DEPARTMENT OF THE INTERIOR 

FRANKLIN K. LANE, Secretary 

UNITED STATES GEOLOGICAL SURVEY 

GEORGE OTIS SMITH, Director 

Professional Paper 95 

SHORTER CONTRIBUTIONS TO 

GENERAL GEOLOGY 

1915 

DAVID WHITE, CHIEF GEOLOGIST 

WASHINGTON 

GOVERNMENT PRJ;NTING OFFlCE 

1916 
RELATION OF THE CRETACEOUS FORMA IONS TO THE ROCKY 

MOUNTAINS IN COLORADO AND EW l\fEXICO. 

By WILLIS T. LEE. 

PURPOSE OF rrHIS PAP R. 

Some time ago, while working on a problern that involved the question of•the presence or 

absence of islands near the close of the Cretaceous period in the region now occupied by the 

southern part of the Rocky Mountains, I was forced to the c nclusion that no land masses or 

islands of any considerable size persisted there throughout t e Cretaceous period, for I found 

no sedimentary rocks that were clearly derived from such isla ds. This result led to a reexamination of available information to see what evidence the edimentary rocks in other areas 

near the present mountains could furnish, and I found ra her unexpected confirmation of 

my conclusion. In the course of this study it became evide t that' there is apparent conflict 

of testimony between different classes of fossils and that he physical evidence, including 

lithology, structure, and sequence of beds, is at variance witbl some of the commonly accepted 

correlations. In this state of uncertainty I tried to apply phyFiographic principles to see if they 

would throw any light on the interrelati.ons of the Cret.aceouf format·i.ons of the Rocky Mountain region and on the events that opened and closed the pe1 od. This led me to a conclusion 

similar to that reached by the paleontologist C. A. White 1 any years ago, namely, that the 

Upper Cretaceous formations up to and including the Laram e extended across the site of the 

mountains. 

I believe that the principles of physiography, as well as those of lithology, paleontology, 

and structure, may be used to good advantage in the cone ation of sedimentary formations, 

and that they may prove effective in some problems whe e the others fail. I propose to 

attempt an application of these principles · to some phases f the stratigraphy of the Rocky 

Mountain region of Colorado and New Mexico, although I rec gnize that to do this adequately 

much more information must be gained, especially in area that are now very imperfectly 

known. The most that can be accomplished a.t the present time is to point out some of the 

relations that seem to exist, in the hope that observation mar thus be stimulated. 

A study of this kind naturally leads into rn.any byways, f nd the paper might be expanded 

indefinitely. I propose, however, at this time to confine th discussion to such facts as may 

have a direct bearing on the correlation of son1e of the Ore aceous formations in the Rocky 

Mountain region and to such facts as will throw some light n the physical conditions of this 

region at the close of the Cretaceous. The significance of the unconformity separating the 

Cretaceous and Tertiary systems in this region--that is, the p st-Laramie, post-Vermejo unconformity-is very largely dependent on the question wheth r any considerable parts of the 

Rocky Mountain area remained above sea level throughout C etaceous time. 

The sections used in this paper were measured indepen ently, most of them for the purpose of showing the stratigraphic relations of the coal-beari rocks. Hence the coal-bearing 

formations have been described more fully than others. Th measurements and descriptions 

were made by different men, and the personal equation must be consid~red; also due allowance 

must be made in the interpretation of results. 

1 U.S. Geol. and Geog. Survey Twelfth Ann. Rept., p. 50,1883. 

27 
28 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

REASON FOR THE INVESTIGATION. 

COAL BEDS AT MANY HORIZONS WITHIN THE CRETACEOUS. 

During the investigations of the Rocky Mountain coal fields coal beds have been found 

at so many horizons within the Cretaceous that it seems probable that conditions favorable 

to their accumulation existed .over extensive areas in one place or another from Dakota to 

Laramie time. There seems to be a tendency on the part of some geologists to explain these 

numerous coal beds on the assumption that the land underwent repeated oscillations which 

carried it sometimes above and sometimes below sea level, whereas it seems equally possible 

to account for them on the assumption that sedimentation was going on contemporaneously 

with the advance of the sea, due either to subsidence of the basin or to rise of .sea level, 

and that the apparent withdrawal of the sea at a given locality as shown by brackish or fresh 

·water sediments was due to the relatively rapid accumulation of sediments in that portion of 

the basin which was thereby filled to the existing sea level. The relation of marine to nonmarine beds would thus depend on the rate of sedimentation relative to the rate of subsidence. 

For example, in some parts of western New Mexico and eastern Arizona no Cretaceous rocks 

younger than Colorado have been found. This has been attributed to the "emergence" of 

these parts, whereas it may as rationally be interpret~d to mean that during Colorado time, 

without any oscillation of the land, these parts of the basin received alternately brackish-water 

and marine deposits, and that during later time sediments were carried seaward over these 

deposits to parts of the basin not yet filled, just as the Mississippi is now filling the Gulf of 

Mexico without materially building up the surface of its delta. 

In the region with which this paper deals the evidence seems to point to art intermittent 

downward movement of the surface (or an intermittent upward movement of the sea level) 

during early Cretaceous time and practical stability during later Cretaceous time rather than 

to oscillations that produced so-called islands in the region of the present Rocky Mountains. 

(The term Rocky Mountains in this paper refers to the ranges of central Colorado extending 

southward into New Mexico and northward into Wyoming.) 

In correlating the sedimentary formations of the several localities which are now more or 

less disconnected because of erosion there is a tendency to correlate rocks that are lithologically 

similar and to neglect.the obvious fact that a coal-bearing sandstone may.have been in process 

of formation at one locality at the same time that marine beds were being formed at another 

locality. 

CORRELATIONS: LITHOLOGY VERSUS PALEONTOLOGY. 

There is a tacit understanding among geologists, more or less precise according to the individual, that a formation name implies a definite age relation. The application of a formation 

name, however, is dependeJ?.t on correlation and is subject to all the uncertainties attendant 

on correlation. For example, coal-bearing sandstones of middle . Upper Cretaceous age at 

several of the localities described in the following pages have been called Mesaverde. There 

are reasons for believing that the so-called Mesaverde of the several localities may occupy very 

different places in the time scale, ranging from upper Colorado to a position well toward the top 

of the Montana group. A sedimentary formation is usually defined as a lithologic unit, but 

this definition is not closely adhered to in practice, and in reality formational boundaries are . 

often drawn on fossil rather than on lithologic evidence. Moreover, it is not possible to adhere 

to this definition in all cases, inasmuch as beds formed at the same time inay change laterally 

from sandstone of fresh-water origin, through brackish-water beds, to shale or limestone of 

marine origin. If the rocks can be traced through these transitions, a fresh-water sandstone 

and a marine shale may be proved to be of the same age; but if for any reason connection can 

not be established by actual tracing and the correlation must depend on lithology and sequence 

or on paleontology, it seems to be practically impossible to determine whether certain freshwater, brackish-water, and marine beds are of the same age or are more or less widely separated 

in time. 
RELATION OF CR-ETACEOUS FORMATIONS TO ROCKY MOUNTAINS. 

NEAR-SHORE VERSUS OFFSHORE DEPOSITS. 

It is obvious that in a basin which is being filled with 

sediment the finer material is likely to be carried far out 

and the coarser material to accumulate near shore and 

that swamp conditions favorable to the formation of coal 

are likely to prevail along the coast. As the shore migrates, either because of the accumwation of sediments 

or because of a change in sea level, the conditions of sediUlentation are likely to remain constant with respect to 

the shore but to vary with respect to given locahties. It 

is possible that in a filling sea like the interior Cretaceous sea of North America brackish-water and freshwater conditions favorable to the accumulation of coal 

persisted near shore throughout the life of the sea, while ~ 

marine conditions existed continuously in the eenter of l 

the basin. It is conceivable, therefore, that a coal- ~ 

bearing sandstone, were its continuity unbroken by erosion, might be traceable without interruption through the 

Cretaceous beds and yet vary in age from place to place, sS as is illustrated in figure 12. For example, the basal ~ 

sandstone at A may be the homogenetic equivalent of ~ {I) the basal sandstone at B, but it is not the time ~ (!) equivalent. o.. 

This illustration is intended to represent some of g. 

g_ the possible relations of the sedimentary rocks formed g· 

in a sea that advanced from the right, spreading over a ;t 

land mass toward the left. A basal sandstone is formed ~ 

at A and is soon covered with marine shale as the sea g 

advances. The san1e thing then take

1

s place at B. Later ~c+

a retreat of the sea, a temporary hat in advance, or ali ~ 

increase in the amount of sediment causes a second sand- ~­

stone to form at A' and a little later at B'. If now the .g 

rocks are exposed at A and at B and the intervening ~ rocks are eroded away or covered, the sections A-A' and ..... 

B-B' appear alike in that, each shows a sandstone sepa- ~ 

rated by a marine shale from a basal sandstone. The W 

sections may be so near each other that correlation by : .g lithology would seem to be admissible, and such fossils ~· 

as happen to have been found may not be adequate to '-<1.f 

show difference in age. There is little doubt that under 

these conditions the formations of the two sections would 

be erroneously regarded as time equivalents, whereas they 

are only homogenetically equivalent and really differ 

in age. 

Sediments accumulating near shore may contain 

fossil plants, fresh-water or brackish-water invertebrates, 

or certain kinds of marine invertebrates, accor:ding to 

local conditions at the time of sedimentation. But the 

fossils in the sandstone at B' are likely to be so different 

from the open-sea forms at C that there is little possibility of recognizing them ~s being o~ the snme age. 

82646,

0 -16-3 

[] 

1'- s--

<I> 

Ci1 

29 
30 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

If the diagram is taken to represent the Cretaceous rocks of the southern Rocky Mountain 

region, the line D-D the separation in time between the Colorado and Montana groups, and the 

line L-L' the beginning of Laramie time, then the sandstone A-B may represent the Dakota; 

the sandstone at A', the coal-bearing rocks at Engle, N. Mex.; the sandstone at B', the coalbearing rocks at Carthage and Zuni, N. Mex.; the sandstone at E, the coal-bearing rocks at 

Pinedale and elsewhere in Arizona; the sandstone at F, the coal measures of Colob Plateau, in 

Utah; the sandstone extending from G to H, the· Mancos and Mesaverde from the Datil Mountains, N. Mex., to Durango, Colo.; the sandstone at I, the coal-bearing "Laramie" formation 

of the San Juan Basin; and the sandstone at J (projected across the spaces where it has been 

removed by erosion), the youngest coal-bearing rocks of Cretaceous age in northern Colorado 

and southern Wyoming-that is; the Laramie formation of the Denver Basin and the formations 

of corresponding age in northwestern Colorado and southern Wyoming. 

It is quite impossible to make a diagram that will illustrate a general principle and at the same 

time adequately represent local details. Three qualifications seem desirable in the application 

just made. First, as the diagram stands, it might be interpreted to mean that the offshore and 

near-shore deposits accumulated at the same rate. If rate of sedimentation alone were to be 

illustrated the lines representing equivalency in age should bend downward toward the right, 

for, as is well known, the great bulk of sediments accumulate near shore, and a thick near-shore 

deposit is likely to be the time equivalent of a much thinner offshore deposit. Second, the representation of formation boundaries in the diagram by straight lines drawn at an angle with the 

lines representing equivalency in age is generalized and should not be interpreted as indicating 

abrupt changes in conditions of sedimentation. There is more likely to be an irregular transitional zone in which the offshore and near-shore deposits meet and interfinger. In this process 

of interfingering marine conditions will sometimes extend well up toward the source of the 

sediments and at other times brackish or fresh-water conditions will extend far out into the 

basin. Third, inasmuch as the basin fills most rapidly near shore a bed of given thickness, 

character, and stratigraphic position near the periphery of the basin is likely to be older than 

a bed of the same thickness, character, and stratigraphic position near its center. As portions 

of the basin near its periphery are built up to the limit of deposition the fresh sediments are transported toward the center over those previously deposited. This process, of course, is further 

complicated by oscillations both of the land and of the sea level. 

PHYSIOGRAPHIC BASIS OF CORRELATION. 

In view of some of the foregoing considerations it would seem wise to emphasize the physiographic conditions, in so far: as they can be ascertained, · that e'xisted during the formation of 

beds whose correlation is attempted. A large volume of exact information is necessary in order 

to lise such data to t}J.e best advantage, but it is my belief that enough is known about the Cretaceous and older Tertiary formations of the Rocky Mountain region to use the physiographic data 

in connection with structure, lithology, and paleontology to a greater extent . than they have 

been used heretofore in making correlations. 

ATTITUDE OF THE CRETACEOUS FORMATIONS TOWARD THE ROCKY MOUNTAINS. 

Some of the most significant facts bearing on the use of physiography in correlating the Upper 

Cretaceous formations of the southern Rocky Mountain region are found in the relation of these 

formations to the mountains. It was the belief of early observers that highlands of considerable 

extent existed in the Rocky Mountain region throughout Upper Cretaceous time. Although for 

years there has been a growing tendency on the part of some investigators to regard this belief 

as unfounded, no one has taken the trouble heretofore to combat it, and it has persisted with characteristic tenacity. Whatever may have been its influence on the establishment of the time scale 

for the Upper Cretaceous formations of the Rocky Mountain region, the fact remains that a stratigraphic column has been established for areas east of the mountains which differs radically from 

that established for areas west of the mountains. If, however, there were in the mountain 
RELATION OF CRETACEOUS FORMATIONS TO ROCKY MOUNTAINS. 31 

region during Upper Cretaceous time no highllands capable of contributing any considerable 

amount of sediment or of forming a barrier to the distribution of sediments derived from the 

continental land masses on either side of the interior sea, the two cofumns should not be so discordant as they seem to be, and it may be well to reexamine the principles on which they were 

established. 

One of the principal objects of this paper is to point out some of the reasons for believing 

that no highlands of any considerable extent existed in the Rocky Mountain region of Colorado 

FIGURE 13.-Paleogeographic map of North Americf~ showing by shading the part of the continent submerged during the Benton epoeh. (After Charles Schuchert.) 

and New Mexic·o during the .Upper Cretaceous epoch and that the sedimentary formations of 

this region were once essentially continuous across the areas now occupied by the rt:wuntains, 

with only such variations in thickness and character as are to be expected in different parts of a 

single basin. Schuchert 1 anticipated this conclusion in some measure when, in his maps, one 

of which is reproduced as figure 13, he extended the Cretaceous sea completely over the Rocky 

Mountain area. However, on page 586 of histext he states that the sea connected the Gulf of 

r Schuchert, Charles, Paleogeography of North America: Geol. Soc. America Bull., vol. 20, pp. 427-606, 1910. 
32 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

Mexico and the Arctic Ocean east of the Rocky Mountains and that throughout Cretaceous' time 

"the Laramie Range seems to have been in slight upward movement with decided elevation 

_toward the close of the period." This statement recalls Eldridge's description 1 of certain arches 

near Golden, Colo., which he supposed to have existed during the Cretaceous period, but whose 

existence Richatdson 2 seems to doubt. If this arching were as important as Eldridge ·and others 

have maintained, indications of similar mid-Cretaceous movements should be present elsewhere, 

but they have not been found in the Rocky Mountain region, although evidences of the proximity 

of land occur all along the border of the continental mass west of the Cretaceous sea, as Stanton 

has frequently pointed out. 

GEOGRAPHIC CONDITIONS P:Jl,IOR TO DAKOTA TIME. 

There is little evidence of notable crustal movement in the Rocky Mountain region for a 

long time prior to the beginning of the Upper Cretaceous epoch. The highlands that occupied 

this region in l~te Paleozoic and early Mesozoic time had been eroded and the region brought 

to a condition of low relief, if not practically to base-level. The interior Cretaceous sea seems 

to have advanced with little interruption over the area now occupied by the southern part of 

the Rocky Mountains. Without entering into a long discussion of the pre~Cretaceous physiography, I may be permitted to call attention briefly to some of the principal arguments for 

this postulate ~flow relief. 

JURASSIC EPICONTINENTAL SEA. 

In late Jurassic time a shallow epicontinental sea covered the northern and western parts 

of the Rocky Mountain region. 3 The distribution, thickness, and character of the sediments 

laid down in this sea are not such as to indicate that the area now occupied by the Rocky Mountains furnished any considerable quantity of coarse material. Moreover, the presence of marine 

Jurassic fossils in western Texas, 4 although they constitute a later fauna than that of Wyoming, 

suggests that the southern part of the Rocky Mountain region may not have been much farther 

above sea level than the northern part. 

PHYSIOGRAPHIC CONDITIONS INDICATED BY THE MORRISON FORMATION. 

The Morrison formation lies with apparent confo~mity on the Jnarine Jurassic beds in southern Wyoming. It overlaps the marine Jurassic both east and west of the Rocky Mountains 

and extends southward into New Mexico with remarkable uniformity in thickness, character, 

and fossil contents. Certain beds in North Park, South Park, and elsewhere in Colorado, in 

the midst of the mountainous area, indicate that the areal distribution of the Morrison was 

essentially the same as that shown later in this paper for the Dakota sandstone. · 

There are, however, a few places where the Morrison is known to terminate against older 

rocks, allowing the Dakota sandstone to overlap it. One such locality near Colorado Sp;ings, 

Colo., is described by Cross, 5 and according to Hayden's atlas of Colorado the Dakota rests in 

several places on older rocks, the Morrison apparently being absent. However, this atlas has 

proved to be inaccurate in this respect in several places, and the Morrison has been found to 

be even more continuous than was formerly supposed. These discoveries led Cross and Larsen 6 

to believe that .the Morrison and its ,western equivalent in the Gunnison formation may have 

been connected across the mountain region. 

THE COMANCHE SEA. 

The Comanche sea appears to have found little obstruction to its passage as far west as 

the present Rocky Mountains. The Comanche sediments of eastern Colorado and New Mexico 

in.dicate that the sea advanced over a graded plain. It is not known to have invaded western 

1 Eldridge, G. H., Geology of the Denver Basin in Colorado: U.S. Geol. Survey Mon. 27, pp. 82-104,1896. 

2 Richardson, G. B., Structure of the foothills of the front range, central Colorado: Washington Acad. Sci. Jour., vol. 2, p. 429, 1912. 

a Logan, W. N., A North American epicontinental sea of Jurassic age: Jour. Geology, vol. 8, pp. 242-273, 1900. 

4 Cragin, F. W ., Paleontology of the Malone Jurassic formation of Texas: U. S. Geol. Survey Bull. 266, 1905. 

o Cross, Whitman, U.S. Geol. Survey Geol. Atlas, Pikes Peak folio (No.7), 1894. 

6 Cross, Whitman, and Larsen, E. S., The stratigraphic break below the Jurassic sandstone in southwestern Colorado: Washington Acad. Sci. 

Jour., vol. 4, p. 238, 1914; Contributions to the stratigraphy of southwestern Colorado: U. S. Geol. Survey Prof. Paper 90-E (Prof. Paper 90, pp. 

39-50), 1914. 
RELATION OF CRETACEOUS FORMATIONS TO ROCKY MOUNTAINS. 33 

Colorado, but, on the other hand, there is no lnwwn evidence that the areas immediately west 

of the present mountains were far above sea level during Comanche time. 

The general character of the Comanche sediments is worthy of note in this connection. 

At their base is a sandstone that is generally conglomeratic in the Rocky Mountain region and 

co'nsists of coarse sand in other regions. It dilffers from many sandstones in being singularly 

free from partings of clay or other fine-grained material. Because of its open texture, due to 

the absence of fine material, it is one of the most important water-bearing sandstones in 

the Rocky Mountain region. The pebbles in the conglomeratic parts consist almost wholly of 

resistant rock, such as chert, quartzite, and argillite. Observations made at many localities throughout the Rocky Mountain region give the general impression that the sandstone is 

due to a washing and reworking of residual silica distributed over a · base-leveled surface. It 

is conceivable that owing to the shallowness of the sea for considerable distances from shore 

broad lagoons were formed that prevented free ttccess of marine waters and that in these lagoons 

'the weathered surface material was agitated by the waves sufficiently long to be thoroughly 

washed and evenly distributed, so that the coarser material was deposited relatively near 

shore and the finer material eventually came . t.o rest beneath the deeper and quieter water at 

some distance out. This sandstone is the lower part of the Dakota in the Rocky Mountain 

region as formerly described. It has only recently been recognized as distinct in age from the 

true Dakota 1 and mapped as the Purgatoire formation. The reference of the Purgatoire to 

the Washita epoch of the Lower Cretaceous is in accordance with the classification used by 

the United States Geological Survey. If, however, the Washita is Upper Cretaceous, as Berry 2 

asserts and as Haug 3 and other European geologists believe, then the Purgatoire is also Upper 

Cretaceous and the Dakota sandstone therefore holds a position somewhat above the base 

of this series. · 

Above this basal sandstone occurs a thin ]layer of clay, carbonaceous in some places, that 

constitutes the so-called "Dakota fire clay." That it is of Lower Cretaceous age is proved 

by the presence in it of certain marine invertebrates. If this thin layer of clay and the underlying sandstone are all that in the Rocky Mountain region can represent the great thicknesses 

of Lower Cretaceous rocks in Texas and Mexico, it is evident either that they represent a long 

period of time or that they represent only a part of the Lower Cretaceous. It is regarded as a 

well-established fact, based on both paleontologic and stratigraphic continuity, that the Lower 

Cretaceous of the Rocky Mountain region represents only the upper part of the Lower Cretaceous 

series. If, therefore, the Morrison is not Lower Cretaceous in age the greater part of Lower 

Cretaceous time must be represented by .a hiatus at the base of the Purgatoire. Little evidenceof such a hiatus has ever been found. If, howeyer, it is granted that the Comanche sea advanced 

over a graded plain and that there was little warping of .this plain prior to the beginning of 

Upper Cretaceous time, there might be no recognizable unconformity at the base of the Purgatoire formation, although the time break might be great. 

From what has been written it is perhaps sufficiently evident that such highlands as may 

have existed in the Rocky Mountain region 2~d been subjected to degradation through all of 

Jurassic and all of Lower Cretaceous time; and, furthermore, that so far as is now known these. 

lands supplied very little sediment during these periods. It is quite otherwise in western. 

Colorado and eastern Utah, where the thick sediments of Jurassic age, including in some places, 

probable equivalents of the Morrison, indicate nearness to the continental land mass farther 

west. In other words, the Rocky Mountain r egion seems to have been reduced to a condition 

closely approximating base-level by the close of Lower Cretaceous time. Such areas as had 

escaped inundation by the Lower Cretaceous sea and had been exposed to the weather through 

long ages must have contained considerable quantities of siliceous material concentrated at 

the surface by the removal of the more .easily decomposed minerals. This residual silica was 

available for the formation of the Dakota sandstone. 

I Stanton, T. W., The Morrison formation and its relation with the Comanche series and the Dakota formation: Jour. Geology, vol. 13, ppL 

657-669, 1905. Stose, G. W., U.S. Geol. Survey Geol. Atlas, Apishapa folio (No. 186), 1912. 

2 Berry, E. W., Correlation of the Potomac formation: Maryland Geol. Survey, Lower Cretaceous, pp. 136-137, 1911. 

a Haug, Emil, Traite de geologie, pp. 1169, 1293, Paris, 1907. 
34 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

GEOGRAPHIC CONDITIONS INDICATED BY THE DAKOTA SANDSTONE. 

EARLY CRETACEOUS PENEPLAIN. 

Probably the best indication of the physiographic conditions prevailing in the Rocky 

Mountain region at the close of the Lower Cretaceous is given by the Dakota sandstone. This 

sandstone is essentially continuous both east and west of the Rocky Mountains and is present 

in many places between the mountain ranges-as, for example, in North, Middle, and South 

parks, Colo. It is upturned on all sides against the mountains in such a manner as to strengthen 

the belief that it once extended continuously over the areas now occupied by them. 

The distribution of the Dakota sandstone, taken from Hayden's atlas of Colorado and from 

later maps of certain areas, is shown on figure 14. East of the mountains its altitude ranges 

from 6,000 to 10,000 feet. West of the mountains it is somewhat higher, attaining such altitudes 

as 12,600 feet in southwestern Colorado, 11,500 to 13,200 feet in central-western Colorado, 

and 13,400 feet near Breckenridge, between Middle and South parks. Inasmuch as the highest 

peaks of the Rocky Mountains are little more than 14,000 feet in altitude, it is reasonabie 

to suppose that the Dakota sandstone may once have extended continuously over this region 

and that it would now be found on the highest peaks had it not been removed by erosion. 

This suggestion becomes especially pertinent when it is considered, for example, that in a district like Breckenridge the Dakota, together with the overlying marine Cretaceous, is found at 

altitudes of more than 13,000 feet, although 45 miles to the east it passes beneath the 

surface of the Great Plains, and if the thicknesses of the strata overlying the Dakota are 

correctly reported, it reaches depths below the present sea level, as shown in the sketch section, 

figure 15. Moreover, this suggestion gains added force from the fact that near Breckenridge~ 

according to Ransome, the Dakota is overlain by remnants of Cretaceous sedimentary rocks 

· estimated as 5,500 feet thick, a thickness comparable with that of the Cretaceous on either side 

of the mountains. Inasmuch as the highest peaks rise less than 1,000 feet above these remnants, 

there seems to be little room for doubt that the Cretaceous sedimentary formations, in this 

part of the range at least, once extended continuously over the area now occupied by the mountains. Also, the Dakota occurs at short intervals from Breckenridge northward and westward 

to the large areas west of the mountains in which the continuity of this sandstone is unbroken. 

In this connection it may be in place to mention the somewhat remarkable uniformity in 

the altitudes of many of the highest peaks of the Rocky Mountains, a large number of which 

are close to 14,000 feet above the sea. This uniformity has been noted by many observers/ 

and the suggestion has been made by some that the tops of these peaks mark the approximate 

position of an ancient peneplain. Others regard this suggestion as visionary. It is conceivable 

that the fiat top of Longs Peak, about 250 feet wide and 600 feet long, may be a remnant of 

the old peneplain, and the domelike summits of other mountains may possibly be similar remnants 

in a more advanced state of erosion. The few facts available can not be taken as a basis for 

definitely postulating an ancient peneplain, but the uniformity in altitude of the high peaks, 

together with the geologic evidence, seems to indicate that they are best explained as the least 

~egraded portions of the base-leveled surface on which the Dakota sandstone was laid down. 

Cross 2 has shown that the post-Cretaceous uplift of the Rocky Mountains was accompanied 

by the outpouring of large quantities of andesite and that although the and'esitic debris form~ 

:a large part of the early Tertiary beds; such as the Denver and equivalent formations, no 

:remnant of the flows has survived. This is somewhat remarkable if the andesite rested on the 

.equally resistant crystalline rocks of the present mountains, but if it was outpoured on weak 

Creta~eous sedimentary beds that once covered the older crystalline rocks, its total removal 

is more readily understood, for these weak beds would erode easily, undermining the igneous 

material and thus facilitating its removal. 

There are few places where the Dakota sandstone abuts against older rocks in such a manner 

.as to indicate that it terminates against land masses that eXisted in Dakota time and few places 

1 Davis, W. M., The Colorado Front Range: Assoc. Am. Geog. Annals, vol. 1, pp. 21-87,1911. 

2 Cross, Whitman, Geology of the Denver Basin, in Color.ado: U.S. Geol. Survey Mon. 27, pp. 201-206,1896. 
j 

109° 

RELATION OF CRETACEOUS FORMATIONS TO ROCKY MOUNTAINS. 

LEGEND 

• Dal<.ota or younger 

D 

Rocks older than Dakota 

Numbers ( 8400, 134DO,etc) /ndicate 

altitudes or Dakota sandstone 

Scale ?L....._.I.....L...J''-2-~.s ___ ---ls1

o Mires 

108" 

9400 0 

1-

2 

::J 

0 

~ <Jl 

~ 

I" s. 

0l 

..:::: 

>- ~ 

-ro 

'-<. 

35 

? 

Jl'IGURE 14.-Map of the southern part of the Rocky Mountain region showing altitudes of the Dakota sandstone and the relations of the Dakota to 

· the Rocky Mountain ranges. (Adapted mainly from Hayden's atlas of Colorado.) 
36 

I 

~ ( 

I 

~\ ~ \ 

/ 

/ 

/ 

C· 

"iii 

ro 

ro 

ro 

+-c 

5 

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

0 

It) 

..... 

'll 

~ 

~ ~ 

«l 

~ u 

U1 tl) 

(\1 

where the succeeding Benton shale overlaps the Dakota and 

rests on older rocks. Cross 1 has described an apparent 

J· overlap of this kind near Pikes Peak, and several are indicated 

:s:1 in Hayden's atlas of Colorado. But these supposed overlaps ~ 

'0 

have been proved to be not overlaps in several places mentioned by the early observers, and the query arises whether 

the observed phenomena were correctly interpreted in other 

places. 

~ The mapping of the Dakota sandstone in the vicinity of 

~ the Rocky Mountains in Hayden's atlas is approximately · 

·; correct so far as it goes, but more recent observations have 

~ . proved the presence of the Dakota in many places not 1 shown on Hayden's map. The Dakota and the overlying 

_s Mancos, with McElmo beds below them, have been traced 

~ by Cross and Larsen 2 up Tomichi Creek, the eastern branch 

~ of Gunnison River, to a zone east of Sargent, where all these 

.Q formations are abruptly upturned. A remnant of the 

~ McElmo and Dakota beds resting on granite was found by 

§ these same observers near Saguache Creek, 16 miles west of 

·~ Saguache, a town on the western border of the San Luis Val- f ley. R. D. George, on his map of Colorado, has recently 

i' added a number of details, such as the occurrence of the 

~ Dakota near Canon City, west of the mountain range 

~ through which the Royal Gorge is cut, and als·o at the crest :S s of the Park Range, in northern Colorado. In brief, recent 

·~ observations are strengthening the belief that the Dakota 

-£1 was originally continuous over the area occupied by the .::f . ~ present moun ta1ns. ~ 

Q;> 

~ A 

<I) 

CHARACTER AND POSSmLE ORIGIN OF THE DAKOTA 

SANDSTONE. 

:S Dakota is a name that has been used in the past for .E s several different formations. In some of the older reports 

~ .the Dakota is described as several hm;tdred feet thick, includ- j ing much more than is assigned to this formation at the 

~ present time. The Dakota of the reports published a few 

~ years ago includes a sandstone and a shale now known to be of <I) 

~ Lower Cretaceous age. Where these can be definitely identi-

~ fied the Dakota sandstone proper has a maximum thickness 

~ of little more than 100 feet, and in many places its thickness ~ '"' is 50 feet or less. It is a coarse-grained sandstone, in some ~ o places conglomeratic and usually hard and quartzose. There 

~ are few places in the Rocky Mountain region where recog-

] nizable fossils have been found in it, and there is no cer-

] tainty that the Dakota of one locality is the exact time 

<I) equivalent of the Dakota of another locality. It seems to 

~ be the custom to assign a sandstone to the Dakota if it 

~ occurs immediately below shale known from fossil evidence 

to be of Benton age. In other words, the Dakota (the 

term being used now in its restricted sense as meaning the 

I Cross, Whitman, U.S. Geol. Survey Geol. Atlas, Pikes Peak folio (No.7), 1894. 

2 Personal communication. 
RELATION OF CRETACEOUS FORMA['IONS TO ROCKY MOUNTAINS. 37 

sandstone resting on the Purgatoire formation, 1 "fhich was formerly regarded as the lower part 

of the Dakota) seems to be the basal sandstone of the Upper Cretaceous series-that is, the sandstone formed near shore in the advancing Upper Cretaceous sea. 

Doubtless objection will be raised to this restriction of the term on the ground that the 

·Dakota is a sandstone of fresh-water origin. It has been so regarded mainly because of its lack 

of marine invertebrates in most places where it has been observed, and the presence in it of fossil 

plants. Apparently because of this assumption that the Dakota is of fresh-water origin, sandstones near the base of the Cretaceous lithologically like the Dakota and once regarded as 

Dakota have, when marine invertebrates were found in them, been assigned to younger formations. The objection loses some of its force when the gradual slope over which the Cretaceous 

sea advanced is considered. It seems entirely possible that a basal sandstone formed on such a 

slope might be essentially of fresh-water character and be followed immediately by beds of 

marine ongm. Thus, as the sea advanced the sandstone deposited one day might the next 

day be buried underneath marine shale. 

According to this hypothesis, it follows th::tt the Dakota of one locality may be the exact 

equivalent in age of some of the Benton shale of another locality. Paleontology has thus far 

- thrown little light on this question, possibly because the fossils may not have been examined 

'with this point in view, and possibly because fossils may not be adequate to show recognizable 

stages in so regular a transgression of the sea. It is especially difficult to make close paleontologic correlations between fresh-water beds and marine beds. The Cretaceous sea may have 

advanced rapidly, because the area that it subrrLerged was nearly level, but there are some facts 

that cast doubt on this possibility. For example, in southwestern Wyoming the ·Bear River 

formation, which, according to report/ ranges from 500 to 5,000 feet in thickness, has been supposed to represent the Dakota, inasmuch as it is the oldest Upper Cretaceous formation of that 

locality and lies beneath shale of lower Colorado age. 3 If the Colorado of the Cretaceous section of 

southwestern Wyoming is equivalent in age to the Colorado of other localities, as the fossils 

tend to prove, the sediments of the Bear River ml!lst have accumulated in about the same time 

that it took the 50 to 100 feet of Dakota sandstone of other localities to form. Even though it 

is granted that the Bear River is a delta deposit, and that its :formation was measurably rapid, 

the accumulation of its thick beds of sediment n1ust have taken a long time. On the other hand, 

if certain marine fossils found near the base of the Bear River. formation indicate Colorado age, 

as Stanton 4 states, most if not all of that formntion must be younger than Dakota. 

All things considered, it seems probable that the interior Cretaceous sea advanced from the 

Gulf of Mexico, and perhaps also from the Arctie Ocean, and completely eovered the region now 

occupied by the Rocky Mountains. (See fig. 1.3, p. 31.) It is possible that areas which were 

never submerged by the Cretaceous sea may be found in the Rocky Mountains, as some geologists · 

believe they Will, but no· such Cretaceous island is known at the present time. West of this 

region the sea seems to have encountered steeper slopes on the flanks of the old continent, where 

for some reason as yet unknown the basal sandstone differs considerably from the Dakota sandstone of localities farther east, and in many places at least is recognized as being of Benton age. 

EVIDENCE OF COAL-BRA,RING FORMATIONS. 

Throughout this paper the term ''near-shore deposits'' is used for rocks made up of rna terial 

more or less coarse, some deposits containing coal beds and land plants, others containing 

brackish-water or littoral marine faunas. InasmlJlch as the coal-bearing rocks prove nearness 

of land at the time when they were formed, they demand consideration, for they occur at one 

place or another in the Rocky Mountain region at almost every horizon within the Cretaceous 

system and at localities far from the rim of the Cretaceous basin. White 5 has pointed out that 

1 Stose, G. W., U.S Geol. Survey Geol. Atlas, Apishapa folio (Ko. 186), 1912. 

2 Veatch, A. C., Geography and geology of a portion of southwestern Wyoming: U. S. Geol. Survey Prof. Paper 56, p. 5~ 1907. 

a Stanton, T. W., The Colorado formation and its invertebrate fauna: U.S. Geol Survey Bull. 106, p. 45,1893. 

4 Veatch, A. C., op. cit., p. 63. 

5 White, David, Physiographic conditions ~ttending the format ion oi coal: U.S. Bur. Mines Bull. 38, pp. 52-84, 1913. 
38 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

in large measure coal beds indicate accumulation of vegetal matter in coastal swamps. Although 

these swamps may have been modified in various ways by barriers, fluctuations of land and 

water level, etc., the accumulation as a rule took place not far from sea leveL It is quite true 

that the presence of sand, littoral faunas, and coal beds 1 indicates deposition near shore, but 

these features do not indicate the character of the shore, nor do they prove that the sediments 

were derived from land near by rather than from land at a distance. 

The formation of the interior Cretaceous basin falls in the class of great epirogenic movements where uniformity of action is to be expected. On the whole this basin seems to have be n 

due to the downward warping of a base-leveled surface. While variations in the rate of warping 

probably occurred, it does not seem likely that upward warping of a magnitude sufficient to 

form land masses of any considerable extent occurred in the middle of the downward-warped 

area during the time when the general subsidence was going on. 

The significance of the coal beds in my reconstruction of the physiography of Upper Cretaceous time may be stated as follows: The Rocky Mountain region reduced to base-level at the 

close of Lower Cretaceous time was included in the area whose subsidence caused the formation 

of the interior basin of Upper Cretaceous time. The basin did not attain at once its maximum 

. depth, nor did the sea occupying it attain at once its maximum extent. Sedimentation pro-. 

ceeded contemporaneously with the advance of the sea, which doubtle-ss was intermittent and 

perhaps even oscillatory. It thus happened that coastal swamps favorable for the accumulation 

of peat developed sometimes near the periphery of the basin and sometimes far out toward its 

center. Thus near the ancient western continent the near-shore deposits with their coal beds and 

other evidences of fresh-water conditions alternating with rocks containing brackish-water and 

littoral marine faunas are found throughout the Upper Cretaceous series, while farther from 

this old land mass the near-shore deposits wedge out and merge with offshore or purely mar{ne 

deposits. Without ~arrying the discussion to unwarranted lengths I may say that it seems 

obvious that the occurrence of all the coal-bearing rocks of Upper Cretaceous age in the Rocky 

Mountain region may readily and rationally be explained without assuming that diastrophism 

reversed its normal processes and produced upward warping in the middle of an area that on the 

whole was being warped downward. 

DISTRIBUTION OF UPPER CRETACEOUS FORMATIONS. 

GENERAL FEATURES . . 

The general distribution of the Colorado formations in the Rocky Mountain region is too 

well known to require more than a passing remark in this connection. In most places where 

the Dakota sandstone occurs the Colorado overlies it with obvious conformity. In a few places, 

as, for example, near Pikes Peak,2 the Benton seems to overlap the Dakota. and to lie on older 

rocks. 

East of the Rocky Mountains the rocks of Colorado age are lithologically separable from those 

of Montana age in some places but not in others and are so uniform in thickness that it seems 

necessary to postulate relatively uniform conditions of sediruentation over extensive areas 

during Colorado time. Certain supposed unconformities near Golden, Colo., described by 

Eldridge, 3 do not support this hypothesis. He explained the thinning out of some of the Cretaceous formations as due to the presence of highlands over which these formations did not at any 

time extend. Doubt has recently been thrown on the existence of these unconformities at Golden 

by Richardson,4 who thinks that tb.e observed relations may be due to strike faults. Others 

who have examined the region with this conception in mind do not accept Richardson's suggestion. It would seem that if these supposed highlands really existed they wer~ of local 

occurrence. Seve!al years ago I advocated Eldridge's hypothesis in explanation of similar 

1 Stanton, T. W., Boundary between Cretaceous and Tertiary in North America, as indicated by stratigraphy and invertebrate faunas: Geol. 

Soc. America Bull., vol 25, p. 345, 1914. 

2 Cross, Whitman, U. S. Geol. Survey Geol. Atlas, Pikes Peak folio (No. 7), 1894. 

a Eldridge, G. H., Geology of the Denver Basin, in Colorado: U.S. Geol. Survey Mon. 27, pp. 82-104,1896. 

4 Richardson, G. B., Structure of the foothills of the front range, central Colorado: Washington Acad. Sci. Jour., vol. 2, p. 429,1912. 
RELATION OF CRETACEOUS FORJ\t ATIONS TO R.OCKY MOUNTAINS. '39 

phenomena near Perry Park, Colo., but I havJ since come to believe that this explanation is 

doubtful. · 

West of the mountains, and also within th rm where isolated remnants of Cretaceous rocks 

occur, there is no definite line of separation betw en the Colorado and Montana groups, although 

the lower part of the shale overlying the Dakot is known to be of Colorado age and the upper 

part to be of Montana age. However, the dis overy of the same fossils on ~oth sides of the 

mountains indica~es that the sea was ~on~inuous eit~er across o.r around the prese~t mounta~no~s 

area, an:d the available data seem to IndiCate tnat It was continuous across. This conclusiOn IS 

reiterated because of certain correlations that ill be suggested later. 

Where the rocks consist of limestone ther(~ could not have been large accessions of sediment such as would be expected from near-by n~ghlands. Arenaceous shales occur in the Color ado group east of the mountains, but the conte~t of sand seems to be no greater near the mountains than far from them, and there is no indi ation that this sand was derived from lands in 

the region of the present mountains rather tha from lands on either side of the interior sea; 

In brief, the general lithologic character of the ormations of the Colorado group argues against 

rather than for the presence of land in the mom tain region during Colorado time . . 

The presence of sedimentary rocks of Colof.ado age in many places within the mountains 

indicates that at least some parts of the mountf 'nous area . were submerged by the sea in Colorado time, butS. F. Emmons and other geolog sts following him have maintained that isl.arids 

persisted in that area throughout the Cretaceou period. This contention is not in accord with 

the evidence that the Dakota was laid down or a practically levelfloor nor with the fact that 

in North Park, South Park, and other inter~~ontane localities the marine Cretaceous rocks 

have essentially the same character and thickn s that they have on either side of the mountains. 

West of the Rocky Mountains the rocks of Polorado age are, on the whole, more arenaceous 

and less calcareous than the corresponding rockF east of the mountains. Beds of limestone are 

thin and few in number, but in some places, ~lpecially near the western margin of the Cretaceous area, the sa.ndstones a~e m~ss. ive. In th , co~ection it is a significant.f~ct that ~est and 

south of the Rocky Mountams the Cretaceous sedimentary rocks are prevailingly argillaceous 

near the mountains and become progressively ore arenaceous away from them, until near the 

margin of the old continent in New Mexico an Utah they consist mainly of sandstone, some 

of which is conglomeratic. This lithologic e dence is in harmony with the postulate of an 

€arly Cretaceous peneplain covering the. Rocky iMountain region and extending far to the west, 

on which the Dakota sandstone was laid down as the Cretaceous sea advanced over it. If this 

advance was due to subsidence of the land, tl e subsidence must have been very regular, for 

the Colorado formations vary but little in thicki ess over wide areas. A rise in sea level allowing 

the water to spread over the surface of a rel tively stable land during Colorado time might 

€qually well explain the observed relations. 

The sea seems to have transgressed the R( cky Mountain peneplain with relative rapidity, 

the thin Dakota sandstone being formed along its advancing front. Against the western land 

mass in central New Mexico, Arizona, and Uta] the advance seems to have been halted and an 

unstable condition of sedimentation establishe( , owing possibly to oscillations of the land but 

more · likely to oscillations of sea level, the ac umulations of sediments being built out from 

shore farther at some stages than at others. ie shorter stages are best indicated by the occurrence of alternating layers of sediments of fres water, brackish-water, and marine origin. The 

longer stages are indicated in the sections give in the several figures accompanying this paper. 

The formations referred to the Montana oup seem to be more irregular in character and 

thickness than those of the Colorado group. !Whether this irregularity is due to oscillations 

and surface warping in late Upper Cretaceous time or to some other cause has not been satis-

. factorily determined. It is a fact worthy of o~nsideration in this connection that. some of the 

greatest variations in thickness, as reported, ~pcur in the soft Pierre shale close to the mountains, where duplication by faulting and thicken:~ng by lateral thrust would naturally be expected. 

Where measurements have been obtained from/ drill records, vertical sections, etc., at sufficient 
40 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

distances from the mountains to make it reasonably sure that original thicknesses were 

obtained, the rocks of the Montana group have been found fairly uniform in thickness and 

character. This uniformity is one of the things that I wish to show by the accompanying 

sections. Another is the probable source of the sediments and the direction in which the 

· formations gradually change in thicknesd and in character. 

Inasmuch as the following descriptions of sections are somewhat lengthy, it may be appropriate to state here that I desire to emphasize my belief that the Cretaceous sediments came 

mainly from the continental land mass west of the interior sea, and that they spread eastward 

over the present site of the RockyMountains-in other words, that no mountains or even islands 

of any considerable size existed there in Cretaceous time. I have already stated that the advance 

of the sea in Colorado time was halted against the western continent. Whether or not this 

continent suffered differential uplift at the close of the Colorado epoch, as certain conglomerates 

and possible unconformities in Utah and elsewhere seem to indicate, there is little question that 

this or whatever land furnished the sedimentary material must have been relatively high or of 

great lateral extent in order to supply the enormous masses of sediment that accumulated in 

the interior Cretaceous sea. On the other hand, sedimentation seems to have been continuous in many places throughout Upper Cretaceous time, and it is not possible to distinguish 

stratigraphically between the Montana and the Colorado. 

Among the conclusions that may be drawn from a study of the sections described below, two 

stand out Tather prominently. First, the area of maximum depth in that part of the basin here 

described, as indicated by the maximum filling shown in the published sections, is in northern 

Colorado and southern Wyoming. It is a fair assumption, borne out in a measure by the 

character of the sediments, that the filling of the basin progressed from. the periphery toward 

this center, and that as time went on the center finally contained the youngest rocks. It may 

be noted in this connection that the youngest coal-bearing rocks of Cretaceous age-that is, 

the Laramie of the Denver Basin and formations of corresponding position-occur over a relatively small area in southern Wyoming and northern Colorado, on both sides of the mountains. 

If these mountains were above sea level during Upper Cretaceous time they must have stood 

near the center of the basin. Inasmuch as the mid-Cretaceous marine formations occur in 

full thickness in North Park, Colo., in the center of the area in which these youngest Cretaceous 

coal-bearing rocks occur, it seems reasonable to assume that there were no mountains there at 

the time and that the marine formations and perhaps also the overlying coal measures once 

covered the region. 

The second conclusion that may be drawn from the sections is that the sandstone formations 

near the ancient western continent thin eastward, toward the present Rocky Mountains, and the 

shale formations are thin to the west but thicken toward the present mountains; also that the 

shales have about the same thickness and character on either side of the mountains and in the 

intermontane areas. If, as is postulated in this paper, the main mass of sediments came from 

the west, the sandstones should thin eastward and finallybe replaced by shale, and it is quite 

natural that the massive mid-Cretaceous sandstones on the west should be replaced farther east 

by thin, inconspicuous sandy layers and that sands in great quantity should have reached the 

center of the basin only in late Cretaceous (Fox Hills and Laramie) time, after its peripheral parts 

had been filled. 

THE SECTIONS ILLUSTRATED. 

The sections on figures 16 to 22 have been selected with a view to showing comprehensively 

the distribution of the Cretaceous formations. They have been generalized in order that the 

features having a bearing OJl the present study may not be obscured by nonessential details. 

References are given to published descriptions, but in some of the sections modifications have 

been made in accordance with information obtained since the time of publication. Two groups. 

of sedimentary rocks are especially emphasized-near-shore deposits, including those of freshwater and brackish-water origin together with sandstones of marine origin; and offshore 

deposits, consisting principally of marine shale with some limestone. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 95 PLATE V 

so o so too tSoMiles ~~~========~~~~~3C=====~==~ 

MAP OF. THE COAL FIELDS IN THE SOUTHERN ROCKY MOUNTAIN REGION, SHOWING THE LOCATION OF THE SECTIONS 

DESCRIBED. 

Adapted from map of the coal fiel ds of North America, by M. R. Campbell. 
~' .. 
RELATION OF CR-ETACEOUS FOR-MATIONS TO ROCKY MOUNTAINS. 41 

The lines connecting the sections. are intended to show homogenetic equivalents rather than 

time equivalents. For example, the Vermejo formation of the Canon City field seems to be 

the homogenetic equivalent of the Laramie of the Denver Basin (section 6), inasmuch as it 

consists principally of coal-bearing rocks of fresh-water origin, but it seems to be the time 

equivalent of the Fox Hills sandstone of this basin. 

The several sections will be considered in groups with special reference to their relation to 

the Rocky Mountains. The approximate location of the sections is indicated on Plate V by 

numbers corresponding to those given in parentheses in the section headings in the text. 

·cARTHAGE, N. l\1EX., TO BOULDER, COLO. 

I 

The sections of figure 16 are so chosen that a line connecting the localities at which they 

were measured will pass east of the Rocky Mountains. to a point well south of the end of 

the main range and near the border of the southwestern continental land mass of Upper 

Cretaceous time. (See fig. 13, p. 31.) 

CARTHAGE (1).1 

Although Carthage is the southernmost locality at which a satisfactory section is available, 

the sedimentary rocks of Upper Cretaceous age extended at least 70 miles farther south, to the 

vicinity of Engle, N.Mex., where coal beds of Benton age havo been found. The rocks of the 

· Carthage field are badly faulted, but the succession, as shown in the section, seems to be well 

established. 

A sandstone at the base of the section is referred with a query to the Dakota because, like 

the Dakota ( ~) of the other sections, it lies below rocks of Benton age. No fossils were found in 

it, and it may represent the Tres Hermanos sandstone men1ber of the Mancos shale of the 

Tijeras section, described below. The Dakota ( ~) is overlain by marine shale with thin layers 

of sandstone of Colorado age, and these in turn by coal-bearing rocks that contain brackishwater invertebrates which range in age from lower Colorado to Laramie. Because of lithologic 

similarity Gardner correlated these rocks with the Mesaverde of the San Juan Basin and referred 

them with a query to the Montana. Later investigations indicate that they are older than 

Montana. The coal measures are ovorlain unconformably by Tertiary red beds containing 

Paheosyops near the base. 

The area between Carthage and Tijeras, a distance of about 80 miles, has not been examined in detail, but no rocks of ·Cretaceous age have been found there. I . 

TIJERAS (2).2 

At the base of the Tijeras section lies a sandstone that is called Dakota on the evidence of 

lithology and stratigraphic position. No fossils have been found in it. The overlying shale, 

the Mancos, consists of a thick shale of marine origin with a sandstone, the Tres Hermanos 

member, near the base. Tho principal part of the coal measures is of Montana age and is correlated by lithology and invertebrate fossils with the Mesaverde of the San Juan Basin. Some 

of the upper part may be younger than Montana, but the few f ossils-found in it were not suffi-

·cient to determine the age. . 

The Cretaceous rocks of the Tijeras field obviously were connected at one time with those 

of the Hagan and Cerrillos fields, to the north, and for the purposes of this paper the formations 

represented in these fields, now separated because of erosion, may be regarded as essentially 

-continuous. 

CERRILLOS (3).a 

At Cerrillos the Galisteo sandstone lies unconformably on the Mesaverde formation and is 

lithologically similar to the beds of Tertiary age in the San Juan Basin. For these reasons it is 

regarded as Tert~ary, but evidence is not at hand for closer correlation. The coal-bearing 

1 Gardner, J. H., The Carthage coal field, N.Mex.: U.S. Geol. Survey Bull. 381, pp. 452-460, 1910. 

2 Lee, W. T., Stratigraphy of the coal fields of northern central New Mexico: Geol. Soc. America Bull., vol. 23, pp. 6.29-633, 1912. 

1 Idem, pp. 633-659. 
SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

Mesaverde formation has yielded a large number of marine and brackish-water invertebrates 

and land plants, all of which indicate Montana age. The Mancos consists of a thick shale with 

marine fossils of Montana age at the top and of Benton age near the base. It contains the 

Tres Hermanos sandstone, here a single layer only a few feet thick. The sandstone below the, 

Mancos is referred to the Dakota because of its conglomeratic character and its position between 

the Mancos shale and the Morrison formation. · 
RELATION OF CRETACEOUS FORMATIONS TO ROCKY MOUNTAINS. 43 

No coal-bearing rocks are known to occur between the Cerrillos and Raton fields, but 

except for about 35 miles east of Cerrillos, whero all rocks younger than the "red beds" have 

been eroded away, the older Cretaceous ·rocks are continuous. The Mancos of the Cerrillos 

section is so similar in thickness and character to the Pierre of the Raton section as. to suggest 

that the Mancos was originally continuous with the Pierre across the intervening area. 

RATON (4),1 

The coal measures of the Raton Mesa region consist of the Raton formation, which is Tertiary in age, and the Vermejo formation, which is of Montana age. The Vermejo flora tends 

to correlate this formation with the Mesaverde of the Cerrillos fie]d and of the San Juan Basin, 

but the marine invertebrates from the top of the underlying shale suggest that the Vermejo is 

younger than the Mesaverde. In brief, the Vermejo is recognized as a Montana formation, but 

it seems to occupy a place in the time scale higher than the Mesaverde. The total thickness 

of the marine Cretaceous of this section is not known, but a well bored near Raton indicates that 

it is more than 3,000 feet. The drill did not reaeh the base of the shale. 

Although Raton is about 100 miles from Canon City, as shown on the plate of sections, the 

coal-bearing formations of the two regions :;tre separated by only about 30 miles and the older 

Cretaceous rocks are continuous between these coal fields. 

CANON CITY (5),1 

The coal measures of the Canon City field lie 1mconformably beneath rocks supposed to be of 

Tertiary age because of their lithologic character and structural relations. The coal measures 

are correlated by fossil plants with the Vermejo formation of the Raton Mesa region and contain 

invertebrates that tend to correlate them with the Fox Hills sandstone to the nouth. The 

thickness of the Pierre shale in this field is estimated as ·4,500 feet. This thickness was obtained 

from well borings in the trough of a syncline, where it seems probable that, because of crushing, 

the shale is now thicker than it was when originnlly .deposited. The thickness of the Colorado 

portion of this section is tnken from the Pueblo folio. 

Cnnon City nnd Boulder are about 125 miles apart, but the coni mensures reprerented m 

the two sections are separated only by the 40 miles between Canon City and Colorado Springs. 

From Colorado Springs to northern Colorado the coal measures are nearly continuous. 

DENVER BASIN-:BOULDER (6),2 

The Laramie formation of the Denver Basin consists of coal-bearing rocks. Beneath it lie 

sandstone and sandy shale of Fox Hills age, which are described as 600 feet thick near Colorado 

Springs 3 and 1,300 feet thick near Boulder. Fenneman states, without explaining how or where 

the measurement was made, that the Pierre shale near Boulder is more than 5,000. feet thick. 

For the snme shale near Colorado Springs Finlay gives a thickness of 2,500 feet, although Eldridge 4 states that its thickness between these two localities is 7,700 to 7,900 feet. Itj is partly 

because of these great variations in reported thickness that I am disposed to question tpe meaning of the figures and to believe that the maxin1um thicknesses as measured represent something more than the original or depositional thickness of the shale. For this reason I have used 

the section giving an intermediate thickness. A relatively thin sandy member knoWn. as the 

Hygiene sandstone occurs 1,000 to 2,700 feet above the base of the Pierre. It has not been 

definitely recognized outside of the Boulder region, and little is known of its relation ~o formations of other fields. 

SUMMARY. 1 

On considering this series of sections together, it seems obvious that shore conditions favorable to the accumulation of coal and of the sediments normally associated with coal ~xisted in 

central New Mexico early in Upper Cretaceous time and progressively later toward the north. 

1 Lee, W. T., unpublished manuscript. 

2 Fenneman, N. M., Geology Of the Boulder district, Colo.: U. S. Geol. Survey Bull. 265, pp. 28-34, 1905. 

a Finlay, G. I., U.S. Geol. Survey Geol. Atlas, Colorado Springs, folio (in preparation). 

4 Eldridge, G. H., Geology of the Denver Basin, in Colorado: U. S. Geol. Survey Mon. 27, p. 69, 1896. · 
\_ 

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44 

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A 

SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

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DATIL MOUNTAINS TO GALLINA, 

N.MEX. 

The series of sections given in 

figure 17 has been chosen for the purpose of showing the change in character of the Cretaceous formations in a 

north-northeasterly direction from a 

point in the Datil Mountains near the 

western Cretaceous land to the present Rocky Mountains-the ,Gallina 

locality. DATIL MOUNTAINS (7).1 

The coal-bearing formations west 

of the Rio Grande are separated by 

·erosion from those to the east, just described, but there are good reasons for 

believing that they are parts of formations that once extended continuously 

overwesternNew1fexico. In the Datil Mountains a thin conglomeratic 

sandstone of irregular occurrence is regarded as the Dakota, and above it are 

sandstone and shale about 3, 700 feet 

thick. The lower half of these rocks 

is referred on fossil evidence to the 

Mancos and the upper half to the 

Mesaverde. The Mancos, which elsewhere is almost exclusively a marme 

shale, here consists of sandstone and 

shale in nearly equal proportion and 

contains several beds of coal in the· 

upper part. The Mesaverde of this 

section is similar to the typical Mesaverde, which is essentially a coal-bearing sandstone. 

The Cretaceous formations are 

practically continuous from the Datil 

1\'Iountains to the Rio Puerco field, but 

little is lllown of them in the intervening areas. 

RIO PUERCO FIELD (8).2 

In the Rio Puerco field the Mesaverde formation lies unconformably beneath rocks that 

are lithologically similar to some of the- Tertiary beds of the San Juan Basin. It 'consists of 

1 Winchester, D. E., unpublished manuscript, 

2 Lee, W. T., Stratigraphy of the coal fields of northern central New Mexico: Geol. Soc. America Bull., vol. 23, pp. 622-629, 1912. 
RELATION OF CRETACEOUS FORMATIONS TO ROCKY MOUNTAINS. 45 

arenaceous. rocks of marine, brackish-water, and fresh-water origin alternating with beds of 

coal. Arenaceous rocks not notably different from those of the coal measures extenld nearly 

700 feet below the lowest known coal and terminate with the Punta de laMesa sandstone, a 

member that can be recognized as far north as Gallina, about 75 miles from Rio Puerco. This 

sandstone was formerly included in the Mesaverde because of its obvious relation to the sandy 

beds above it, but is now regarded as a member of the Mancos shale. The name Mancos shale 

is somewhat misleading for this field. Only the lower part of the formation is shale, and fossils 

indicative of Benton age occur practically to the top of this part. Near the base of the.Mancos 

occurs the Tres Hermanos sandstone member, whieh is recognizable from the Datil Mountains to 

Casa Salazar but which was not found farther north, near Gallina. This sandstone is coal bearing in the Datil Mountains, but in the Rio Puerco field only a single layer of carbonaceous shale 

was found in it. The sandstone below the Tres IIermanos is called Dakota ( ~) because it is at 

the base of such Upper Cretaceous rocks as occur here. No fossils have been found in it. 

CASA SALAZAR (9).1 

The section near Casa Salazar differs from the Rio Puerco section mainly in the character 

of the rocks above the Punta de laMesa sandstone member. They consist of shaly sandstone 

that seems to be transitional between the shale of the upper part of the Mancos at Cabezon and 

the lower part of the arenaceous beds of the Rio Puerco field. Only" the base of the Mesaverde 

formation is ·exposed here. 

The Mancos shale is exposed continuously between Casa Salazar and Cabezon and the continuity of the Mesaverde is only slightly interrupted. 

CABEZON (10),2 

. At Cabezon the Puerco formation lies unconformably on the "Laramie,". and the Lewis 

seems to be the attenuated edge of a shale formation that is very much thicker farther :P-orth. 

The Mesaverde seems to thicken as the Lewis thins and vice versa. 

From Cabezon to Gallina the Cretaceous formations are continuous, but for a few miles they 

are not exposed at the surface because of the Tertiary overlap. 1 

GALLINA (ll).a l . Near Gallina the Tertiary rocks lie unconfor1nably on the "Lara ie" formation, which is 

much thinner than it is near Cabezon. This formation, though regardJd by some as of Laramie 

age, contains fossil plants which indicate that it is Monta:o.a. The Mesaverde also is much 

thinner, but the Lewis has greatly increased in thickness. The Manpos shale is reported" as 

500 to 1,000 feet thick and has a sandy bed (the Punta de laMesa (~)sandstone) 275 feet from 

the top. The Dakota underlies the Mancos, but the Tres Hermanos sandstone, if represented 

at all, is inconspicuous. 

SUMMARY. 

The Datil Mountain locality is nearest the western continental land mass of Cretaceous time 

and the Gallina locality farthest from it. As in the sections farther east, the sandstones shown 

at the base of the several sections are homogenetically equivalent but may not represent equivalency in time. Near-shore deposits, including coal, began to accumulate in the Datil Mountain 

region early in Colorado time while marine shale was accumulating farther north. The coal 

measures of the Datil Mountains, being principall:y if not wholly of Colorado age, were in process 

of formation while marine shale was accumulating near Cabezon and Gallina. 

During Mesaverde time the strand line seems to have moved northward along this line of 

sections because of the filling of the sea and perhaps in part because of change of sea level. 

However, the open sea could not have been far away, for marine invertebrates occur at several horizons in the Mesaverde of the Rio Puerco field. A second notable advance of the sea 

occurred in Montana time, resulting in the formation of the Lewis shale. Inasmuch as this 

shale is notably lenticular, according to the reported measurements, there must either have been 

1 Lee, W. T., unpublished manuscript. 

2 Gardner, J. H., The coal field between San Mateo and Cuba, N. Mex.: U. S. Geol. Survey Bull. 381, pp. 461-473, 1908. 

a Gardner, J. H., The coal field between Gallina and Raton Spring, N.Mex., in the San Juan coal region: U.S. Geol. Survey Bull. 341, pp. 

335-351, 1907. . 

82646°-16--4 
46 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

a local downward warping of the basin in which it was deposited or else the advance of the 

sea was so slow that near-shore sediments now included in the upper part of the Mesaverde 

and perhaps also in the lower part of the "Laramie" were accumulating near Cabezon while 

c-- . (/) 't.O §3 offshore or marine clays were accumula- '00 Ql .-e c j ., 

j ~~} ] .§l j ting ~~~~:~~~~alities included in this 

~ 8 ~-o ~ series Gallina is nearest to the present 

g 

N 

CDo 

~­ro 

0 

1:9 highlands of the Rocky Mountains and .~ 

~ the Datil Mountain locality is farthest 

~ from them. An inspection of the sec2l u; . tions of figure 17 shows that the offshore d..., 

:8 ~ deposits (the Mancos and Lewis shales) 

·~ ·s increase and the near-shore deposits de-

~ : crease in thickness toward the Rocky ~ s ,.q :o Mountain area. Had this area furnished 

ZJ l=l 

~ ,g the sediments this relation would have o+> 

~ $ been reversed . ...... ..., ~ e : .g PINEDALE, ARIZ., TO GALLINA, N.MEX. 

d$ 

., "' d > 

~ ·~ l=l ., Q) <:) ,.q ..... 

8~ ,....._ bll 

> s p:;,g 

Q) ~ 

~~ :3£ 

~as .S"O 

,_, l=l 0 Q) ~..., '-' .s 

The sections shown in figure 18 are 

chosen to illustrate the change in character of the formations along a line crossing the San Juan Basin from a point in 

Arizona near the border of the western 

continental land of Cretaceous time 

northeastward to the present Rocky 

Mountains. Because of the absence of 

other published sections for the eastern 

part of this basin the Cabezon and Gallina sections are here repeated. 

PINEDALE (12),1 

The coal measures of the Pinedale 

region occupy a small area south of Holbrook, Ariz., and seem to have been continuous at one time with those of the San 

Juan Basin, although they are now separated py erosion. They are about 500 

feet thick and contain coal near the base. 

No sandstone referable to the Dakota 

was found. Just above the beds of coal 

were found fossils of Benton age that 

correlate these rocks with the coal measures of southern Utah. The Cretaceous 

rocks lie unconformably on shaly sandstone of probable Paleozoic age. Two 

other small areas have been described by 

Gilbert.2 

FORT WINGATE (13).a 

The section at Stinking Spring, 12 miles west of Fort Wingate, N. Mex., was measured 

long before the present formational nomenclature of this region was adopted. The coal-bearing 

1 Veatch, A. C., Coal deposits near Pinedale, Navajo County, Ariz.: U.S. Geol. Survey Bull. 431, pp. 239-242, 1911. 

2 Gilbert, G. K., Report on the geology of portions of New Mexico and Arizona examined in 1873: U.S. Geog. and Geol. Expl. W.100th Mer., 

vol. 3, p. 544, 1875. 

a Idem, pp. 503-567. 
RELATION OF CRETACEOUS FOHMATIONS TO ROCKY MOUNTAINS. 47 

rocks seem to be what are now called Mesave:rde by some authors and are referred to the upper 

part of the Mancos shale by others. They arJ underlain by a marine shale that is doubtless the 

Mancos, and certain sandstones near the base of this shale hold a position similar to that of 

the Tres Hermanos sandstone. The lowest sandstone is I called Dakota, but there is nothing 

aside from stratigraphic position to indicate !hat it is equ~valent in 11ge to the Dakota of other 

localities. GALLUP (14).1 

The coal measures near Gallup are supposed to be approximately equivalent in age to the 

original Mesaverde and hence of Montana age,[ but the coal of this region may extend downward 

into the Colorado, as it is known to farther ::~o uth and east, and its lower beds are supposed 

to be Mancos. Below the coal-bearing rocks occurs typical Mancos shale, but it is described as 

only 300 feet thick. A lower sandstone 200 ~eet thick is called Dakota. This thickness seems 

to be too great for the Dakota, and some of 1Jhe sandstone may be equivalent to the sandstone 

of the Fort Wingate section near · the base of the Cretaceous, which contains marine invertebrates and is therefore regarded as younger tlfan Dakota. It is probable that here, as in many 

other places in western New Mexico and Colorado, where the lowest sandstone of the Upper 

Cretaceous is called Dakota, the name has little time significance beyond the fayt that the 

sandstone lies below shale referable to some art of the Benton. It seems probable that this 

sandstone may, in some places at least, be of Benton age. 

The coal measures of the Gallup section ]are homogenetically equivalent to the Mesaverde 

formation of the Cabezon section, but obviously they are not equivalent in age if the reference 

of their lower part to the Mancos is correct. 

SUr :IIIARY. ·. 

In this series of sections, as in those already described, purely marine conditions persisted 

longest near the present mountains. If the time correlations are correct, near-shore deposits 

were accumulating in the Wingate-Gallup region while offshore deposits were being formed in the 

Cabezon-Gallina region. 

NORTHERN ARIZONA TO RATON, N.MEX. 

Although the coal-bearing rocks of Blac"k Mesa, in northeastern Arizona, are not now connected with those of the San Juan Basin, thJl two areas were probably connected at one time 

and are now separated only because of erosion!. Hence, the sections of figure 19 show the variations in character of the several formations in the northern part of this basin, from its most 

westerly exposure eastward to t~e Rocky Mountains. At one time these formations probably 

extended across the space now occupied by ~he mountains and were continuous with those of 

the Raton section, which is included here for comparison. 

NORTHEASTERN ARIZONA (15),2 

The first section of this series was measurl~d in the northeastern part of the Black Mesa coal 

field, near Chilchivito Spring, and shows the general relations of the formations throughout the 

field., although their thicknesses vary from place to place. From the unpublished not~s of members of Gregory's party another measureme t near this spring shows Mancos 565 feet thick 

and Mesaverde 865 feet. About 20 miles fitrther southeast the Mancos is reported as 470 

feet thick and the Mesaverde formation as 82Cl feet. Near Oraibi, about 70 miles southwest of 

Chilchivito, the Mancos was found to be about 225 feet thick and is overlain by an unknown 

thickness of Mesaverde. In the western part of the field, 6 miles east of Blue Canyon store, the 

Mancos is 490 feet and the Mesaverde more t~tan 221 feet thick. 

The coal measures of the Black Mesa field are referred with doubt to the Mesaverde, the 

underlying shale to the Mancos, and the lowest sandstone of the Cretaceous to the Dakota. 

This correlation is doubtless homogenetically porrect, as is indicated by the connecting lines In 

1 Gardner, J. H., The coal field between Gallup and San Mateo, N.Mex.: U.S. Geol. Survey Bull. 341, pp. 364-378, 1909. 

2 Campbell, M. R., and Gregory, H. E., ~he Black Mesa coal field, Ariz.: U.S. Geol. Survey Bull. 431, pp. 229-238, 1911. 
48 . SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

figure 19, but few da;ta for determining the age relations of these formations have beeh obtained. 

The lowest sandstone seems to have been called Dakota only because of its position. It is 

possible that the coal measures above the Mancos shale are the western continuation of the 

Mesaverde in the San Juan Basin, but 

the authors cited regard them as equivalent in age to the coal measures of the 

Mancos near Gallup. 

SAN JUAN BASIN (16 to 20). 

The coal measures on San Juan 

River consist of two formations, the 

"Laramie" above and . the Mesaverde 

below, separated by a marine shale. 

This shale, the Lewis, has _a thickness 

of 1,600 feet or more at locality 17 (see 

Pl. V) and farther east but thins to 250 

feet 35 miles to the southeast, at locality 

16.1 These formations are exposed 

continuously eastward around the San 

Juan Basin. The original descriptions 

of the Mancos shale, the Mesaverde 

formation, and the Lewis shale were 

based on their occurrence near Mancos, 

Colo.2 (section 18). 

The Durango section 3 (No. 19) 

was prepared by J. H. Gardner, who 

examined the sedimentary rocks of the 

Ignacio quadrangle for folio publication. Although the name ''Laramie" 

is retained for the upper coal measures 

they contain fossil plants regarded by 

Knowlton as of Montana age. . 

From Durango eastward to Pagosa 

Springs 4 (locality 20) and beyond the 

several Cretaceous formations have 

been traced continuously, and near 

Pagosa Springs the ''Laramie" formation contains plants indicative of Montana age. SUMMARY. 

The Lewis shale seems to have its 

maximum thickness and the Mesaverde 

its minimum thickness in the northeastern part of the San Juan Basin. In 

this group of sections, as in those previously described, the marine shales are 

thickest near the Rocky Mountains and 

the coal-bearing sandstone formations 

become progressively thicker away 

from the mountains. It is obvious 

1 Shaler, M. K., A reconnaissance survey of the western part of the Durango-Gallup coalfield of Colorado and New Mexico: U.S. Geol. Survey 

Bull. 316, pp. 37&--426, 1907. 

2 Cro~s, Whitman, U.S. Geol. Survey Geol. Atlas, La Plata folio (No. 60), 1899. 

a Lee, W. T., Stratigraphy of the c:>al fields of northern central New Mexico: Geol. Soc. America Bull., vol. 23, p. 584, 1912. 

~Idem, pp. 615-619. Lee, W. T., and Knowlton, F. H., unpublished manuscript. 
RELATION ,OF CRETACEOUS FORMATIONS TO ROCKY MOUNTAINS. 49 

that the Mesaverde of the Pagosa Springs section is to be correlated with the Mesaverde 

of the San Juan River section, but it seems equally obvious that the 400 feet of the former does 

not represent the same interval of time as the 1,450 feet of the latter. It also seems oovious that 

the 250 feet of Lewis shale south of San Juan River 

does not represent the same time interval as the 

2,000±feet of Lewis shale in the Pagosa Springs 

section. 

The Lewis contains invertebrates that tend to 

correlate it with the upper part of the Pierre of the 

Raton field, east of the mountains (section 4). On 

the other hand, plants similar to those from the 

Vermejo formation if not i~entical with them occur 

in both the Mesaverde ·and the "Laramie" formations of this basin. It has been shown on page 42 

that the Verniejo formation is probably the homogenetic equivalent of the Mesaverde formation of 

central New Mexico. The evidence of the fossils 

seems to indicate that the Vermejo of the Raton 

field is intermediate in age between the Mesaverde 

and the "Laramie" of the San Juan Basin. If we 

assume that the Cretaceous formations once extended across the area now occupied by the mountains, it seems probable that the Trinidad sandstone 

and the Vermejo formation are the homogenetic 

equivalents of the Mesaverde. It is possible, however, that could the formations· be restored it would 

be seen that the Mesaverde thins out to the east 

before reaching Raton and that the Trinidad and 

Vermejo formations are the homogenetic equivalents of the "Laramie" of the San Juan Basin. 

The 'absence of marine deposits above the Vermejo 

renders it difficult to determine which alternative 

is more likely to be correct. 

COLOB PLATEAU TO HENRY MOUNTAINS, UTAH. 

The group shown in figure 20 contains the 

most westerly sections obtainable of the Cretaceous 

coal-bearing rocks. of the southern Rocky Mountain region. However, the Henry Mountain section shows the probable connection of some, if not 

all, of the Cretaceous rocks of the Colob Plateau 

with the Mancos shale, the Mesaverde formation, 

etc., farther east. 

COLOB PLATEAU (21 to 24).1 

C':> 

0 

[ 

~~.lllilll=W.W.U. (}> 

The lower part of the coal-bearing and asso- ~· 8_ 8_ g 8.: g 8 

ciated rocks of Colob Plateau is of Colorado age. [ ~~ g-g !f g-g {3~ gi 

The top of the Colorado is drawn at a conglom- ~~J- ~~· ~~ 3·- ~ ~ ~~ ~ ~ ~ erate that seems to be persistent throughout the ~ . oo. 

Colob Plateau region, but its significance has not been determined. The rocks above 1t contain 

fossil leaves, but unfortunately their affinities are not sufficiently well known to determine the 

1 Richardson, G. B., unpublished manuscript. 
·50 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

age of the rocks. Richardson has suggested that the sedimentary rocks above this conglomerate may be either Montana or Tertiary. At the westernmost locality for which a section 

is given (No. 21) the rocks consist of alternating layers of sandstone and s~ale with little 

evidence of differentiation into separate formations. Farther east the coal-bearing rocks 

occur in an upper and a lower group, separated by a marine shale, which seems to be the Mancos 

of localities still farther east. The coal measures above the shale seem to be homogenetically 

equivalent to Gilbert's Blue Gate sandstone of the Henry Mountains and to the Mesaverde of 

localities farther east. 

The coal-bearing rocks of the Colob Plateau seem to have been connected at one time 

with those in northeastern Arizona but have been separated from them by erosion. 

HENRY :MOUNTAINS (25),1 

The Henry Mountain section seems to be comparable in some ways to the section farther 

north, at locality 26, and in other ways to the sections in the San Juan Basin. Gilbert's Tununk 

sandstone is comparable to the Ferron sandstone member of the Mancos shale, his Tununk 

shale to the shale underlying the Ferron as described by Lupton, his Blue Gate shale to the 

main body of the Mancos shale, and his Blue Gate sandstone to the Mesaverde formation. The 

Masuk shale of Gilbert holds a position suggestive of the Lewis and his Masuk sandstone a 

position suggestive of the "Laramie" as developed farther south, but no satisfactory evidence 

of age has yet been found in these upper formations. 

A group of sandstones at the base of the Cretaceous, called the Henrys Fork group, is of 

Benton age at the top, according to the fossil evidence, but may be Dakota and older below. 

By the method of correlation suggested in this paper the Cretaceous portion of this group of 

rocks is regarded as homogenetically equivalent to the Dakota, but it may be very different 

in age from the Dakota of other localities. 

SOUTHERN PART OF UINTA BASIN • . 

The coal-bearing rocks are exposed continuously from central Utah eastward through 

central-western Colorado to the mountains, where they are upturned in the Grand Hogback. 

They have been observed throughout this distance and examined in detail at several localities, 

as shown by the sections in figure 21. 

EMERY (26).2 

The southwestern extremity of the coal measures of the Uinta Basin is exposed near Emery, 

Utah, about 75 miles northwest of the Henry Mountains. The Cretaceous formations of the 

two localities seem to be comparable both in the order of succession and in lithologic character, 

as just suggested, although Lupton regards his Mesaverde as probably equivalent to Gilbert's 

Masuk sandstone and correlates his Ferron sandstone member of the Mancos shale with Gilbert's Blue Gate sandstone. This correlation, assuming that the formations were continuous, 

requires that the 500 feet of Gilbert's Masuk shale represent the 2,500 feet of shale above the 

Ferron sandstone, and by comparison with sections farther south it would seem to necessitate 

the cor~elation of the Ferron with the original Mesaverde. This correlation is rendered doubtful by the nature of the fossils. 

WELLINGTON (27),3 

. The same formations are exposed in the Wellington and Sunnyside areas, Utah, that were 

found by Lupton 50 miles farther southwest. Here also few data regarding the age of the beds 

were found in the upper portions of the Mesaverde. It is significant that the base of the arenaceous 

rocks of this formation occurs progressively lower in the section toward the west. - In tracing 

exact horizons Clark found that the base of the Mesaverde is stratigraphically about 150 feet 

lower in the western part of the area that he examined than in the eastern part. Here, then, 

is foun~, in an area examined in detail, the same transition laterally from shale to sandstone 

1 Gilbert, G. K., Report on the geology of the Henry Mountains: U.S. Geog. and Geol. Survey Rocky Mtn. Region, 1877. 

2 Lupton, C. T., unpublished manuscript. 

a Clark, F. R., unpublished manuscript. 
,.....j 

lQ 

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Coal Bai;rin:. 

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Color-ado Suri:ru?s, southern -:r;>art 61:' 

Denver Basin 

Ccv 

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Grand Mesa1

1 

I 

I 

/ s·.ooo'wid '- South.Park 27s'coal 

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Wellington 

27 

'i"t:.:RIII'-t3i- ---& ------ ---.-~~,;roe 1 

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Book Cliffs 

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/ 

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ss ~ _!;)re;'ckis h-wat.er 

deposits 

0 

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etc 50 miles . '" 300 r ·· -· .L;~a~bF;---

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600'ma rin e . depos1-t:_ __ '- ~ 

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FIGURE 21.-Group of sections from Emery, Utah, along the southern border of the Uinta Basin eastward through South Park to the Denver Basin, Colo. (For location see Pl. V .) The names at the left of each section 

indicate correlations that have been made. The lines connecting the sections are intended to show homogenetic equivalents rather than time equivalents. 
52 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 19'15. 

that was described for the San Juan Basin, where points of observation were far apart. The 

Ferron sandstone member of Lupton's section occurs in the Wellington area, but is notably 

thinner and seems to disappear farther east. 

BOOK CLIFFS (28).1 

The generalized section given for the Book Cliffs of Utah and western Colorado does not 

differ essentially from the one just described, except that Richardson mak~s no mention of 

the sandstone later named Ferron by Lupton. A well drilled 10 miles northeast of Grand 

Junction, Colo., indicates that the Mancos is more than 2,800 feet thick. The drill did not 

reach the Dakota sandstone. The sedimentary rocks between the Mancos shale and the rocks 

of Eocene age are all referred to the Mesaverde, with some qualifying statements, and no attempt 

is made to divide them into members. Some fossil plants and invertebrates were found, and 

dinosaur bones were obtained east of Green River, about 500 feet above the top of the Mancos 

shale. 

GRAND :MESA (29),2 

A basal conglomerate lying unconformably on the Gunnison formation in the Grand Mesa 

field is referred to the Dakota because of its stratigraphic position. Certain coal-bearing rocks 

. above this conglomerate heretofore regarded as Dakota are referred on fossil evidence to the 

Mancos. Above these the Mancos through an estimated thickness of 3,000 feet is principally 

shale. All the rocks between the Mancos shale and the base-of the Tertiary are referred to the 

Mesaverde, although an unconformity is described as separating them into a marine and brackish-water division below and a fresh-water division above. The Mesaverde comprises a small 

sandstone member, the Rollins, overlain by a coal-bearing sandstone and shale member, the 

Bowie, both containing marine and brackish-water invertebrates; and a higher coal-bearing 

member, the Paonia, containing fossil plants and fresh-water shells. The Paonia member lies 

unconformably on the Bowie in some places and on the Rollins sandstone in other places. 

These members have not been differentiated in neighboring fields. 

COAL BASIN (30),3 

Beekly examined the formations in the neighborhood of Coal Basin, Colo., south of Glenwood Springs. His work consisted mainly in obtaining data for the classification of the coal 

lands, but his measurements of the formations were made with care. The section was measured 

at the Grand Hogback and is therefore at the eastern margin of the Uinta Basin. The thickness 

of the sedimentary formations in the Grand Hogback renders it obvious that they once extended 

much farther east over the area now occupied by the mountains. It seems probable that they 

once connected with those near Como, in South Park/ about 60 miles e~st of the Grand Hogback, where coal-bearing rocks· overlie marine shale of great though unknown thickness. It 

seems to .be a debatable question whether the coal measures of South Park shall be correlated 

with the Laramie to the east or with the Mesaverde to the west. From the fact that at the 

north end of this park, near Breckenridge,5 about 30 miles north of Como, these marine rocks 

are estimated as 5,500 feet thick, it is reasonable to suppose that near Como they are comparable in thickness to the Mancos shale on the west and to the marine Cretaceous about 45 

miles to the ·east, where they are reported as about 5,500 feet thick at Canon City (section 5, 

fig. 16) and more than 7,000 feet thick at Boulder (section 6). 

SOUTH PARK (31) AND COLORADO SPRINGS. 

Although the coal measures of the Denver Basin east of the mountains are known as Laramie 

and those west of the mountains as Mesaverde, the suggestion seems pertinent that could the 

eroded portions of the formations be restored the Mesaverde would be found to extend east1 Richardson, G. B., Reconnaissance of the Book Cliffs coal field between Grand River, Colo., and Sunnyside, Utah: U.S. Geol. Survey Bull. 

371, pp. 12-23, 1909. ' 

2 Lee, W. T ., Coal fields of Grand Mesa and the West Eft Mountains, Colo.: U. S. Geol. Survey Bull. 510, pp. 18-19, 1912. 

3 Beekly, A. L., unpublished manuscript. 

4 Washburne, C. W., The South Park coal field, Colo.: U.S. Geol. Survey Bull. 381, pp. 307-316, 1910. 

5 Ransome, F. L., Geology and ore deposits of the Breckenridge district, Colo.: U. S. Geol. Survey Prof. Paper 75, p. 39,1911. 
RELATION OF CRETACEOUS FORMATIONS TO ROCKY MOUNTAINS. 53 

ward and include the coal measures of South P ark and the Laramie of the Denver Basin. In 

other words, the coal measures east and west of the mountains may be homogenetically equivalent although differing in age, just as the coal measures of the San Juan Basin seem to differ 

in age, although continuous from place to place. It may be equally pertinent also to inquire, 

first, if the Mesaverde of the Grand Hogback is the time equivalent of the original Mesaverde 

of the San Juan Basin; and second, if there is as great a difference in age between the Mesaverde 

of the Grand Hogback and the Laramie of the Denver Basin as the 'time significance of the 

names indicates. 

NORTHERN PART OF UINTA BASIN AND AREA TO THE EAST. 

The group of sections shown in figure 22 represents the Cretaceous formations that now 

extend from the Wasatch Mountains, in Utah, eastward to the Rocky Mountains and probably 

at one time extended eastward across the site of the Rocky Mountains. They are obscured 

in many places west of Axial, Colo., by the overlapping Tertiary strata, but from that point 

eastward to the mountains they are continuously exposed. Most of them have been eroded 

from the mountainous area farther east, but remnants have been preserved in the center of 

this area that serve to show the possible connection between the Cretaceous formations of the 

Uinta Basin and those of the Denver Basin. 

BLACKTAIL MOUNTAIN (32).1 

The sandstone at the base of the Cretaceous is referred with some doubt to the Dakota. 

Above this sandstone is a shale that is correlated with the Mowry shale member of the Benton 

of Wyoming. Above the shale is a coal-bea~n1g formation that seems to correspond to the 

Ferron sandstone member of the Mancos shale, and this in turn is overlain by the marine shale 

that constitutes the main body of the Mancos. The Mesaverde is describedas 3,263 feet thick 

and contains coal throughout the upper h alf. Unlike the Mesaverde of other localities, it is 

not coal-bearing in the low.er part. The coals are described as·lignitic' or subbituminous like 

those above th~ unconformity within the Mesaverde of the Grand Mesa field. This suggests 

the question whether all the rocks here called Mesaverde constitute a single formation. 

DEEP GREEK (33).2 

Deep Creek is about 15 miles northwest of Vernal, Utah, and 70 miles east of Blacktail 

Mountain, described above. A sandstone at the base of the Cretaceous in this district is referred 

doubtfully to the Dakota, and above this sandstone occur other sandstones and shales 390 feet 

thick, with coal near the top, which obviously represent the lower shale and coal measures of the 

Blacktail Mountain field, but are notably thinner. Above this coal lies the main mass of the 

Mancos shale, 2,100 feet thick. The younger Cretaceous rocks have been eroded away at 

. Deep Creek and the Wasatch formation rests unconformably on the Mancos sh-ale. Near 

Vernal, however, the Mesaverde is present in normal development. Gale 3 states that it IS 

about 1,500 feet thick at Green River and increases in thickness toward the east. 

AXIAL (34).4 

Axial 1s m the Yampa coal field described by Gale 5 several years ago. More recently 

Hancock has mapped the part of this field near Axial for folio publication. His section measured near Axial differs only in detail from that given by Gale for the whole field. The coal 

measures that occur near the base of the Mancos in the sections farther west a.re represented 

here by an inconspicuous sandstone tha-t contains no coal and that disappears toward the east. 

The Mancos shale has an enormous thickness, and the marine conditions indicated by its fossils 

1 Lupton, C. T., The Blacktail (Tabby) Mountain coal field, Wasatch County, Utah:, U. S. Geol. Survey Bull. 471, pp. 595-655, 1912. 

2 Lupton, C. T., The Deep Creek district of the Vernal coaJ fie}d , Uinta County, Utah: U. S. Geol. Survey Bull. 471 , pp. 579-595, 1912. 

a Gale, H. S., Coal fields of northwestern Colorado and northeastern Utah: U. S. Geol. Survey Bull. 415, pp. 204-219, 1910. 

• Hancock, E. T., unpublished manuscript. 

5 Gale, H. S., op. cit. 
(For.mation 

not dEiterm i ned) 

,.Lar~n-.ie" 

formation/ 

IQ) 

I] 

I ~ 

>-I ·~ ;; _J 

/if ~ ~/ {:§ I / g5 

I / ~= E~ I '-<ll 

0> 

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Blacktail.Mount.ain. "0+' 

LCO 

/ ~ ; Wosatch 

formet.ion 

32 I cno 

<lla. 

I ::;;:~ 

I -~ \ I ~ \ I 

\~ I // 

\'?.>. I / 

~~ I / \A / 

3,263' \ 1 / ' coal \ / 

bearing \ 1 / 

'\ I / \. I I 

\ DeepCrik/ 

\ . 3_3:-,// 

---_ -=~ira~tl>/ \ 

z,too'marine · deposits 

<ll 

Iii 

~ Ill 

g 

fO 

~ 

.Axial 

:Hayden: 

35 

,~ ----

Coal b · ~~J!P.I formation .::: !?00 . coal - eanng ·· ··· ···· \ bearing· 

.. \ 

~-

1.20b' marine 

1,6oo'marine ;:~ \ depo1;1its 

deposits j \ 

---------g 44 ___ \ .-r-~ ·.·.·.·.·.·: t,2oo'coal\ 

3,000'coal bearing· 

IOO' 

beai\ng 

\ 

\ 850'coal \ 

bearing \ 

100' 

1250'coal bearirtg 

\ 

\ 

\ 

\ 

La rami' 

for/!ion 

/ 

€/ 

",'I/ 

/.«-'/ ;<,_V/ 

/ 

~: :::· :1 , North.Park ... . . .. . _§Q... ____ CoBlmont\ 36 

s.soo'±marine 

· deposits 0 

0 

c 

fO 

~ 

formation \1:\ :;: 

I,OOO'±marine (§ 

deposits 

v 

~ 10 

a: 

-t 

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~ 

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4,500' "iij 

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Cool bearing 

s,ooo' .... marine 

deposits 

Hygiene 

sandstone member 

---;:.;-'~lEE§• Somiles n::=:::::::J, 25miles 1<-'------som.iles I~==Jr SOmiles H=:::= 

NiOu• en-a •u• rr•au-.. 

and Benton shale~ ~..::§ 965' 

:1=:: = =-::.-=--::.. = :'l= .::-= = .=.=-= :_JED=. -::£>a-Kc>ta sar.asrone-= 

400'marine 

deposits 

soo'marine deposits 

FIGURE 22.-Group of sections from the Blacktail Mountain coal field, Utah, along the northern border of the Uinta Basin eastward through North Park to Boulder, Colo. (For location, see Pl. V.) The names 

at the left of each section indicate correlations that have been made. Ths lines connecting the sections are intended to show homogenetic equivalents rather than time equivalents. 

<:.7t 

~ 

lJ). 

p:j 

0 

~ t:J 

~ 

0 

0 

~ 

1-3 

el 

t;d 

c:i 

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o r 

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~ 
RELATION OF CRETACEOUS FORMATIONS TO ROCKY MOUNTAINS. · 55 

did not wholly cease with the beginning of tie deposition of the overlying Mesaverde, which 

cont~ins marine invertebr.ates at several horizlns, the marin~ beds alternating with coal and its 

assomated .fresh or brackish water beds. ThF Mesaverde Is separated from a younger coalbearing formation by a thick marine shale 'fhich, because of its stratigraphic position, has 

been called the Lewis. This shale becomes sahdy toward the west and in part at least merges 

into the " Laramie" above and into the Me~averde below. This suggests that the enormous 

thickness of Mesaverde in other parts of the Yj"ampa field-over 5,000 feet, according to Galemay be due to the fact that the Lewis in the~e parts has not been. distinguished because of its 

sandy character and that what has been calli d Mesaverde in that region may include equivalents of both Lewis and "Laramie." I · 

HAYDEN (35).1 · 

The section near Hayden, Colo., in T. 5 [N., R. 89 W., was measured several years ago 

and the description was prepared for publicatwn, but it has not yet appeared in print. The 

"Laramie" and Mesaverde formations were cleasured with considerable care because of their 

coal beds, which were being examined in deta~, but the thickness assigned to the Lewis shale 

is believed to be less reliable, inasmuch as the shale occupies a broad valley and is not continuously exposed. In a reconnaissance trip throVgh this field I received the impression that the 

Lewis shale thickens toward the east and . the Mesaverde becomes thinner and less sandy 

in the same direction, but while this impression accords with other observations in western 

Colorado no definite statement to that effect lean now be made. 

The Cretaceous formations extei1d astw d to Steamboat Springs, about 25 miles from 

Hayden, where they are upturned, the Dakot~ forming a prominent hogback. Thus, although 

the distance between the Hayden and North Park sections is about 50 miles, as shown on 

figure 22, the Cretaceous rocks are lacking ov r a mountainous ar a only about 25 miles wide. 

• NORTH ~ARK (36).2 · 

The base of the Cretaceous inN orth Park cbnsists of a sandstone that is presumably Dakota. 

Above this sandstone is 965 feet of shale refefred on the evidence of the invertebrates to the 

Niobrar~ and Benton formations and 4,500 fe~t referred to the Pierre- shale. The shales have I 

the character and approximately the same thic~ess as the Mancos, to the west, and apparently 

differ but slightly from the Pierre and associat d shales to the east. Unconformably overlying 

the Pierre shale are 4,000 to 6,000 feet of coal bearing rocks of nonmarine origin consisting of 

shale, sandstone, and conglomerate, to which reekly has given. the name Coalmont format~on. 

Although this formation is classed as "Cretacepus or Tertiary" it contains a flora said to be of 

Tertiary age and is correb,ted with the "Upper Laramie" of Veatch, which in turn is regarded 

by some as essentially equivalent in age to ~ Denver formation. These facts strengthen the 

~sumption that the unconformity between th'- Pierre shale and the Coalmont formation is . the 

same as the post-Laramie unconformity of the,!Denver Basin. . 

On the assumption that the upper part of t}e Mancos and the upper part of the Pierre occupy 

different positions in the time scale, according o current usage, there seems to be serious doubt 

as to whether the beds in North Park should be correlated with those east of the mountains, 

in which case the 4,500 feet of beds of Montana age would be Pierre, or whether they are more 

closely allied with those to the west, in whicK case all the marine Cretaceous beds of North 

Park would be referred to the Mancos. wHatever differences in age may be indicated by 

the invertebrates, it seems obvious that the !Mancos is the homogenetic equivalent of the 

older marine Cretaceous rocks-in other wor 's, that the Mancos of the Uinta Basin and the 

Pierre, Niobrara, and Benton of North Park and of the Denver Basin are parts of the same 

formation and were once continuous across the areas now occupied by the mountains. 

1 Davis, J. A., unpublished manuscript. 

J Beekly, A. L., Geology and coal resources of Nort Park, Colo.: U.S. Geol. Survey Bull. 596, p. 20, 1915. 
56 SHORTER CONTRIBUTIONS TO GENERAL ·GEOLOGY, 1915. 

SUMMARY. 

The sandstones at the babe of the several sections of figure 22 are doubtless homogenetic 

but not necessarily time equivalents. The sandstones associated with the ·lower coal to the 

west lose their coal-bearing character toward the east and finally thin out altogether. Although 

the horizon is recognizable near Axial it has not been distinguished farther east. 

The principal shale of the Mancos is relatively thin in Utah and thickens toward the east. 

The resemblance of the shale in North Park to the Mancos on the west and the Pierre on the 

east strengthens the hypothesis that no barrier . existed in the Rocky Mountain region during 

Cretaceous time and that the Mancos and the Pierre are parts of a once continuous formation. 

An inspection of the sections also suggests that the base of the Mesaverde of the Blacktail 

Mountain locality is older than the base of this formation as developed farther east and that 

this formation may be the homogenetic equivalent of the Fox Hills of the Boulder region, 

although the latter has generally been regarded as much younger than the Mesaverde. 

The Lewis shale is well defined in some places, but toward the west it seems to become 

arenaceous and coal bearing and to merge into the Mesaverde below and the "Laramie" above. 

This fact, considered in connection with the occurrence of marine invertebrates at several horizons in the Mesaverde of the Uinta Basin, suggests that the Lewis shale of this basin may not 

be as definitely separable from the Mesaverde as some have supposed and that the Mesaverde, 

Lewis, and "Laramie" together of this basin may be comparable with the Fox Hills and 

Laramie of the Denver Basin. At first glance it seems difficult to reconcile this hypothesis with 

the great differences in thickness of the formations in the Uinta Basin and in the Denver Basin. 

But if the sediments came mainly from the lands to the west arenaceous beds should be thick 

in the western fields and should thin eastward. This difference seems to be even more conspicuously shown in the Cretaceous beds of southern Wyoming, north of the areas discussed 

in this paper. 

The youngest coal-bearing rocks of Cretaceous age in this region occur on either side of the 

mountains at localities less than 100 miles apart, and there is good reason for beli'eving that 

they were originally deposited in North Park. Remnants of them occur farther north, so 

distributed as to indicate that they originally occupied a relatively small area which may 

represent, so far as the region here described is concerned, the last part of the Cretaceous basin 

to be filled. 

GENERAL CONCLUSIONS. 

No claim is made that the views expressed in this paper are final. Many of them will 

doubtless be modified when more detailed work is done in the several areas described, but it is 

believed that they are worthy of consideration as field work is continued. Little is known of 

some of the sections discussed~ while others have been examined carefully, but all are believed 

to be sufficiently accurate to show the broad general relations of the Cretaceous formations. 

The points to which especial attention is directed are as follows: 

1. It seems evident that no notable crustal movement affected the southern Rocky Mountain region for a long period prior to the beginning of Upper Cretaceous time, and there was 

ample time for the peneplanation of this region before the invasion of the Cretaceous sea. 

2. The distribution of the Dakota sandstone about the present mountains, its attitude 

toward them, its presence upon them and within them at great altitudes, and its relation to the 

older formations all indicate that the Dakota originally extended continuously over the area. 

now occupied by the Rocky Mountains. 

3. There are good reasons fo.r believing that the marine Cretaceous formations overlying· 

the Dakota also extended uninterruptedly over the areas now occupied by the Rocky Mountains. 

If islands persisted there throughout the Cretaceous period, their location is now not known. 

Among these reasons may be mentioned (a) the absence of conglomeratic or arkosic material 

in these formations near the mountains, (b) the fact that .the marine formations are comparable 

in thickness and character on either side of the mountains as well as within them but do not 

in the mountain region contain intercalated beds of fresh-water origin, as they do farther west,. 
RELATION OF CRETACEOUS FORMATIONS TO ROCKY MOUNTAINS. 57 

and (c) the fact that the marine shales in western New Mexico and Colorado thicken toward the 

present mountains while the sandstones thin in the same direction. 

4. The several so-called basins containing Cretaceous rocks all seem to be of post-Cretaceous 

origin. Little evidence has been brought forward to show that they were separate basins of 

deposition during Cretaceous time. The physical evidence thus far produced seems to favor 

the hypothesis that the interior Cretaceous sea occupied a single basin extending from Utah and 

Arizona eastward over the present site of the Rocky Mountains. Certain paleontologic evidence, 

however, seems to oppose this hypothesis. 

5. When rocks at two or .more localities are correlated and designated by the same name, 

there may be no intention of implying that they are exact time equivalents, but that is the 

impression usually conveyed to the reader. There seems to be need of some definite term to 

express the fact that rocks at two or more localities may be equivalent in lithology and appear 

to hold the same stratigraphic position and even to contain essentially the same fossils and yet 

differ in age. Until a better term is proposed this relation may be expressed as tthomogenetic." 

For example, the Mesav-erde, a sandy, coal-bearing formation, known to be of Montana age in 

its type locality, seems to thicken downward at the expense of the underlying shale and has 

been described as continuous with coal-bearing rocks near Gallup, N.Mex., that contain fossils 

indicative of Colorado age. These rocks near Gallup are homogenetically equivalent to the 

Mesaverde. To follow this conception still further and apply it to beds that can not be traced 

through intervening areas, this term still seems useful, inasmuch as there may be less uncertainty 

in homogenetic correlation than in paleontologic correlation, especially where there is conflict 

of opinion arising from different classes of fossils. For example, there seems to be little doubt 

that the Vermejo formation of the Raton coal field is the homogenetic equivalent of the Mesaverde 

of the Cerrillos field (section 3), to the south, and of the Vermejo of the Canon City field (section 5), to the north; and yet some of the.paleontologic evidence indicates that the Vermejo 

at Canon City is of late Montana age, while the Mesaverde at Cerrillos is early Montana. 

6. The conceptions herein presented have an intimate bearing on the problem of the 

Cretaceous-Tertiary boundary in the Rocky }.tfountain region. Certain conglomerates that rest 

unconformably on Cretaceous beds are regarded as basal Tertiary by some geologists and as 

Cretaceous by others. These conglomerates contain great numbers of pebbles of crystalline 

and metamorphic rocks such as are now found in the mountains, and they are so distributed 

as to prove that they were derived from the present mountainous areas. Inasmuch as the 

Cretaceous for~ations were originally continuous over the site of these mountains, it follows 

that there must have been uplift and erosion sufficient to remove them and to reach the preCretaceous rocks before the materials for the conglomerates could be obtained . . In the Rocky . 

Mountain region of Colorado and New Mexico all deposits above these conglomerates are of 

the nonmarine type that characterizes the undisputed Tertiary formations of the same regions. 

Such a measure of this unconformity has been severely criticised. Whether or not the postCretaceous erosion removed 20,000 or 14,000 feet of sediments, less or more, the uplift was obviously. sufficient to cause the streams to cut through whatever Cretaceous formations were deposited over the areas now occupied by the mountains and enough more of the formations that 

underlie the Cretaceous to obtain the large quantities of coarse material now found in the 

lower part of the Tertiary. 

Briefly stated, it is conceived that the interior Cretaceous basin was formed by the slow 

and somewhat -intermittent downward warping ·of a nearly base-leveled surface, probably 

accompanied by a rise of sea level. This subsiding area, reaching from the Gulf of Mexico to 

the Arctic Ocean and from Utah to the Mississippi, is so large that the subsidence may be better 

called an epeirogenic movement rather than a downward warping, a term usually applied to 

smaller areas. The rate of sedimentation in this basin was comparable with the rate of 

subsidence. The sediments and the fossils they contain are of such a nature as to render it 

improbable that the sea was at any time very deep. It was probably for the most part so 

shallow that the sediments were somewhat uniformly distributed over its floor by waves and 
58 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

currents. The long period of quiescence was terminated and a new period of very different 

character was begun by an orogenic movement that resuscitated the mountains which had 

been worn down and whose roots had been buried beneath the Cretaceous strata. This movement was followed at short intervals by ·other similar movements, which produced the present 

highlands of the Rocky Mountain region and which also introduced the long series of volcanic 

events that characterized the Tertiary period in western America. As nearly as can now be 

determined, this movement corresponds in time with movements in other parts of the wo:dd 

which are universally recognized as terminating the Cretaceous period and introducing the 

Tertiary. It naturally follows that the conglomerates and other sediments derived by erosion 

from the newly uplifted mountains-such as those of the Denver, Arapahoe, Dawson, Raton, 

and related formations-belong to the Tertiary system.