upon reaching the 

foot of that slope, do not enter Butler Wash directly but first turn 

south and run for considerable distances before entering the main 

stream.

Comb and Butler Washes and their branches are all intermittent 

streams, usually flowing only in prompt response to rainfall. A 

single heavy, though local, storm may send a flash flood many feet 

deep down a side canyon. When this flood pours into the main stream 

it widens out but still may fill the channel bank to bank with a muddy 

stream several feet deep persisting for perhaps half a day.

CLIMATE AND VEGETATION

The area lies within a semiarid region and with its relatively high 

altitudes has the characteristic scanty rainfall (averaging perhaps 

8 or 10 inches a year) and large daily and yearly range of temperatures (from a winter minimum below zero to a summer maximum of 

well above 100 degrees). Readings are not taken within the area 

itself,' the nearest Weather Bureau stations are at Blanding, Bluff, 

and Mexican Hat, a few miles northeast, east, and southwest of ItS 

limits, respectively; and the approximate figures given above are

based on the available records frona tKose three stations over the past

decade. Most of the rain falls in brief local showers, some oi wmcn

are very heavy. Only a minor part of the precipitation is absorbed"

to become ground water. In a few of the canyons, there are small

water seepages, but most of them are only large enough to cause wet 

patches for a short distance downstream and few merit the name of 

"spring." To the writer's knowledge, the largest spring within the 

area was the long-known and frequently used "Navajo spring," below 

the old dugway on the west side of Comb Ridge. This spring was
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 175

damaged and largely covered by the rocks that tumbled down the 

steep slope during the rebuilding of State Highway 47 in the spring 

of 1953 and would require considerable work to make it again usable. 

Timber in the area is limited chiefly to the numerous junipers on 

Grand Gulch Plateau and a fair number of cottonwoods in the upper 

part of Comb Wash. Elsewhere, a few scattered cottonwoods are 

found in lower Comb Wash and in Butler Wash, and sparse piiion 

pines and junipers are seen on some of the sandstone ledges. 

Tamarisks grow along the edge of Comb Wash valley at a few places. 

In general, however, the vegetation of the area includes only sagebrush, greasewood, prickly pears, and other low-growing plants 

typical of a semiarid region. Forage plants are sufficient in both 

Comb and Butler Washes to supply a moderate amount of winter 

grazing for cattle.

SUPPLY POINTS AND TRAVEL ROUTES

There are no towns or villages in the mapped area indeed, so far 

as known to the writer not one person is now living within its borders. 

Until 1951 or 1952 a few Indian families lived in hogans in the upper 

part of Butler Wash; but at that time they were required by court 

decision to move to the reservation south of the river.

The nearest communities are Bluff, an early Mormon settlement 

on the river a few miles east, and Mexican Hat some 15 miles to the 

southwest. More varied supplies and facilities are available in 

Blanding, a town of some 1,200 population about 30 miles by State 

Highway 47 from the southeast corner of the area. For other services and supplies recourse is had to the more distant towns such as 

Monticello, Moab, and Cortez or to Grand Junction, Colo., some 

250 miles distant by road.

The only main road crossing the area is State Highway 47, usually 

referred to locally by the briefer term "U-47." It begins at Monticello, where it connects with paved U. S. Highway 160, which extends 

northward 88 miles through Moab to Crescent Junction, located on 

U. S. Highways 6 and 50 and the Denver & Rio Grande Western 

Railroad, and extends eastward and southeastward 60 miles to Cortez, 

Colo. From Monticello, State Highway 47 runs south through 

Blanding and Bluff, thence west and crosses the southern part of the 

mapped area. Still farther southwest it crosses the San Juan River 

by a bridge at Mexican Hat and reaches Monument Valley, where it 

connects with Arizona roads leading via Kayenta and Cameron to 

Flagstaff on The Atchison, Topeka, and Santa Fe Railway. By 

these roads the mapped area is about 140 and 215 miles, respectively, 

from the railroads to the north and south. Despite the remoteness 

of the area, however, State Highway 47 brings to it a rather surprising amount of tourist as well as other traffic. In the spring of
176 CONTRIBUTIONS TO GENERAL GEOLOGY

1953 that part of State Highway 47 between Blanding and Mexican 

Hat was improved. It was relocated in places, widened to permit 

continuous 2-way traffic, and surfaced throughout with crushed 

rock. Particular improvements were the construction of a bridge 

over Butler Wash, eliminating the descent into its deep rock-cut 

canyon; the construction of a new dugway with much gentler grade 

down the west side of Comb Ridge; and just west of Comb Wash, 

bypassing of the former twisting route up Snake Gulch. In the fall 

of 1953 that part of the road from Monticello to Blanding was surfaced with "black top," and a new bridge was being built over the 

river at Mexican Hat.

Another main road extends east and west several miles north of the 

northern edge of the mapped area. This new road, also surfaced 

with crushed rock, was completed in the spring of 1953. It leaves 

State Highway 47 at a point about 3 miles south of Blanding, runs 

roughly west to the Natural Bridges National Monument, and connects with the road down White Canyon to the Colorado River ferry 

at Hite. This road is much lower than, and an alternative for, the 

old road from Blanding to the Bridges, which, because of its altitude 

of more than 8,500 feet near the Bear's Ears is blocked by snow or 

made impassable by mud during much of the year. The new road 

descends Comb Ridge by a steep dugway down its west side (see 

pi. 20A), crosses Comb Wash just upstream from the Perkins Ranch, 

climbs the dip slope of sandstone to the top of Grand Gulch Plateau, 

and goes west some 15 miles to join the Bear's Ears road a few miles 

east of the Bridges.

Between these 2 main roads extend 3 dirt roads through or across

the mapped area. The first and most important of these was the 

old "Mormon Trail" road. This leaves State Highway 47 and runs

north on the west side of Comb Wash for about 5 miles, crosses Road 

Canyon, and turns northwest for a few miles, where it climbs to the 

top of Grand Gulch Plateau. Some 25 miles farther northwest it 

joins the 2 roads between Blanding and the Bridges the new low 

road and the older high road by way of the Bear's Ears. In the past 

the "Mormon Trail" road was much used by travelers to and from

the Bridges and White Canyon, because it served both as a short cut

from or to the south on State Highway 47 and as a substitute for the 

Bear's Ears road, when impassable, for those going from or to

Blanding and. tlie north..

Just south of Koad Canyon a road branches from the "Mormon

Trail" road, crosses to the east side of Comb Wash, and runs north

some 15 miles to the Perkins Ranch just outside of the mapped area.

The third road leaves State Highway 47 and goes up the east side

of Butler Wash to a point just north of the mapped area, then turns
northeast out of the wash, climbs over a divide, and drops into Cottonwood Wash where it joins the new low road from Blanding'to the 

Bridges. That part of the road in Butler Wash is in general passable 

only by vehicles with 4-wheel drive, even in dry weather; and after 

storms it becomes virtually impassable.

Indeed, all 3 of these dirt roads suffered so badly from the heavy 

rains and floods in July and August 1953, with washed-out culverts, 

deep rutting, cave-ins, and boulder cover, that some doubt was expressed about their repair and further maintenance, especially in view 

of the fact that the new low road from Blanding to the Bridges now 

gives the necessary alternative for the Bear's Ears road and also 

shorter and better access to the Perkins Kanch.

STRATIGRAPHY

GENERAL SECTION

Within the area described in this report are exposed beds aggregating 5,000 feet or more in thickness and ranging in age from Pennsylvanian to Recent. The oldest rocks the Hermosa formation, of 

Pennsylvanian age are seen in the deep canyon of the San Juan 

Kiver. The next overlying formation the Rico, of Pennsylvanian 

and Permian (?) age is exposed in the same canyon walls, and its 

hard upper limestone layers form the surface of the uplift in the 

southwest part of the area. Because of the easterly dips in the Comb 

monocline, bands of successively younger formations are found toward 

the east; the youngest named formation within the area the Bluff 

sandstone, of Late Jurassic age lying nearly horizontal forms a 

west ward-facing escarpment along the east margin of the mapped 

area. Recent alluvium, terrace gravels, slope wash, and windblown 

sand were noted at a number of places within the area, particularly 

along the upper part of San Juan River and in the valleys of Comb 

and Butler Washes; but these were not studied in detail and are not 

shown on the geologic map because in places they would too fully 

obscure the bedrock.

The stratigraphy is in part complicated by marked lateral variations of several formations and members in both lithology and thickness, as brought out in the descriptions of those units. The regional 

setting and broader relations of these strata, as well as many pertinent 

details about them, have been presented by Gregory (1938). Many 

helpful details are furnished also .by Baker (1936) for the Monument 

Valley-Navajo Mountain region, which approaches within 10 miles 

of the southwest corner of the area described herein.

The areal distribution of the formations is shown on plate 17. 

Their succession and character are summarized in the following 

general section.
178 CONTRIBUTIONS TO GENERAL GEOLOGY

General section of sedimentary rocks exposed in mapped area

Systems and 

series

Quaternary

S

! &

_c>

I

Upper Jurassic series

Middle andUpper Jurassic series

Jurassic Lower

series

a 

f<n 

_o

I

H»

TrLassic system

Triassic series Upper 

Lower andMiddle(?) Triassic 

series

Group, formation, and member

Rafael San(upper part) group 

Rafael Sangroup part) (lower 

Canyon Qlen group part) (upper 

Qlen Canyon group 

part) (middle 

II9°

cj^ 

0&^

H t-3 § s| gMft

Bluff sandstone

Summerville formation

Entrada sandstone

Carmel formation

Navajo sandstone

Kayenta formation

Wingatc sandstone

Cninle formation

Moenkopi formation

Thickness 

(feet)

100+

146-162

150

108-114

341±

126db

27S±

956

140(?)- 

222

Character of rocks

Alluvium of clay and conglomeratic 

lenses in Comb and Butler Washes; 

slope wash of clay and of sandstone 

boulders on east side of Comb Wash; 

windblown sand on west side of 

Comb Wash and elsewhere; terrace 

gravel near mouth of Comb Wash 

and on San Juan River.

Grayish-brown, thick, massive, resistant sandstone, in part crossbedded.

Grayish-brown or tan thin-bedded 

sandstones, sandy siltstones, and 

minor shaly beds. Northward, 

upper part becomes reddish brown, 

includes several thin grayish-white 

sandstones, and is less thin bedded.

Lower unit, medium-bedded gray to 

buff sandstones. Upper part, soft 

thin-bedded brick-red shaly sandstones to south, more massive red 

sandstone to north.

Soft, thin-bedded red sandstones and 

mudstones, with a few thin gray 

sandstones, in southern part. Gray 

to tan, thicker, more massive sandstones, with a softer reddish zone at 

middle, in northern part.

Grayish-white sandstone, in part 

weathering light buff; a few softer 

reddish sandstones in lower part.

Large-scale tangential crossbeduing.

Alternating sandstone, siltstone, and 

limestone, buff, tan, gray, white, 

and light red; essentially flat bedded; 

medium to thin bedded.

Massive, cliff-making ]ii;ht-orange-red

or brick-red to reddish-buff sandstone, crossbedded on large scale.

Variegated shale ana clay; brown-gray

and orange-red sandstones; thin Jiuiusivuw.

Chocolate-brown and brownish-red 

thin-bedded sandstone and siltstone; 

ripple marked at numerous horizons.
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 179 

General section of sedimentary rocks exposed in mapped area Continued

Systems and 

series

8O>

I 

I

a 

(2

 irtSw t>» 05 d d

agaS B.SS« £S|£

>OH

gj.ll «£?§« * Ps?

Group, formation, and member

Cutler formation '

Hoskinnini tongue

De Chelly sandstone 

member

Organ Rock tongue

Cedar Mesa sandstone 

member

Halgaito tongue

Rico formation

Hermosa formation

Thickness 

(feet)

0-30±

0-135

560(?)- 

820

830

406

390±

330+

Gha'racter of rocks

'Alternating thin , layers;.6f ''orange-red, .brownish-red, chocolate-brown, and grayish-white sandstone.

Red-brown sandstone, -crossbedded in 

upper part, more massive in lower 

part.

Brownish- to orange-red rather thinbedded fine-grained sandstones and 

sandy mudstones that weather into rounded surfaces and thin slabs. Grayish-green mottling in patches 

and stringers is common.

Thick-bedded, massive gray and buff 

sandstones with some thinner layers and lenses of gray, buff, and reddish sandstone and clay, in northern part; locally, basal 130 feet is softer, with 

thin beds of variegated clay, sandstone, gypsum, ana limestone. Toward south, whole member grades into soft facies, of pastel-colored clays, friable sandstones, and gypsum.

Hard and soft thin-bedded brownish- red sandstones, slltstones, and mudstones, with a few thin beds of gray 

limestone and white to buff sandstone.

Hard gray limestone and massive sandstone in beds up to 20 feet thick, interspersed with thicker zones of 

prevailingly red softer sandstones and thin-bedded mudstones.

Gray, cherty, very siliceous limestones, interbedded with gray shale.

PENNSYLVANIAN SYSTEM 

HERMOSA FORMATION

Above river level in the canyon of San Juan River in the southwest 

part of the mapped area is exposed some 330 feet of beds identified 

as belonging to the Hermosa formation. They consist chiefly of 

gray cherty very siliceous limestones, interbedded with gray shale. 

The limestones contain some fossils, principally brachiopods and 

echinoid spines. The exposed Hermosa occupies roughly the lower 

half of the canyon wall and has the general appearance of a gray 

slope interrupted by harder, ledge-making layers. Its upper contact 

was selected as the top of a light-blue-gray sandy shale zone 15 to 

20 feet thick (the upper of two such light-toned bands) just below 

a conspicuous ledge of tan-weathering sandy limestone about 25 feet 

thick. (See pi. ISA.)

352502 56 3
ISO CONTRIBUTIONS TO GENERAL GEOLOGY

The beds thus exposed represent only the uppermost and minor 

part of the Hermosa of this region, for Baker (1936, p. 19) records 

an incomplete thickness of 1,020 feet farther down the river, 3 miles 

northwest of the Honaker Trail.

PENNSYLVANIAN AND PEKMIAN(?) SYSTEMS 

RICO FORMATION

Lying above the Hermosa formation with apparently full conformity is the Kico formation, now classed by the Geological Survey 

as of Pennsylvanian and Permian (?) age. Based upon several elevation readings by Bell and Yochelson, it is estimated in this area 

to be about 390 feet thick. It includes numerous hard gray limestones 

and massive sandstones up to 10 or even 20 feet thick, interspersed 

with thicker zones of softer and prevailingly red sandstones and thinbedded mudstones. At its top is a thin dense blue-gray limestone with 

local zones of fragmental crinoid remains. This limestone is so resistant that it forms the surface of most of the domelike anticlinal 

upland in the southwest part of the area. Nearer the river, however, 

this limestone and the subjacent 130 feet of beds have been eroded, 

and the next resistant gray limestone (characterized by included, 

irregular masses of red chert) forms a wide bench and the rim of the 

canyon. (See pi. 1&#.) In the walls of the canyon, the Kico contrasts markedly with the underlying gray slope of the HePHlOSa, for 

it has a generally red tone and a greater number of thick ledge-forming beds that give a step-and-siope profile to the upper canyon walls. 

As previously stated, its contact with the Hermosa was selected as the 

base of a tan-weathering ledge of sandy limestone some 25 feet thick.

The uppermost 133 feet of the Kico in this vicinity was recorded 

by Yochelson as follows:

Section O/ ttj)J)er part of Rico formation measured just north of entrenched 

oxbow in southwest corner of mapped area

Feet

Limestone, fine-grained, light-gray, massive, resistant; uppermost inyer of 

Rico, forms surface of upland 2.5

Sandstone, staiy, weathering dark red__ ________________ 55

sandstone, upper part massive, ledge-forming, weathering light tan; lower

PRrt thin bedded, shaiy, weathering dark red_____________- 1S- ° 

Limestone, sandy to shaly, weathering gray l- ® 

Sandstone, mostly mm-beaded, slm]y, weathering dark red 1, includes 5 or 

G more resistant layers eatil 2 to 2 feet thick_______________ 111. 0

Total__.___________________:______ . 133.0

Limestone, fine-grained, gray, with included irregular masses of red chert; 

resistant, weathers to rough surface; forms a prominent bench and the 

rim of San Juan River canyon.
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 181

PERMIAN SYSTEM 

CUTLER FORMATION

In his report on the Monument Valley-Navajo Mountain region, 

Baker (1936, p. 28) states, "All the strata between the top of the Rico 

formation and the base of the Moenkopi formation, of Lower Triassic 

age, are included in the Cutler formation. * * * The Cutler formation is divided into five distinct units, which, from oldest to youngest, 

are the Halgaito tongue, Cedar Mesa sandstone member, Org&n Rock 

tongue, De Chelly sandstone member, and Hoskinnini tongue." 

Gregory (1938, p. 41) brought this usage of the Cutler formation and 

its five subdivisions northward across the San Juan River into the 

San Juan country, and it is appropriate and applicable, for the area 

discussed in the present report.

HALGAITO TONGUE

Lying upon the Rico formation with no evident break is the basal 

unit of the Cutler formation, the Halgaito tongue. In the southwest 

sector of the mapped area, the Halgaito makes a prominent, rather 

bright-red zone above and curving around the arched gray-limestone 

surface of the Rico. Farther north, varying amounts of its upper 

part are seen in the several branches of Road Canyon, Fish Creek, 

and Dry Wash, but the lower part and the underlying Rico are not 

exposed.

In this area the Halgaito tongue is made up chiefly of hard and soft 

thin-bedded sandstone in layers from a foot to 10 feet thick, interspersed with thin-bedded siltstones and mudstones, of a general reddish tone. A few scattered thin beds of gray limestone and white to 

buff sandstone were noted. Some 10 feet above the base is a red 

sandstone several feet thick whose upper surface weathers into 

separate rounded blocks; where mounds or "islands" capped with this 

layer remain on the gray-limestone upper surface of the Rico the bed 

viewed from a little distance has the appearance of a low manmade 

stone wall of piled-up small boulders. As a whole, the alternating 

hard and soft thin-bedded material in the Halgaito gives to its outcrops a distinct layered effect. Yochelson noted that near the 

river the unit contains somewhat thicker and more resistant ledges of 

sandstone than it does near State Highway 47. He also recorded 

that south of the highway a few local lenses of conglomerate were 

observed.

In the small canyon half a mile south of Snake Gulch, near State 

Highway 47, the Halgaito was found to be 406 feet thick, as measured 

by Bell and Yochelson in the following section:
182 CONTRIBUTIONS TO GENERAL GEOLOGY

Section of Halyaito tongue measured in canyon half a mile south of Snake Giilch

Cedar Mesa sandstone member of the Cutler formation.

Halgaito tongue of the Cutler formation : Feet 

10. Sandstone, platy, red, poorly exposed________________ 10. 5

9. Gypsum in beds 1 to 2 feet thick, interbedded with gypsiferous

shale____________________________________ 8. 0

8. Largely covered; gypsiferous sandy shale in upper part, and some

fine-grained platy calcareous sandstone in lower part 41. 0

7. Sandstone, fine-grained, somewhat platy; white to red, weathering

brick red______________________ ___________ 6. 0

6. Sandstone, very calcareous, or limestone, very sandy; fine-grained,

platy; brown with purplish cast, weathering red brown_____ 1. 5

5. Sandstone, fine- to medium-grained, brick-red, in 1- to 6-foot layers, 

interbedded with more friable layers. Lower quarter of zone has 

massive and softer beds of about equal thickness; upper threequarters averages 10 to 15 feet of soft sandstones between ledges. 140. 0

4. Shale, sandy, dark-red, interbedded with sandier dark-tan layers 

weathering red. A 2-foot ledge of sandstone in lower third of 

zone, and a prominent 5- to 6-foot sandstone ledge slightly above 

middle of the zone____________________________ 190. 6

3. Sandstone, fine-grained, light-gray, weathering gray with bluish

cast_____________________________________ .2

2. Sandstone, very fine-grained, blue-gray, weathering tan______ 1. 2

1. Shale, weathering red; mostly covered_________________ 7.0

Total _____________________________________ 406. 0 

Kico formation.

This thickness of 406 feet checks closely with the 402 feet nearby at 

Navajo Springs as recorded by Gregory (1938, p. 42). On the same 

page Gregory gives 203 feet as the thickness in Fish Creek Canyon, 

and two paragraphs later says, "In Fish Creek Canyon strata assigned

to the Halgaito are about half the thickness of those 10 miles farther

south and are weathered at the top." For this and several other

recorded reasons, he suggests the possibility of erosion and an unconformity at the top of the Halgaito, On the next page (1938, p, 43)

he suggests a "northward thinning and final disappearance of the 

Halgaito." This suggestion has since been confirmed by studies of 

other geologists to the north and northwest; but his cited thickness 

of 203 feet in Fish Creek Canyon could scarcely be considered evidence

of that thinning in. vie-vv of the f net stilted, earlier that, the base of the

tongue is not there exposed.

CEDAE MESA SANDSTONE MEMBER 

The Sharp color change between the red Halgaito tongue and the

predominantly light-huecl strata of the overlying Cedar Mesa sandstone member makes the belts of outcrop of the two units readily visible 

and separable where seen from vantage points. Furthermore, in that 

part of the mapped area where the Cedar Mesa consists chiefly of
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 183

thick, massive sandstones, the separation is emphasized by the change 

from the sloping surface of the less resistant Halgaito to the abrupt 

ledges and cliffs of the Cedar Mesa.

Gregory's suggestion (1938, p. 42, 43) that there was erosion at 

the top of the Halgaito tongue and at least minor unconformity between it and the Cedar Mesa is given some support within the mapped 

area by the presence locally of a thin limestone-pebble conglomerate 

at the base of the Cedar Mesa, as noted in 1953 by Fuller near Snake 

Gulch and by Yochelson near Comb Wash between State Highway 47 

and the river. Gregory points out, however, that the conditions he 

describes may be due to lateral gradation between the intertonguing 

Halgaito and Cedar Mesa, as they were considered by Baker and 

Reeside (1929).

The Cedar Mesa sandstone member was named by Baker and Reeside 

(1929, p. 1443) from an upland north of the San Juan River and north 

and west of Lime Creek known as Cedar Mesa, which is part of the 

broader Grand Gulch Plateau. Strikingly exposed in the mesa walls 

the Cedar Mesa member consists of many hundreds of feet of lightcolored sandstone in beds 10 to 50 feet or more in thickness, with some 

thinner layers and lenses of softer sandstone and clay, some of which 

are red. The member can be followed far to the north and northwest 

under Grand Gulch Plateau (see pi. 19J.) and to and beyond Elk 

Ridge, the Natural Bridges, and White Canyon, with essentially similar thickness and lithology.

Along the east margin of Grand Gulch Plateau, where its southeastward-facing escarpment extends across the northwest corner of the 

mapped area discussed in this report, about T25 feet of the Cedar 

Mesa member remains (as calculated from elevation readings) ; and 

about 100 feet of its uppermost beds are estimated to have been eroded. 

The upper, major part of the member consists chiefly of the usual sequence of thick, massive, somewhat crossbedded white to buff sandstones, so resistant that the walls of the upland and of the deep canyons 

cutting it are unscalable at most places. The basal, minor part of 

the Cedar Mesa member, averaging about 130 feet thick immediately 

above the Halgaito, shows a markedly different and unusual lithology. 

It is much less resistant than the upper part and of a generally grayer 

tone, so that viewed from a distance its soft rounded slopes below the 

ledge-forming sandstones appear to be carved from a zone of gray clay 

with a few harder layers. Examined more closely, it was found to 

contain not only gray, reddish, and purplish shale or clay but also 

a number of soft fine-grained friable sandstones, some of them gypsiferous; several thin beds of gypsum; and three thin beds of gray 

limestone. The uppermost limestone, occurring at an average distance 

of some 20 feet below the top of the zone, contains many small irregu-
184 CONTRIBUTIONS TO GENERAL GEOLOGY

lar masses of red chert; it is sufficiently resistant to hold up benches 

and dip slopes. (See pi. 19B.) The other two limestones are less 

conspicuous. The middle limestone, which is absent at places, was 

found to contain scattered fossils (apparently most of them pelecypods) that, however, were unfortunately too fragmentary and poorly 

preserved for identification or for use in determining the age of the 

beds.

This interesting and striking contrast between the "lower soft" and 

"upper hard" parts of the Cedar Mesa (apparently not noticed by 

previous workers during their reconnaissance in the area) is well exposed in the valleys of Mule Canyon, Dry Wash, Fish Creek, and 

nearby smaller streams. The sharp contact between them, above a 

series of thin beds of sandstone and below a massive, ledge-forming 

buff sandstone, makes a clear horizon marker. (See pi. 195.) It was 

therefore mapped in the field, a number of elevation readings being 

taken on it for structure-contour purposes (pi. 17). Farther south, 

in the northern two forks of Eoad Canyon, the distinction is still visible ; and the contact was mapped, though with less certainty because 

the lower part is much more sandy. Still farther south, in the other 

two main forks of Road Canyon and along the Cedar Mesa cliffs above 

the Halgaito, the lower part is again a series of ledge-forming sandstones like the upper part, and tracing of the contact was no longer 

feasible.

A larger scale and even more striking departure from the usual 

lithology of the Cedar Mesa sandstone member is found to the southeast. This is clearly seen within the crude triangle of Cedar Mesa 

oxitcrops approximately bounded on the east by the Mormon Trail 

road, the northwest by the longest branch entering Road Canyon

from the southwest, and the south by a line drawn due east from the

head of that branch. Within this triangle and in the narrow belt of

outcrops southward to the river, the whole Cedar Mesa is a series 

of very soft pastel-colored rocks that bear virtually no resemblance to 

the typical series of thick resistant sandstones in the escarpment and 

canyons to the northwest. Unfortunately, the great lateral change in 

lithology toward the southeast cannot be followed or studied in detail,

bed by bed, because the drainage in relation to the structure (nearly

horizontal beds under the upland and steep easterly and southeasterly

dips nearer Comb Wash) has served to erode most of the Cedar Mesa

within the transition belt. However, that transition belt must be only 

a mile or so wide and must extend in a northeasterly direction on the

northwest side of the crude triangle described above; and the change

must wow rapidly toward the southeast within the belt,

This unusual facies of the Cedar Mesa sandstone member, the softness of which has resulted in a land surface cut by an intricate leaf-
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 185

vein pattern of small streams (not shown on the map, because of its 

small scale), has aroused the interest of geologists earlier visiting the 

area. Probably the first to notice it was Woodruff (1912, p. 86), who 

described it briefly. Gregory (1938, p. 44) gives the following graphic 

description 

Along lower Comb Wash the Cedar Mesa member, as represented elsewhere, is 

scarcely recognizable. The dominant resistant sandstones give way to weak 

strata and, in consequence, the usual cliffs are replaced by slopes and mounds 

of crumbling rock. * * * the Cedar Mesa here includes not only beds of white 

sandstone but also red-pink gypsiferous shales, brown shaly sandstones, conglomeratic and even-bedded dense limestones, chert, and much gypsum. Though 

the section includes beds of typical tan-colored Cedar Mesa sandstone, the thickest measures only 22 feet. Most of them are sheets a few inches to a few feet 

thick that appear as if inserted between layers of shale, of shale and gypsum, 

and of limestone and dark-red thin sandstones. Some of them taper to extinction ; others grade laterally into thin beds or thick beds that differ from the 

type only in color; still others grade along the strike and down the dip into 

gypsiferous or arenaceous shales with or without change in color.

Most of the sandstones in this facies of the Cedar Mesa are so soft 

and friable that from a distance of a few hundred feet their existence 

is scarcely suspected. They were studied more closely by Bell and 

Yochelson, who measured, the following section of the member in the 

small canyon half a mile south of Snake Gulch, near State Highway 

47. (This is an upward continuation of the section, the Halgaito part 

of which is recorded on p. 182.)

Section of Cedar Mesa sandstone member measured in canyon half a mile south

of Snake Gulch

Organ Rock tongue of the Cutler formation.

Cedar Mesa sandstone member of the Cutler formation : feet 

60. Limestone, sandy, medium-grained, pale-green to lavender,

weathering dark gray in lower half, almost white at top;

laminated near base___ _. _ _______ 1.0 

59. Shale, dark- to brick-red, weathering into small blocks and

chips_____________________________________ 27.0 

58. Poorly exposed; probably mostly soft reddish sandstone_____ 28.0 

57. Sandstone, medium-grained, brownish-red, weathering light red;

crossbedded at low angle__ _ _ _ _ ____ 1.2 

56. Shale, weathering dark purple to brick red and into small blocks;

thin layer of more consolidated claystone 6 feet above base___ 9. 0 

55. Limestone, fine-grained, light-greenish-gray, weathering light gray;

in two beds separated by 0.2 foot dark red shale_________ 1.2

54. Sandstone, very friable, weathering brick red; very poorly

exposed____________________:____________ 9.0 

53. Sandstone, fine-grained, white, weathering tan; lower 0.5 foot,

platy; upper 1.7 feet, a ledge former weathering into large

blocks___________________________________ 2.2

52. Sandstone, friable, soft, weathering dark to brick red_______ 1.1 

51. Sandstone, medium-grained, white, weathering gray brown____ 1. 5
186 CONTRIBUTIONS TO GENERAL GEOLOGY

Section of Cedar Mesa sandstone member measured in canyon half a mile southof Snake Gulch ^Continued

Cedar Mesa sandstone member of the Cutler formation Continued Feet

50. Sandstone, fine-grained, chocolate-brown, weathering very dark

red brown ; less resistant than beds above and below__ _ 1. 5-

49. Sandstone, medium-grained, white, weathering gray brown; a

ledge former; weathered surface shows fine laminae______ 2.4

48. Sandstone, soft, white, weathering dark red; upper 2 feet more

shaly__________________________________ 10.1

47. Limestone, slightly sandy, fine-grained, very light-gray, weathering gray with dark-red streaks________________:__ . 8

46. Sandstone, platy, weathering purple_________________ 1. 5

45. Sandstone, medium-grained, white, weathering gray brown; a

ledge former___ _____ _ 2. 0>

44. Shale, sandy, weathering red to purple, upper foot weathering

white___________________________________ 7.0

43. Sandstone, platy, fine-grained, very light-gray, weathering nearly

white _ _______________________________ .8

42. Sandstone, gypsiferous, weathering red________________ 18. 4

41. Limestone, fine-grained, dark-gray, weathering dark gray, blocky_ . T

40. Limestone and chert, thin, at bottom; overlain by gypsiferous beds

like No. 38; distinguished from No. 39 chiefly by color_____ 82. 3

39. Shale, gypsiferous, and gypsum; like No. 38, but weathering somewhat darker red without pinkish cast; bed of white gypsum at 

base- ___________________________________ 109.6

38. Gypsum, some very sandy, weathering with pinkish cast; somewhat sandier and darker red at top________________ 82. 3

37. Limestone, in two beds separated by gypsiferous shale, lower bed

weathering light blue gray, upper bed, dark gray__________ 1. 0

36. Gypsum, weathering pink ; locally with a foot of white gypsum

at base_____________________________________ 32. 7

35. Sandstone, gypsiferous, and gypsum, weathering red faintly tinted

with pink or orange; upper 4 feet, slightly more resistant, darker 

red sandstone ^ 81. 4 

34. Limestone, very thin-bedded, medium-grained, lavender, weathering gray to purple : _ _ ________________ 1. 8

33. Gypsum, sandy, white, weathering red-____________ ft0

32. Limestone, fine-grained, light-gray _ ___ ______________ . 3

31. Sandstone, gypsiferous, platy, weathering with pinkish cast; top

marked by 1-foot gypsum bed above 2 feet of purple shale_ 36. 4

30. Sandstone, moderately resistant, siabby to platy, weathering purplish near base, brick red in middle, gray to white above____ 6. 0

29. Shale, sandy, gypsiferous, and gypsum____________________ 37. 9

28. Limestone, fine-grained, dark-purplish-gray, weathering light blue

gray .5 

27. Sandstone, fine-grained, purplish-brown, weathering very dark red;

more calcareous upward___________________________ 4. 0 

2O. Limestone, thin-oeaaetl, Ilne-grainea, darK-gray, weutnering mue

gray; some white chert____________________ 1.0 

25. Shale, gypsiferous, and sandstone, weathering red with pinkish

to orange cast _ ________________________ 15.9 

24. Limestone, very sandy, fine-grained, dark-brown, weathering

brownish gray, rubbly__________________________ 1. 0
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 187

Section of Cedar Mesa sandstone member measured in canyon half a mile 'south 

of Snake Gulch Continued

Cedar Mesa sandstone member of the Cutler formation Continued

23. Sandstone, fine-grained, dark-brown, weathering brick red ; in beds 

a quarter to a half inch thick ; crossbedded at low angle ; soft, 

but somewhat more resistant than No. 22 53. 3

22. Poorly exposed. Lower 3 feet, sandstone, platy, weathering brick 

red; upper part, gypsiferous shale and sandstone, weathering 

brownish red _ _ 48. 6

21. Sandstone, medium-grained, light-buff to tan, weathering light

brown with orange cast ; locally a ledge former. 8. 0

20. Sandstone, soft, platy, crossbedded at low angle, weathering dark

red _______________ _ 27. 1

19. Poorly exposed. Interbedded thin limestones, weathering white,

and shale, weathering purplish _________________ 4. 5

18. Limestone, very sandy, medium-grained, light-blue-gray, weathering brown __ _ _______ __ __ ______ __ _____ 2. 0

17. Sandstone, white, and gypsum ; very soft ; poorly exposed 4. 5

16. Sandstone, fine-grained, white, weathering white to red with

orange cast _ _ ___ ____________________ 1. 3

15. Limestone, very sandy, medium-grained, blue-gray, weathering 

dark gray ; at top a 0.1-foot layer of sandy limestone conglomerate ; changes laterally into sandstone of No. 14 _ _______ . 8

14. Sandstone, fine-grained, very light-gray, weathering blue gray;

splintery _ 5. 5

13. Sandstone, medium-grained, dark-brown, weathering brownish 

red; crossbedded at low angle; locally indurated, but mostly 

weathering soft, platy ; top of No. 13 is a 1-foot massive sandstone over 2 feet of shale ___ _ ___________________ 13. 9

12. Sandstone, very gypsiferous, medium- to fine-grained, friable, 

weathering pinkish red; platy; upper half includes several 

thin sandstone layers, weathering white ____________ 29. 2

11. Limestone, medium-grained, dark-purplish-gray, weathering dark 

gray, in 2 beds, the lower 0.4 foot thick and separated by 0.4 

foot of purplish-weathering shale __________________ 1. 8

Total________________________________ 830. 0 

Halgaito tongue of the Cutler formation.

The thickness of 830 feet measured in the foregoing section is consistent with the thickness of 825 feet (calculated from elevation readings for the lower part and estimated for the eroded upper strata) 

already given for the member in the northwest corner of the mapped 

area.

The top of the Cedar Mesa was somewhat arbitrarily drawn above 

the highest white layer below the uniformly red mass of Organ Rock 

strata, even though this places in the upper part of the Cedar Mesa 

several tens of feet of beds some of which resemble those of the Oreran o

Rock tongue in color and lithology. This white layer, though only 

from 6 inches to 2 feet thick, is quite persistent and conspicuous among
188 CONTRIBUTIONS TO GENERAL GEOLOGY

the darker beds. It is a light-gray limy sandstone or sandy limestone, 

weathering white on its upper surface, which is platy and somewhat 

crinkled. At places it breaks into thin slabs an eighth to a half inch 

thick. It is underlain by nonresistant red to brownish-red siltstone 

with greenish mottling, suggestive of material in the Organ Rock.

At no place in the area was there noted any evidence of erosion or 

unconformity at the top of the Cedar Mesa, described by Gregory 

(1938, p. 45) for the region, although he refers to the opinion of Baker 

and Reeside (1929) that the Cedar Mesa is "an integral part of the 

Cutler formation, conformable with red beds above and below." On 

the contrary, where the contact crosses Sweet Springs Draw, the 

uppermost thin white layer, there about 2 feet thick, was observed by 

Mamay to thin eastward and pinch out within about 20 feet, the 

underlying red siltstones thus merging with those of the overlying 

Organ Rock.

ORGAN BOCK TONGUE

Just as the prevailingly light-colored strata of the Cedar Mesa overlie the thick red zone of the Halgaito, so they are in turn overlain by 

another sequence of red beds, the Organ Rock tongue of the Cutler 

formation. The small-scale intertonguing between Cedar Mesa and 

Organ Rock that was noted at Sweet Springs Draw is described in 

the preceding paragraph. At no place within the mapped area was 

there seen any example of marked erosional or depositional irregularities of the contact such as had previously been observed by the 

writer in White Canyon. Thus the area contributed no further evidence bearing on the possible unconformity between Cedar Mesa 

and Organ Rock discussed by Gregory (1938, p. 45, 46).

The Organ Rock tongue is rather uniform, both vertically and 

laterally, in lithology as well as in its brownish-red or orange-red 

color. The unit is not very resistant, and for the most part weathers

into slopes broken by steplike thin harder ledges. It is composed

chiefly Of Sandy mudstone and fine-grained sandstone in rather thin

continuous beds that weather into rounded surfaces, thin slabs, and

angular chunks. Much of the material is calcareous. Grayish-green

mottling in Small patches, spots, and. stringers is ecfloamon tliroughout

the unit.

Th0 following Section Of the Organ Rock tongue and the overlying 

iippox-nxost v»o<is of tixe Cutler formation was measured by Bell and 

IB thO Small canyon half a mile south of Snake auich,

State Highway IT. (This is an upward continuation of me

section, the Halgaito part of which is recorded on page 182 and the 

Cedar Mesa part on pages 185-187.)
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 189

Section of Organ Rock tongue and the overlying uppermost. , beds of the Cutler 

formation measured in canyon half a mile south of Sna'

Moenkopi formation. . ,,.... 

Hoskinnini tongue of the Cutler formation : ' , _ ... . ', ' ' '-.JFeet 

70. Sandstone, thin-bedded, somewhat wavy or crumpled ; alternating 

dark-brick-red and light-gray-white lensing layers ; Bonmicaceous ; sand grains very irregular in size, mostly well r^un,ded, 

V poorly cemented with iron oxide _ _ _ __ _ ___ .j.^ r_-r_;r_ , , 20. 0

De Chelly sandstone member of the Cutler formation:

69. Sandstone, massive, crossbedded throughout; near top, crossbedding in festoons; fine-grained, red-brown to tan; grains somewhat larger than any noted below; some grains very dark, 

probably heavy minerals ____________________ 20. 0 

68. Mostly sandstone, fine-grained, red-brown, well-cemented; broken 

by several layers or lenses a foot thick of silty, finely bedded 

sandstone; massive except for the silty'layers, and weathers 

in the gulch as a solid sandstone mass with smooth rounded 

ledges; at top a 15-inch shale layer separates massive sandstone 

below from overlying red-brown crossbedded sandstone. (In 

the field, Bell favored including No. 68 in the Organ Rock; the 

writer has placed it in the De Chelly as seemingly more logical.) _ 115.0

Total De Chelly sandstone member_____________ 135. 0 

Organ Rock tongue of the Cutler formation:

67. Sandstone, red-brown, in layers that differ in hardness so as to 

develop a series of minor ridges about 2 feet thick and about 

12 feet apart; for most of unit, bedding planes marked by thin, 

1- to 2-inch layers of light-gray sandy clay which give striped 

appearance to outcrop; near top of unit, striping more closely 

spaced, bedding appears varved; in uppermost 4 feet 40 percent 

of rock light colored-______________________ __ 133.0

 66. Sandstone 8 to 10 feet thick at top, fine-grained, brown ; iron oxide 

cement, some small mica chips; separated into 4 layers by 12- 

inch to 14-inch lenses of silty sandstone, somewhat fissile; soft, 

but forms a low ridge. Rest of unit is like No. 64_________ 89. 0

 65. Sandstone 6 feet thick at top, fine-grained, red-brown, only a little 

mottling; soft, but forms a low ridge. Rest of unit is like 

No. 64_______ _ _______________________________________ 60.0

«64. Sandstone 5 feet thick at top, separated into 2 layers by 6 inches 

of finely banded soft sandstone; red-brown; each sandstone layer 

shows light-gray mottling in its upper part; forms a low ridge. 

Most of the unit is sandstone, silty, fine-grained, thin-bedded, 

red-brown, with gray-white clay balls and chips along bedding 

5 planes and within the more massive sandstones; soft and does

not form ridges _____-_____________________ 227. 0

63. Mostly sandstone, red-brown, soft, weathering to small chips. At 

top, a 2-foot sandstone, red-brown with some minor light-gray 

blotches, fine-grained, somewhat fissile, weathering platy. About 

10 feet lower, a 3-foot sandstone, fine-grained, smooth-weathering, with light-gray blotches as much as 6 inches in diameter; 

forms a low ridge____________________________ 43. 0
190 CONTRIBUTIONS TO GENERAL GEOLOGY

Section of Organ Rock tongue and, the overlying uppermost beds of the Cutler 

formation measured in canyon half a mile south of Snake Gulch Continued

Hoskinnini tongue of the Cutler formation Continued Feet 

62. Sandstone 2 feet thick at top, red-brown, with iron oxide cement; 

makes a low ridge. Most of unit fine-grained brown-red siltcemented sandstone with some light-greenish-gray mottling along 

bedding planes _______ _ _ ___ 18. 0

61. Sandstone 2 to 3 feet thick at top, very fine-grained, silt-cemented, 

red to red-brown, mottled in part by circular and oval spots of 

light greenish gray, irregularly distributed, from % to 3 

inches in diameter. About 40 feet lower, a 2-foot sandstone, 

dark-chocolate, fine-grained, slightly micaceous, with iron oxide 

cement. At base, 2 feet of reddish-tan siltstone followed by 2 

feet of purplish siltstone. Most of interval light- to mediumred and brownish-rod Siltstone, sandy, thin bedded, generally 

soft, weathering platy_____-________________ 72. 0

Total Organ Rock tongue _ _ _________________ 642. 0 

Cedar Mesa sandstone member of Cutler formation.

Within the mapped area the thickness of the Organ Rock tongue 

seems to differ widely, as measurements and calculations of it range 

from 560 to 820 feet. The section recorded above shows that (with 

unit 68 being included in the De Chelly sandstone member) the thickness is 642 feet. This is consistent with the 650 feet calculated from 

structure and elevation readings half a mile to the south. Calculations from similar readings farther north (several of which are open 

to question because of poor exposures) give 710 feet, 720 feet, and 780 

feet, respectively, half a mile, iy2 miles, and 2% miles north of Snake 

Gulch ; 820 feet near the mouth of Eoad Canyon, 660 feet a mile north 

of that point, and about 560 feet near the mouth of Sweet Springs 

Draw and midway between the mouths of Fish Creek and Dry Wash.

At a point near the mouth of Sweet Springs Draw and extending

northward for about a mile, Mamay observed outcrops (nonCOntinUous, because of alluvial cover) of a sandstone which he and the writer 

at first thought to be the De Chelly. Later they found it to be near 

the middle of the Organ Rock tongue and overlain as well as underlain by beds characteristic of that unit, although lithologically it 

differs from those beds and resembles the De Chelly. It differs from

the usual Organ Rock beds by the absence of the typical grayish-green

mottling, by its rather sandy texture as contrasted with the usual

more silty texture, and by its distinct crossbedding. The sandstone

was observed to reach a thickness of 20 feet. At places the lower channels ana it contains conglomeratic lenses near its

base. It is more resistant than the typical Organ Bock beds below; 

hence, where exposed, the contact is clearly defined. The temporary 

Confusion of this sandstone with the De Chelly was even easier because
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 191

the latter thins northward and seems to pinch out just before reaching 

this locality.

The uppermost beds of the Organ Rock tongue and the contact with 

the overlying De Chelly sandstone member differ somewhat from 

place to place within the mapped area. In the northern part of the 

area, where exposures in the midst of the alluvium of Comb Wash are 

poor, the De Chelly and Hoskinnini appear to be absent, and the 

Organ Rock seems to be followed directly by the Moenkopi formation. 

For the belt from Road Canyon northward for some 2 miles, Mamay 

recorded that in the few exposures the Organ Rock-De Chelly contact 

is clearly denned; the upper beds of the Organ Rock show the typical 

coloring, mottling, texture, and mode of weathering, and they are 

overlain by a massive poorly bedded medium-grained red sandstone 

with very irregular weathering features. To the south, between Road 

Canyon and Snake Gulch, Fuller recorded at several points a "gradational contact between crossbedded massive De Chelly and flat-bedded 

fissile Organ Rock." In that vicinity L. C. Craig, in a section measured by him and T. E. Mullens in 1951, identified as the De Chelly a 

partly massive and partly crossbedded sandstone zone 53 feet thick 

and recorded that it is underlain by a 37-foot "transition interval 

between De Chelly sandstone member and Organ Rock tongue." This 

transition interval he described as follows 

Sandstone, interbedded moderate reddish brown and dark reddish brown, 

abundant pale green mottling in streaks, bands, and %- to 1-inch spots, very fine 

to fine grained, angular to sub-angular, non- to slightly calcareous, firmly cemented, composed of clear and amber-stained quartz, uncommon black accessory 

minerals, very fine white mica; parallel bedded, faint sub-parallel laminations; 

weathers to ledgy cliff; darker colored parts less resistant.

Still farther south, in the section measured by Bell and Yochelson 

half a mile south of Snake Gulch (see p. 189), doubt was felt as 

to whether the Organ Rock-De Chelly contact should be drawn below 

or above unit 68. That a difficulty in selecting this contact is not 

limited to this small area is emphasized by Baker (1936, p. 36), who 

in writing about the Monument Valley region says, "The determination of a precise boundary between the De Chelly sandstone and the 

underlying red beds of the Organ Rock tongue is usually impossible."

DE CHELLY SANDSTONE MEMBER

In northeastern Arizona and in adjacent areas in southeastern Utah, 

the De Chelly sandstone member is a thick prominent cliff-making 

unit. It is known to thin greatly toward the northwest and north. 

.Baker (1936, p. 36) measured 375 feet in Wide Butte at Monument 

Pass but found it is "90 feet on the San Juan River near the mouth of 

Nokai Creek * * * and is absent near the San Juan River in the 

vicinity of Clay Hill Crossing." He adds, "the De Chelly sandstone
192 CONTRIBUTIONS TO GENERAL GEOLOGY

is about 500 feet thick in Comb Ridge, near the Arizona-Utah State 

line, but it is only about 100 feet thick a few miles to the north, near 

the mouth of Comb Wash." Another measurement, in geographical 

location and thickness intermediate between the last two quoted from 

Baker, was made by Craig (written communication, 1953) in a section 

by him and others in 1951 "about 2 miles south of place where San 

Juan River crosses Comb Ridge"; there he determined the De Chelly 

to be 294 feet thick and underlain by a 66-foot "De Chelly-Organ Rock 

transition interval."

Northward from the San Juan River, in the lower reaches of Comb 

Wash, the thinning De Chelly member becomes less and less conspicuous. Though it is more massive and somewhat more resistant

than the units above and below, it forms only a low rounded ridge.

Northward from State Highway 47 it is still lower, more moundlike, 

and less conspicuous; and from the mouth of Road Canyon northward 

for 2 miles only 3 small isolated outcrops of it were seen. At this last 

point it seemed to have thinned almost to disappearance, and it was 

mapped as wedging out beyond that point. In the northern quadrangle of the mapped area, where the strata near Comb Wash are very 

largely covered by alluvium, slope wash, and windblown sand, no conclusive evidence was found of any De Chelly or overlying Hoskinnini 

between the Organ Rock and the Moenkopi.

Peterson noted at two places one opposite the mouth of Dry Wash 

and the other a mile and a half to the north below small outcrops of 

the, Moenkopi, patches of a medium-grained sandstone containing 

coarse grains, which he suggested might possibly be remnants of the 

De Chelly.

As shown in the section measured by Bell and Yochelson half a mile 

south of Snake Gulch (see p. 189), the upper part of the De Chelly 

(unit 69) is 20 feet thick and the lower part (unit 68, which Bell suggested be included in the Organ Rock) is 115 feet thick, a total of 135

feet. Three miles north, where Craig and Mullens in 1951 measured

a section (referred to above in the discussion of the Organ Bock), 

they found the De Chelly sandstone member to be 53 feet thick and

noted that it is oi-ossbedded in its middle half and more massive in itS

upper and lower quarters. 

Because of tne narrowness of their bands of outcrop and the scale

Of the mapping, the De Ckelly sajndstone member and the Hoskiimini

tongue are undifferentiated on the map. (Se& pi. 17.)

HOSKINNINI TONGUE

The Hoskinnini tongue was named by Baker and Reeside (1929, p. 

1443) "for the relatively thin red-bed unit above the De Chelly sandstone and beneath the Moenkopi, the name being derived from the 

exposures on the north face of Hoskinnini Mesa, several miles west
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 193

of 01 jeto trading post on Moonlight Wash." Among many details 

about the unit in that region later reported by Baker (1936, p. 38, 

39), the following are pertinent to the present discussion 

The Hoskinnini tongue * * * is the highest of the three red-bed tongues of the 

Cutler formation. It * * * is composed of red beds similar to those of the 

Organ Rock and Halgaito tongues of the Cutler .formation. It * * * where 

present is less than 100 feet thick. * * * Where the De Chelly sandstone 

wedges out * * * the Hoskinnini is inseparable from the Organ Rock tongue.

Within the mapped area north of the San Juan River, the Hoskinnini is a very minor unit, even thinner and less conspicuous than "it is 

in Monument Valley. Above the massive and crossbedded red De 

Chelly sandstone member is a zone some 20 to 30 feet thick of alternating thin layers of orange-red, brownish-red, chocolate-brown, and 

grayish-white sandstones having a banded or striped appearance. 

This zone was interpreted to represent the Hoskinnini tongue. Its 

contact with the De Chelly is rather sharp, but contact with the overlying Moenkopi is less certain and was placed somewhat arbitrarily 

at the horizon where the banded beds of several hues give way to more 

generally chocolate-brown thin platy beds of sandstone that bear ripple marks at many places.

North of the point where the De Chelly is shown as pinching out, 

no trace of the Hoskinnini was noted; and it, too, is mapped as pinching out before it reaches the northernmost quadrangle. ' However, in 

that northern part of the mapped area, the exposures are so sparse and 

poor that it may be present but indistinguishable from the underlying 

Organ Rock.

As stated above, because of the narrowness of their bands of outcrop 

and the scale of mapping, the De Chelly sandstone member and Hoskinnini tongue were mapped together.

TRIASSIC SYSTEM

LOWER AND MIDDLE(?) TRIASSIC SERIES 

MOENKOPI FORMATION

In general, the Moenkopi formation is readily distinguishable from 

the red beds of the Cutler units below by its prevailingly more chocolate-brown color, its thin platy bedding, and the presence of ripple 

marks at a number of horizons, particularly on the more sandy layers. 

Viewed more closely, it is seen to be more micaceous and somewhat 

coarser grained. It is composed chiefly of fine- to medium-grained 

somewhat calcareous sandstone and siltstone or shale. Here and there 

it shows whitish and greenish-gray spots and a few thin white layers.

The section measured by Craig and Mullens about 2*^ miles north 

of Snake Gulch, shows a thickness of 222 feet for the Moenkopi. From 

that point northward in Comb Wash, the exposures are so poor that
194 CONTRIBUTIONS TO GENERAL GEOLOGY

no satisfactory measurements could be made; hence, the thickness 

of 222 feet is assumed to persist. The same section shows 32 feet for 

the Hoskinnini and 53 feet for the De Chelly, making a total of 307 

feet for the 3 units. Southward from the vicinity of:,-the section, the. 

Moenkopi appears to thin at about the rate of thickening of the De 

Chelly-Hoskinnini zone; and therefore, for the purpose of preparing 

structure contours, the combined thickness of the 3 units, 307 feet, 

was assumed to continue unchanged south to the river.

Within the mapped area no convincing evidence was noted of the 

unconformities that elsewhere have been found to exist at the base 

and top of the Moenkopi.

The Chinle formation crops out in a continuous belt from the north 

edge to the south edge of the mapped area. Its upper, major part is 

fairly well exposed in the slope flanking the west side of Comb Ridge. 

Its lower, minor part is largely concealed by the talus and slope wash 

from the ridge and by the alluvium of Comb Wash.

In his report on the Navajo country, Gregory (1917, p. 42) subdivided the Chinle formation into four units called A, B, C, and D in 

descending order. In 1951, Witkind (1954, in preparation) found 

that in Comb Ridge and adjacent areas in Monument Valley, Ariz., 

these four units are rather distinctive, and therefore he names and 

separates them on his map. Within the mapped area of the present

report, approximate representatives of these four units can be observed. However, because they are not clearly distinctive, because

their lower part is so poorly exposed, and because with the small scale

of the map it is undesirable to furtW divide the already narrow belt, 

the Chinle is mapped as a single formation. Its character in this, 

area and the surrounding San Juan country is well described by 

Gregory (1938, p, 49,50) as follows 

* * "'" the Chinle formation * * * 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 weatner in tne manner of marls, in 

the San Juan country, as in regions immediately adjoining, measured sections

Of the Chinle Show foxir rough subdivisions (1) at tne base, bro-wn n»<a gray

sandstones that weather into steps and benches; (2) variegated sandy and

Calcareous Shales or "marls" that weather as mammillary mounds and immature 

mesas with typical badland expression; (3) light-red, dark-red, and mottled

stales and. massive or conglomeratic limestone in alternating beds, weathering

as a stepped slope on which the resistant limestone forms the top Of lOflg, narrow 

benches and small mesas; (4) red and brown sandstone and sandy shale, weathering as a cliff that, continued upward, includes the Wingate sandstone. Although 

these subdivisions are generally recognizable and the beds that compose them
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 195

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 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.

At no place within the mapped area did the writer and his associates 

find any unquestionable remnants of the Shinarump conglomerate, 

which at most places over a wide region lies, with some intertonguing, 

at the base of the Chinle formation and unconformably above the 

Moenkopi. Here and there gray to tan sandstone lenses occur up to 

several hundred feet in length and 10 feet or more in thickness, some 

showing a little faint cross lamination, some coarser grains, and a few 

widely scattered pebbles up to a quarter of an inch in diameter. As 

these lenses are so near the base of the Chinle, they were at first 

assumed to represent the Shinarump. However, on closer inspection 

and some digging, they were found to be underlain by several to 10 

feet of light-greenish clay or whitish to lavender calcareous mudstone 

of Chinle type, above characteristic chocolate- or reddish-brown thinbedded Moenkopi. It was therefore concluded that these lenses are 

in the Chinle and that the Shinarump conglomerate is almost if not 

wholly absent within the mapped area. The nature of the contact is 

such that it yielded no clear evidence as to whether the top of the 

Moenkopi was eroded before further deposition, but an unconformity 

between Moenkopi and Chinle formations seems probable.

In the section measured by Craig and Mullens about 2% miles north 

of Snake Gulch, the thickness of the Chinle was found to be 956 feet. 

That thickness is assumed to be constant throughout the mapped area, 

and the party in the summer of 1953 undertook no further measurements.

GLEN CANYON GROUP (LOWER PART)

The Glen Canyon group, consisting of the Wingate sandstone at 

the bottom, the Kayenta formation, and the Navajo sandstone above, 

is very prominent over a wide region in southern Utah and northern 

Arizona; and a wealth of detail about it is contained in many published reports, including those by Gregory (1938) and Baker (1936). 

Because the three formations are so well known and because of the 

difficulty of reaching most parts of them within the mapped area, the 

party made no detailed study of them in the summer of 1953.

The U. S. Geological Survey formerly considered the whole Glen 

Canyon group to be of Jurassic (?) age. It now places the lower part, 

the Wingate sandstone, in the Upper Triassic; the middle part, the 

Kayenta formation, in the Jurassic (?) ; and the upper part, the 

Navajo sandstone, in the Lower Jurassic.
196 CONTRIBUTIONS TO GENERAL GEOLOGY

WINGATE SANDSTONE

As in most parts of the Colorado Plateau, the Wingate sandstone is 

one of the most conspicuous formations. On the west side of Comb 

Ridge it forms a nearly vertical cliff, at most places from 250 to 300 

feet high. (See Surface features and drainage.)

The formation is an essentially single, homogeneous mass of lightorange-red or brick-red to reddish-buff sandstone. (See pi. 20A.) 

Its crossbedding is on a large scale but is not obvious in the rather 

smooth or vertically jointed cliff face. It is composed chiefly of fine 

rounded quartz grains.

The lower limit of the formation in this area is placed at the base 

of the sheer massive cliff. Below this is a zone a few feet thick of 

softer sandstone beds that make a small continuous break or slope 

logically separating the Wingate from the several thick red ledge- or 

cliff-forming sandstones in the upper part of the Chinle.

At most places the top of the sheer cliff plainly marks the top of 

the Wingate and the transition to the overlying resistant but thinnerbedded material of several kinds that makes up the Kayenta.

As thus defined, the Wingate sandstone has a rather uniform thickness of about 2Y5 feet throughout the mapped area. This thickness 

was calculated from a large number of pairs of altitude readings on 

the top and base of the unit; most of these pairs gave thicknesses quite 

close to this averaged figure.

JTTRASSICC?) SYSTEM 

GLElf CANYON GBOUP (MIDDLE PART)

KAYENTA FOEMATION

In contrast to the underlying and overlying more massive, crossbedded Wingate and IsTavajo sandstones, the Kayenta formation in

this area, as in most parts of the Colorado Plateau, stands out across 

country as a zone of thinner and flatter bedded alternating sandstone, 

siltstone, and lenticular limestone, of buff, tan, gray, and light-red 

tones. At only a few scattered localities, where it is protected by 

small high remnants of basal Navajo sandstone, is the full thickness

of the Kayenta formation preserved in the west face of Comb Ridge.

(See pi, 204.) Generally the upper part of the Kayenta and the lower

part of the Navajo are eroded to lower levels down the east slope of 

the ridge. The lower part of the Kayenta seems to be much more resistant, however, and weathers as thick ledges that at many places

are nearly vertically flush with the sheer Wingate cliff below, form

the upper segment of the steep west wall of Comb-Ridge, and hold up 

the highest parts of its serrated crest.
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 197

Although when viewed as a whole, the Kayenta is strikingly different from the Wingate and the Navajo below and above; when seen 

at closer range, its exact boundaries in much of the area cannot be 

selected with confidence. This is particularly true for its upper 

boundary, as one or more thick white to buff sandstones of Navajolike appearance are found in the upper part of the Kayenta, and one 

or two reddish zones occur in the lower part of the mostly white or 

gray Navajo, causing confusion where only small parts of the two 

formations are in sight.

By calculations based on air photographs, W. R. Hemphill, of the 

Photogeology Section, U. S. Geological Survey, found the thickness 

of the Kayenta at 6 places in the northern quadrangle of the mapped 

area to average 126 feet. This thickness accords reasonably well with 

the average figure obtained by the writer from two sets of elevation 

readings on the top and base of the formation.

JURASSIC SYSTEM 

LOWER JURASSIC SERIES

GLEN CANYON GROUP (UPPER PART)

The Navajo sandstone forms the major part of the east slope of 

Comb Ridge, being exposed in a belt ranging generally from a third 

of a mile to a mile or more in width. At a very few places small 

remnants of the lower part of the Navajo are preserved above the full 

thickness of the Kayenta in the west wall of Comb Ridge. (See pi. 

20A.) Generally, however, the Navajo and the upper part of the Kayenta are eroded eastward down dip. This gives the Kayenta-Navajo 

boundary on the map (see pi. IT) an unusual saw-edged pattern, in 

which the "teeth" or noses of Navajo between the closely spaced drainage lines point uphill westward. Many of these noses stand at high 

altitudes, and some of them are higher than the top of the west wall 

where it is held up by sandstone ledges in the Kayenta.

The eastward-sloping surface of the Navajo is not a dip slope. 

On the contrary, successively higher horizons are crossed downhill 

until the upper boundary is reached near Butler Wash. South of 

State Highway 47 the dip near the wash is nearly horizontal, and 

the belt of Navajo widens to reach the river.

As is usual with the Navajo sandstone in the Colorado Plateau, its 

surface is very irregular, rounded, and hummocky. The sandstone is 

quite friable in fact, it breaks down into much loose sand that at 

many places drifts to fill hollows and form dunes. Large-scale tangential crossbedding is so conspicuous that only at a few places, where 

views are extensive, can there be seen breaks between materials of
198 CONTRIBUTIONS TO GENERAL GEOLOGY

differing color and lithology that appear to represent parallel 

bedding.

In this area the Navajo is prevailingly grayish white, in part weathering to tan or buff. It contains one or more zones of softer sandstone of a reddish hue. Nowhere does it show the general light 

orange red that characterizes the formation in many parts of the 

region. Its light tone in this area brings it into strong contrast with 

the red or other darker hued strata below and above and makes doubly 

striking the long-range views of Comb Ridge from the east for example, from the road between Blanding and Bluff.

Because of its extensive crossbedding and paucity of true bedding' 

planes, its width of outcrop, and the great difference in dips between 

the steep basal beds high on Comb Ridge and the gently dipping 

uppermost beds near Butler Wash, trustworthy field measurements 

of its thickness in this area are scarcely feasible. For the same reasons, calculations of the thickness from air photographs are difficult 

to make and can be considered only approximate. Such calculations 

made by Hemphill at 6 places within the northern quadrangle of themapped area average 341 feet, which, though recognized as not precise, indicates the magnitude of the true thickness.

SAN RAFAEL GROUP (LOWER PART) 

CABMEL FORMATION

The Carmel formation is a relatively thin zone of soft rocks, lying 

on the slightly irregular surface of the Navajo sandstone and forming a slope capped by the protecting ledge of the basal sandstone of 

the Entrada sandstone.

Its nature and thickness in the lower part of Butler Wash are

shown by the following section, measured half a mile north of State 

Highway 47 by Conti and the writer.

Section of Carmel formation measured, on east side of Butler "Wash half a milenorth of State Highway Jfi 

Entrada sandstone.

Carmel formation: ^eet 

Mudstone, red, soft, weathering to clay __ _ 5. 5

Sandstone, red_____________ -- - _ ______ _ 4. OMudstone, red, soft, weathering to clay ____ 4.5

Sandstone, red, blocky 3.0 

MuQstone, red, soft, weathering to clay ____________ q. o

Sandstone, red, rounded or knobby 3. OMuastone, red, sort, weathering to clay __ 2,0

Sandstone, red, knobby _ 2. 0

Clay, red, and a few very thin layers of red sandstone_________ 14. 0

Sandstone, red, weathering rounded or knobby ___________ 2. 0
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 199

Section of Carmel formation measured on east side of Butler Wash half a mile 

north of >State Highway 47 Continued

Carmel formation Continued

Clay, reel, and a few very thin layers of red sandstone __ _ ____ 18. 5

Sandstone, gray, in two layers _ __ 3. 0

Clay, red, and thin sandy layers _ _ _______ _ _. _______ 4.0

Sandstone, grayish below, pinkish above__ _ __ _ _ _____ _ _ 5. 0

Clay, red _____ _____ _ _ _ _ _ __ _ 3. 0

Sandstone, gray . 5

Clay, red ___ _ _ __ _ _ __ __ _ __ _ _ 4. 0

Sandstone, gray . 5

Clay, red ____________ _ _ ________ _ __________ 2. 5

Sandstone, thin-bedded, red ___ __ ___ _ _ _ ______ _ . 5

Mostly sandy red clay _______ _ _________________ ____ 8. 0

Sandstone, fine-grained, gray, soft, friable ___ 1.5 

Partly concealed ; mostly red clay and some soft reddish sandy mudstones _ _ ___ ___ _ __ _ 11. 5

Total_________ _ 108.0

l^avajo sandstone.

Northward in Butler Wash the thickness of the Carmel formation 

remains nearly constant. The composition gradually changes, however; the unit becomes more and more sandy and lighter in color, 

until it is made up chiefly of gray to buff sandstones with a notably 

reddish soft band in the middle. Despite these changes, the Carmel 

is still readily separable from the gray-white crossbedded Navajo 

below and the darker buff, more resistant basal sandstone of the 

Entrada above. Its nature and thickness in the northern area are 

recorded in the following section measured by Bergendahl and the 

writer.

Section of Carmel formation measured on west side of Butler Wash 2 miles 

south of northern boundary of mapped area

Entrada sandstone.

Carmel formation: feet

Mostly buff clay, with several thin soft gray to buff sandstones, 

thickest about 5 feet___________________________ 38. 0

Sandstone, gray__:_______ _ ____ 2. 0

Conspicuously red zone; mostly red clay and thin platy red sandstones ; about 4 feet above base, a 0.5-foot buff sandstone; about 

at middle, a 1-foot gray to buff sandstone ______________ 24. 0

Sandstone, buff, massive; laterally lower part changes to interbedded 

red clay and buff sandstone layers___ _ _________ 25. 0

Sandstone, gray to buff; somewhat less resistant than thick sandstone 

next above_______ _ _____ ________ ______ 10. 0

Poorly exposed; lower part, buff sandy clay; middle, buff sandstone; 

upper part, buff sandy clay, some faintly reddish _________ 15. 0

Total______________________ __________ 114.0 

Nava/jo sandstone.
200 CONTRIBUTIONS TO GENERAL GEOLOGY

Bergendahl inspected the Carmel formation and related strata 

about 4 miles north of the .mapped area, where the upper end of 

Butler Wash is crossed by the new "low" road between Blanding and 

the Natural Bridges. The Navajo is as usual white to light-gray 

massive and crossbedded sandstone. The Carmel appears to be of 

approximately the same thickness and appearance as in the section 

just given. It is composed of mostly buff massive sandstones that at 

places form ledges up to 15 or 20 feet high, but includes some thinner 

beds (up to 3 or 4 feet) of red mudstone weathering to clay. The 

mudstones and some of the thin, platy sandstones are wavy or crinkled 

at places.

UPPER JURASSIC SERIES

SAN RAFAEL GROUP (UPPER PART)

ENTEADA SANDSTONE

The Entrada sandstone crops out in a narrow band up to half a 

mile wide in Butler Wash valley from State Highway 47 northward 

for more than 16 miles to the northern edge of the mapped area.

The upper and lower parts of the formation differ markedly from 

each other in lithology. Furthermore, though the lower unit is fairly 

constant throughout its length, the upper unit changes considerably 

from south to north.

The lower unit, making up roughly a third of the formation, is composed chiefly of medium-bedded gray to buff sandstones, of which 

the lower are the more"resistant ledge-formers and the upper are the 

more friable. Near State Highway 47 the basal sandstones form a 

low bench. Half a mile to the north the more resistant lower sandstones form a steep ledge or rim above the slope of soft Carmel rocks.

This rather pronounced escarpment, near and on the east side of Butler Wash, continues unbroken northward for about 3 miles and with

gaps for another 214 miles. The upper sandstones of this lower unit 

are somewhat thinner bedded and softer and are found either as irregular remnants above the escarpment or eroded back to soft steplike 

slopes to the east. In this southern part of the area, the upper unit 

of the Entrada is made up of soft thin-bedded brick-red shaly sandstones that lie in a poorly exposed zone between tKe lower light-hued 

unit and the overlying tan thin-bedded strata assigned to the Summerville formation. 

Between points, respectively 6 and 8% miles north of State Highway

4?7', tHe Entrada, sandstone is almost -wholly concealed by the alluvium

of Butler Wash. In that concealed stretch, its lower boundary crosses 

the wash; and from the latter point almost to the north edge of the

mapped area, the boundary is west of the wash, its lower, most resistant sandstones again making a conspicuous westward-facing 

escarpment above the Carmel and their upper surface making a promi-
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 201

nent dip slope eastward. Bergendahl records that within this northern stretch of Butler Wash the upper unit of the Entrada "is a massive 

red sandstone which is in contact with the Summerville. The thinbedded unit, conspicuous in the south, is not visible in the north; however, it may be present as a middle unit, buried in alluvium. The 

actual f acies change cannot be seen, owing to the discontinuity of outcrop."

The Entrada sandstone has a uniform thickness of about 150 feet in 

Butler Wash.

SUMMERVILLE FORMATION

In the southern part of Butler Wash, north of State Highway 47, 

the Summerville formation is composed of grayish-brown or tan thinbedded sandstones, sandy siltstones, and some minor shaly beds. Its 

lowest part is poorly exposed; its middle and upper parts, though 

rather soft and nonresistant to weathering, form a slope and above that 

a banded cliff protected by the massive rim of Bluff sandstone. In 

general, the thin layers are parallel and horizontally bedded; but at 

places they are somewhat crumpled, and at others they turn upward 

for a few feet and are truncated by later beds, even in some instances 

by the Bluff sandstone. At one locality, on the east side of Butler 

Wash about 4 miles north of the highway, the beds exposed in the 

bottom of the cliff show two conspicuous sags (see pi. 20#) within a 

hundred feet of each other; these are interpreted to have been caused 

by slumping during deposition while the material was unconsolidated. 

These sags affect several beds; but higher in the cliff both sags are seen 

to have been fully filled in and leveled across, each at a different 

horizon, so that the next bedding plane above shows no bend.

In this vicinity some of the upper thin-bedded sandstones coalesce 

for considerable distances into a single thick, massive, ledge-forming 

sandstone that looks just like the Bluff sandstone. Generally it is 

separated from the Bluff by a few feet of the thinner beds; but at one 

place this zone of thin beds wedges out, and the two thick sandstones 

merge into an unbroken cliff. This intertonguing, in marked contrast 

to the more usual type of boundary between the two formations, suggests a close relationship in age and depositional history between the 

Summerville and Bluff in this area.

Beginning near the boundary between the middle and the northern 

quadrangles of the mapped area, within a short distance the upper 

part of the Summerville changes from grayish brown to a distinct 

reddish brown or brownish red and includes several thin layers of 

grayish-white sandstone. It also becomes less thin bedded toward 

the north.

As calculated by the writer from a number of pairs of altitude 

readings taken by Conti, the thickness of the Summerville formation
202 CONTRIBUTIONS TO GENERAL GEOLOGY

in Butler Wash ranges from 146 to 162 feet and averages 152 feet. 

This average is almost identical with the average of a number of 

readings obtained by Hemphill from air photographs.

BLTJFF SANDSTONE

The youngest named formation in the mapped area is the Bluff 

sandstone, a grayish-brown thick massive resistant sandstone, in part 

crossbedded. It upholds the high bench east of Butler Wash, and its 

lower part forms the west rim of that bench as a prominent unscalable 

escarpment on the east side of the wash (see pi. 205) from near State 

Highway 47 northward for some 6 miles, where the dwindling escarpment still is more than TO feet high. Farther northward its topographic expression is less bold, and its outcrop is interrupted by 

alluvium in the numerous stream valleys that drain westward into 

Butler Wash.

The formation was not measured or studied in detail within the 

mapped area; but a brief description of it a few miles to the east, at 

and near the village of Bluff from which it is named, is given by 

Gregory (1938, p. 58).

The Bluff sandstone was formerly classed as a member of the Morrison formation; but because of present uncertainties as to its stratigraphic relations, it is now treated as a separate formation. East of 

the upper reaches of Butler Wash, it is overlain by a thick series of 

sandstones and clays fully accepted as Morrison.

QUATERNARY SYSTEM

In Comb Wash and in Butler Wash northward from a point 2 miles 

north of State Highway 47, the bottom lands are made up of alluvial 

fill of clay and very sandy clay with lenses of coarse grit and pebbles.

In Comb Wash the belt of alluvium is flanked on the east by clayey 

slope wash and large and small sandstone boulders eroded from Comb

Eidge and on the west by mounds of windblown sand presumably 

derived from the Cedar Mesa. 

Yochelson noted that in the lower reaches of Comb Wash, south of

State Highway 4T, two low-lying terraces are well developed. Their

surfaces are covered with gravel and cobbles up to 6 inches in

diameter 5 most of the material is chert and limestone, and few 

boulders are of sandstone. At one locality the upper surface of the

lower terrace deposits has been cemented,

The alluvium and other deposits here described largely mask the 

older formations for long stretches and make the outcrops of their 

boundaries difficult to find and identify The concealing material is 

therefore omitted from the map (pi. IT), and the approximate position 

of the older boundaries is drawn with dashed lines so as to bring out 

the bedrock geology more clearly.
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 203

IGNEOUS ROCK

The only igneous rock observed within the mapped area is a small 

dike noted by Bell just north of the San Juan River canyon in the 

southwest corner. This dike, a few feet wide and less than a quarter 

of a mile long, extends northwest and protrudes through the limestone 

bench in the Rico. A specimen taken by Bell was examined by Charles 

Milton, of the U. S. Geological Survey, who reports as follows:

The rock is a lamprophyre, with the usual bronze biotite flakes in a fine-grained 

but holocrystalline gray-green mafic groundmass. With a hand lens pale-green 

diopsidic pyroxene may also be seen; there are also numerous whitish small 

vesicles filled with clayey and calcitic material. This drops out in the process 

of making the thin section (unless special precautions are taken), so that in 

the thin section there are only holes representing the vesicle fillings.

In thin section, pale-brown biotite and colorless pyroxene, both fresh and 

euhedral, are abundant in a crystalline dark groundmass too.fine grained to be 

decipherable mineralogically.

STRUCTURE

METHODS OF REPRESENTING STRUCTURE

The present attitude or structure of the sedimentary rocks in this 

area is shown on the geologic map (pi. IT) by strike and dip symbols, 

two cross sections, and contour lines drawn through points of equal 

altitude above sea level on a selected stratigraphic horizon. For all 

that part of the area west of the wall of Wingate sandstone in Comb 

Ridge, the top of the Rico formation was selected as the horizon for 

contouring. In the eastern part of the area, the base of the Wingate 

sandstone was chosen as the key horizon.

In the southwest corner of the area, where by uplift and erosion the 

top of the Rico is exposed at the surface, many altitude readings were 

taken on it by planetable and triangulation or stadia traverses, and 

these readings gave accurate figures directly usable for contouring. 

Similar readings on the top of the Halgaito tongue of the Cutler and 

the "lower soft" part of the Cedar Mesa in the northern part of the 

area were recalculated to altitudes of the Rico by subtracting the 

stratigraphic thickness of the upper units adjusted at each place for 

the degree of dip; the results are considered reliable. Altitudes of 

the Rico formation calculated from readings on still higher contacts 

(from the top of the Cedar Mesa up to the top of the Chinle) are 

progressively less accurate the higher each contact is in the geologic 

column, for there are variations in and questions about the stratigraphic thickness of several of the formations and members, and also 

there is possibility that the forces causing the uplift and steep easterly 

dips squeezed and thinned some of the softer zones. Therefore, for 

those parts of the area where the dips are steeper and the strata from 

Organ Rock to Chinle are at the surface, the contour lines are less
204 CONTRIBUTIONS TO GENERAL GEOLOGY

accurately placed but give an approximate picture of the structure. 

For this reason and because the Rico is so deeply buried under and 

cast of Comb Ridge, it was deemed preferable to use the base of the 

Wingate sandstone as the key horizon for contouring in the eastern 

part of the area.

FOLDS

As described and partly mapped by Gregory (1938, p. 85-86, pi. 1), 

the Monument upwarp is one of the dominant structural features of 

the San Juan country. This broad fold has a north-south length of 

some 110 miles and a width, eastward from the Colorado River, of 

40 to 60 miles. Gregory says: "in general the Monument upwarp 

is characterized by a broad, nearly flat top, gentle westward slope,

steep eastward slope * * *." Its "steep eastward slope" is the Comb

monocline, so named because the more resistant of its sharply dipping 

beds form the conspicuous hogback aptly called Comb Ridge.

Gregory points out further that the Monument upwarp is modified 

by a number of lesser anticlines and synclines. Only one of these 

subordinate folds is of concern to the present report. It is described 

by Gregory as follows 

The fold defined in published reports as the Raplee anticline is an elongated 

dome sharply bounded on the east and west by monoclines. Beginning near the 

mouth of Comb Wash, the beds of Rico limestone that form the cap rock of the 

anticline rise abruptly westward from beneath higher beds with dips of 15° to 

40°, flatten to less than 3° across the crest, then bend sharply downward and 

disappear beneath beds of the Cutler formation in the Mexican Hat syncline, 

As a structural and topographic unit the anticline is 15 miles long and 4 to 6 

miles wide * * *. As mapped by Woodruff, its top is not a continuous structural 

ridge. Instrumental surveys show low anticlines with ends that overlap. 

Gregory indicates this compound minor folding on his map by showing two parallel axes, tlie pair being labeled "Raplee anticlines." 

The area discussed in the present report is in the eastern part of

the Monument upwarp, and its main structural features can be readily 

fitted into the broader structural pattern described by Gregory and

summarized above. From the contour lines and the strike and dip 

symbols on the geologic map (pi. 17), the steep eastward-dipping

Comb monocline can be at once recognized in a belt extending from 

the north to the south boundary of the area. Westward from the

monocline tne dips become progressively less steep, and. in the north.-

west part of the mapped area, in and behind the escarpment rimming

the Grand Gulch Plateau, the capping sandstones of the Cedar Mesa 

member have the nearly horizontal bedding that characterizes the 

broad central part of the Monument upwarp. In the southwest part 

of the mapped area is the steep eastern limb and a small part of the 

crest of the compound fold mapped by Gregory as the "Eaplee anticlines," to which his description quoted in the preceding paragraph 

is applicable. Small flexures on this fold are shown by the slightly
GEOLOGY OF COMB RIDGE NORTH OF SAN JUAN RIVER, UTAH 205

undulating surface of the top limestone of the Rico and are particularly evident near where State Highway 47 crosses the west edge 

 of the map.

FAULTS

The only faulting of considerable magnitude found within the 

mapped area is that observed and studied by Bell and Yochelson in 

the west slope of Comb Wash valley south of State Highway 47. (See 

pi. 18A.) At places there is a single fault; at others there are two 

side by side, which approach and merge, or elsewhere are connected 

by small cross faults. (Owing to the small scale of the map, the 

fault as drawn is slightly generalized from the conditions noted in 

the field.) The faulting is normal but dips steeply. Downthrow is 

on the south side and is greatest near the contact between the Rico 

.find Halgaito, where it exceeds 100 feet and perhaps is as much as 

200. The fault is made conspicuous by the substantial lateral offset 

of the gray limestones of the Rico, the red sandstones of the Halgaito, 

and the light-colored beds of the Cedar Mesa, all of which, dipping 

steeply to the southeast, make many small ridges. Eastward, the 

throw on the fault becomes progressively smaller, and the contacts 

of the upper formations appear not to be offset. The fault heads 

directly toward the small reentrant .in the west wall of Comb Ridge 

that was utilized for the dugway to carry the State road and toward 

JsTavajo Spring near the bottom of the reentrant, but it seems to die 

out before reaching them, as no trace of either break or bending could 

be found in the massive Wingate sandstone or the several sandstone 

ledges in the upper part of the Chinle at that locality. However, 

it seems reasonable to suggest that both the erosion of the reentrant 

and the existence of the spring reflect some weakening or even undetected shattering of the otherwise hard, resistant strata by the same 

fcarth force that to the west brought the more obvious fault and 

movement.

Of a different and unrelated type are two small faults noted by 

Fuller some 2 miles to the north. These lie close to but on opposite 

sides of State Highway 47 about a quarter of a mile north of the point 

where the road crosses Snake Gulch, in the stretch where it runs in 

the lower Halgaito tongue before turning southwest and climbing the 

Rico slope. Both are overthrust faults, trending north-northwest 

and traceable for a few hundred feet. In both faults the overthrust 

side is on the northeast, and the throw reaches a maximum of perhaps 

15 feet. The southwest fault, very near the road, runs along and cuts 

back and forth across the "normal" line of outcrop of the Rico-Halgaito contact. This is at the foot of the prominent dip slope on the 

hard, uppermost gray limestone of the Rico. On the hanging wall 

that limestone is repeated in outcrop by being shoved southwest over 

the dip-slope bed. Its lower surface, dipping northeast, is scored by
206 CONTRIBUTIONS TO GENERAL GEOLOGY

large grooves or slickensides. Along the strike the fault cuts obliquely for a small distance into the Halgaito, and there is duplication 

also of a thin buff sandstone near the base of that member. The 

second fault is northeast of and a little farther from the road and 

cuts across the southwestward-pointing rounded promontory of lower 

Cedar Mesa beds that surround the small inlier of Halgaito. As the 

beds are softer, the details are less sharp, but the general pattern of 

overthrust toward the southwest on a steep plane is clear.

These two faults, though too small to be shown on the general map, 

are of interest because they suggest that, after uplift, the anticlinal 

nose pointing toward this spot subsided slightly; and the resulting 

lateral compression caused a small buckling and an over-thrusting inward from the flanks. This hypothesis finds support from a third 

feature found nearby. Southwest of the road in this vicinity, about 

a hundred yards up the dip slope on the Eico and approximately 

parallel to its boundary with the Halgaito, is a peripheral narrow belt 

of flattening, almost to reversal, of the prevailing northeasterly dip. 

This belt of modified dip shows clearly in the hard surf ace of the 

uppermost limestone of the Rico; viewed from the road, it gives the 

impression of an old rounded, continuous mound of earth tossed out 

on the downhill side of a ditch dug around the curve of a hill at a 

constant level. A diagrammatic'cross section of this belt of flattened 

dips and the two overthrust faults is given in figure 41.

sw

Rico formation

Appr 600 Feet

FIGURE 41. Diagrammatic cross section of two small overthrust faults and of a flattening

of dip near State Highway 47 in the southern part of the mapped area. (The section 

from end to end represents a distance of about 1,000 feet ; tbe structural details lire

disproportionately enlarged to emphasize them.)

LITER -A.TUKE CITED

Baker, A. A., 1936, Geology of the Monument Valley-Navajo Mountain region,

San Juan County, Utah: U. S. Geol. Survey Bull. 865.

Baker, A. A., and Reeside, J. B., Jr., 1929, Correlation of the Permian of southern 

TJtuli, northern Arizona, nortnwesrern New Mexico, and southwestern Colorado: Am. Assoc. Petroleum Geologists Bull, v. 13, p. 1413-1448.

Gregory, H. E., 1917, Geology of the Navajo country: U. S. Geol. Survey Prof. 

Paper 93.

Gregory, H. E., 1938, The San Juan country : TJ. S. Geol. Survey Prof. Paper 188.

Witkind, I. J., 1955, Uranium deposits at base of the Shinarump conglomerate, 

Monument Valley, Arizona: U. S. Geol. Survey Bull. 1030- (in preparation).

Woodruff, E. G., 1912, Geology of the San Juan oil field, Utah: U. S. Geol. Survey 

Bull. 471, p. 76-104.
INDEX

Page 

Abstract.....------------ .---- 167-168

Acknowledgments__.- ---- - 171

Alluvium, Recent-------.-.------------ 177,202

Baker, A. A., cited. -. 180,181,183,191-192,193 

Bibliography............. .-- . ....... 206

Bluff sandstone..... .......- 177,178,201,202

Butler Wash................... 173-174,200,201,202

section of Carmel formation measured at 198-199

Carmel formation...-.-.......-..--.. 178,198-200

Cedar Mesa sandstone member. 173,174,179,182-188 

ChinlC'formation................... 172,178,194-195

Comb monocline........______ 204

Comb Wash.-...--.....__-..- 173-174,202

Craig, L. C., cited.....___... . 191,192,193,195

Cutler formation...-..-.-......--- 172,179,181-193

Cedar Mesa sandstone member__ 179,182-188 

Pe Chelly sandstone member. __ 179,191-192 

Halgaito tongue.. ---------- 179,181-182

Hoskinnini tongue 179,192-193

Organ Rock tongue............. 179,188-191,192

De Chelly sandstone member... 179,190,191-192 

Entrada sandstone...._._....___ 178,200-201

Faults . .- 205-206

Fieldwork. ............................. 170-171

Folds . - -- 204-205

Geography....--.-.. ................... 171-177

climate. 174-175

drainage. (See surface features this section.) 

supply points and travel routes_ _ 175-177 

surface features.__.._.....__ . 171-174

vegetation._______.____-.... 175

Glen Canyon group, lower part._._._ 195-196 

lower part, Wingate sandstone___ 172,178,196 

middle part, Kayenta formation- 172,178,196-197 

upper part, Navajo sandstone.. 172,173,174,178,

197-198

Goosenecks.-.. .. .. . 173 

Grand Gulch Plateau.. . 172

Gregory, H. E., cited ...... .......... 181,

182,183,185,188,194-195,202,204

Halgaito tongue.. ._.. . 174,179,181-182 

Hcrmosa formation......__..._ 172,177,179-180

Hoskinnini tongue.... . 179,192-193

Igneous rocks _...__ . 203 

Introduction-________________ 168-171

Jurassic system.....___________.. 197-202 

Lower series, Navajo sandstone 172,173,174,178,

197-198

Middle and Upper series, Carmel formation.. .. 178,198-200 

Upper series--.-.------.-----.-- . 200-202

Jurassic system Continued

Upper series Continued Page 

Bluff sandstone ..... 177,178,201,202

Entrada sandstone------------- 178,200-201

Summerville formation......... 178, 201-202

Jurassic(?) system______________ 196-197

Kayenta formation._________ 172,178,196-197 

Location of area..___..___ ____ 168-169

Mexican Hat syncline .__________ 204 

Milton, Charles, quoted..________.._ 203 

Moenkopi formation________ 178,192,193-194 

Monument upwarp....._....._..___. 204

Navajo sandstone... ... 172,173,174,178,197-198 

Organ Rock tongue..... 179,187,188-191,192

Pennsylvanian system, Hermosa formation_ 172,

177,179-180

Rico formation. 172,177,179,180, 203 

Permian system, Cutler formation (see also

Cutler formation).............. 181-193

Permian(?) system, Rico formation (see also

Rico formation)......_.......... 180

Personnel...._________ ______ 170

Physiography. (See Geography.)

Purposes of report______________ 168-169

Quaternary system_______ .. ...'_ 202

Rainfall. .. 174-175 

Raplee anticline____ ___ 204 

Rico formation.. . 172,177,179,180,203

San Juan River ..... 172

San Rafael group, lower part, Carmel formation. . 178,198-200 

upper part-________________ 200-202 

Bluff sandstone ..--. 177,178, 201,202 

Entrada sandstone..______ 178,200-201 

Summerville formation. .. 178, 201-202 

Shinarump conglomerate. . ...._ 195 

Snake Gulch, section of Cedar Mesa sandstone

measured at_____. ____ 185-187 

section of Halgaito tongue measured at... 182 

section of Organ Rock tongue measured at_ 189-

190

Stratigraphy (see also specific formations) _ 177-202 

Structure, methods of representing....__ 203-204 

Summerville formation_________ 178,201-202

Triassic system.......... ..__......... 193-196

Lower and Middle(?) series, Moenkopi

formation......------ 178,192,193-194

Upper series _.._ 194-196

Chinle formation.... . 172,178,194-195

Glen Canyon group flower)........ 195-196

Wingate sandstone. . 172,178,196

O 207