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 
EOCENE GLACIAL DEPOSITS IN SOUTHWESTERN COLORADO. 

By wALL ICE w. ATWOOD. 

INTR DUCTION. 

At the northwest base of the San Juan Mountains, not far from the village of Ridgway, 

Colo., there is a series of exposures that in ude a remarkable section of glacial till, which is 

overlain by formations of early Tertiary age. These exposures were· found in. September, 1913, 

while I was conducting an areal geologic s rvey of the southwest quarter of the Montrose 

quadrangle. The areal work was being don with a .class of advanced student~ from the University of Chicago, and credit for collecting any of the data presented in this report is due 

to these able and enthusiastic young men. The party consisted of Messrs. F. B. Plummer, 

Frank Selfridge, W. J. Coleman, H. R. Benn tt, M. M. Leighton, Walter R. Miller, L. E. Wells, 

Lloy~ Le Due, and V. L. Wooten. Messrs. B1ennett and Miller first located what is now referred 

to as the type section, and they, with Mr. LF Due, were with me when the material was first 

recognized to be of glacial origin and to be bTied.beneath Tertiary formations. Mr. Kirtley F. 

Mather, who is assisting me in the physiographic survey of the San Juan district, also aided 

in working out the characteristics and age o these glacial deposits. 

After the completion of the work with he student party physiographic studies were continued in this area under the auspices of the United States Geological Survey, and at the close 

of the season Mr. Whitman Cross joined me d critically examined several of the best sections. 

I feel especially indebted to Mr. Cross for h s very careful analysis of the available field data 

arid for his judgment that the overlying T rtiary formations, with which he is so familiar, 

have been correctly recognized. 

PHYSICAL GEOGRAPHY. 

The area within which these early Ter iary glacial deposits have been found is in the 

foothill belt at the northwe t base of the m untains, in the same region where three distinct 

stages of Pleistocene glaciat on among the ountains were demonstrated in 1912.1 

At the south margin of this area are th~ precipitous slopes of the high mountains. Many 

of the summits ris.e to altituf es of over 13,0d0 feet, and Sneffels Peak, the highest ·peak in this 

mountain front, attains 14,1 8 feet. At the orth is a bold escarpment, bounding the southern 

face of the Uncompahgre P ateau. To the est are Hastings Mesa and Howard Flats, both 

of which are broad, flat-top ed areas somewl at below the level of the Uncompahgre Plateau, 

and far below the mountain summits to the south. The valley of Dallas Creek and its tributaries occupies the central p rtion of the are ., and the dissection of this area by these streams 

has developed a topograph of late maturit and uncovered many of the exposures to be described in detail in this pa er. The conspifuous topographic features in the central portion 

of the area, between the mo ntains and the lateau escarpment, are West Baldy, South Baldy, 

and Miller Mesa. West Bal y rises somewha boldly above the generally even surface of Howard 

Flats. South Baldy is a d me-shaped mo , tain in the upper portion of the Dallas Valley. 

MillerMesa is at the easter margin of the a ea and adjoins the valley of Uncompahgre River. 

In many of the escarpments of this mesa the Eocene till is exposed. 

Dallas Creek drains e stward into U compahgre River, which · is the master stream 

on the northwest slope of t e range. The ncompahgre rises far south of the region under 

1 Atwood, W. W., and Mather, K. F., vidence of three distinct glacial epochs in the Pleistocene history of the San Juan Mountains of Colorado: Jour. Geology, vol. 2J, pp. 385-409, 19 3. 

82646°--16----2 13 
14 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

consideration among high mountains, and flows northward, skirting the east base of Miller 

Mesa and the Uncompahgre Plateau. The western po!'tion of the area i~ drained by Leopard 

Creek and its tributaries, and Leopard Creek joins the Dolores. All the streams of this area 

are tributary to Colorado River. 

DESCRIPTIVE GEOLOGY. 

MESOZOIC SECTION. 

The Mesozoic section here exposed includes, from the top downward, the Mancos shale, 

the Dakota sandstone, and the series of sandstones and shales belonging to the McElmo formation. These formations decline northward with very gentle dips, but are broken by a great 

east-west fault a short distance north of Ridgway. The block north of this fault line has been 

elevated relative to that at the south, and the elevated block is a portion of the Uncompahgre 

Plateau. The Mancos shale is preserved below the capping of the glacial drift in Horsefly 

Peak, near the southern margin of that plateau, but most of the surface of the great table-land 

is formed by the Dakota sandstone, and the slope of the surface corresponds very closely to 

the inclination of the sandstone. In the escarpment bordering the plateau, below the Dakota 

sandstone, the McElmo formation is exposed. South of the fault escarpment, in the valley 

of Dallas Creek, the dominant formation is the Mancos shale, which also forms the basal portion of Miller Mesa. To the west the Dakota sandstone forms the surface of much of Howard 

Flats and of portions of Hastings Mesa, but there are remnants of Mancos shale upon these 

upland surfaces, and in each locality where any of the earliest of the Pleistocene glacial deposits 

are present small areas of the shale are preserved beneath those glacial deposits. 

TERTIARY ROCKS. 

Above the Mesozoic section the geologic succession differs from place to place within the 

area. The Eocene · till, where found, rests unconformably upon the Mancos shale. The till 

is usually overlain unconformably by the Telluride conglomerate, which, in turn, is overlain 

unconformably by the San Juan tuff. Locally the Telluride conglomerate or the San Juan 

tuff rests upon the Mancos shale. At the south margin of the area the San Juan tuff is buried 

by great thicknesses of later Tertiary volcanic rocks of the Silverton and Potosi volcanic series. 

The Telluride conglomerate and the basal portion of the San Juan tuff extend northward 

over Miller Mesa, to which the tuff forms a resistant capping. The upper volcanic series probably extended much farther northward, but they have since been removed by erosion. 

PLEISTOCENE FORMATIONS. 

The three distinct Pleistocene drift sheets which have been recognized in the San Juan 

region must be frequently referred to in this paper and in aU subsequent reports on the glacial 

studies of these mountains. For convenience in description the following names have been 

selected: 

The oldest known Pleistocene drift sheet in this region will be called the Cerro till. The 

name Cerro is selected from Cerro Summit, near the north margin of the Montrose quadrangle, 

at which there are heavy glacial deposits of this earliest Pleistocene stage. The Pleistocene 

glacial deposits of the intermediate stage will be called the Durango till. A little northeast 

of Durango, a city on the south slope of the range in the valley of Animas River, there are 

heavy morainic deposjts of this intermediate stage. These deposits rest on a high rock bench. 

They retain a distinct morainic topography and are associated with extensive outwash deposits 

of the same age. They are l'8adily distinguished from the later glacial deposits which rest 

upon the valley floor several m11es upstream. In the Montrose quadrangle deposits of Durango 

till occur on the hilltops east and west of the village of Dallas, which is about 3 miles north of 

Ridgway, also near the headwaters of Dallas Creek, and a short distance to the southeast of 

Dallas Divide. The most recent Pleistocene glacial deposits in the region will be referred to 
Base from Montrose 

topograph ic atlas sheet MAP OF 

l l5 

PROFESSIONAL PAPER 95 PLATE I 

Geology b 

Wallace W. A wood 

LEGEND 

Wi con in till 

Latest glacial d rift) 

Cerro till 

( Ol<lest Pl ist<>cet~e ola..cial 

d1Vt in thia r {Ti.on) 

/ 

~ EJ ~ ~ 

~ o Ridgway till ~ t Upper m.emh ,. is bb~e W 

till., w wer member t.q lbo11Jlder t iU) 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 95 PLATE II 

A. 

' ,.- . .· '\, ''"/. I •yt 

B. 

RIDGWAY TILL AT THE TYPE LOCALITY, 1 MILE W OF RIDGWAY, COLO. 

For expr at;on of d;agram seep. 16. 

I 
l 
EOCENE GLACIAL DEPOSITS IN . SOUT WESTERN COLORADO. 15 

as the Wisconsin till, in accordance th a u,sage alre dy establi hed in describing the latest 

of the till sheets in the Rocky Mountains. The correl tion of th deposits of the last glaciers 

in the Rocky Mountains with those of the last great c ntinental ice sheet has been established 

in the study of the deposits at the east ibase of the Rod y Mounta ns near the Glacier National 

Park,t but the correlation of the deposits of the pre- isconsin glaciers in the Rocky Mountains WI.· th the pre-Wisconsin drift s.heetk of the Central States is ¢1 certain, and hence the loc.al 

names Cerro and Durango are introduded. · 

The Pleistocene glacial deposits thin this area e at the s rface and must not be confused with the buried glacial deposits tp be herein des 1 ribed. T e remnants of the Cerro till, 

or oldest known Pleistocene glacial dewosits of this re ion, are at widely separated localities, 

on the higher portions of the lowland dountry. Thus, West Bal ly and South Baldy are both 

capped with the Cerro till. Certain of the higher hills rising abo e Hastings Mesa are also so 

capped, and Horsefly Peak, a promin~nt feature on e Uncom ahgre Plateau, rising to an 

elevati.on o.f 10,338 feet above the sea ~evel, or about 2,500 feet hove the bordering plateau 

surface, is also capped with Cerro till. j The moraines f the Dur ngo or intermediate Pleistocene glac. ial stage occur in association jith the mode n valleys r canyons just north of. the 

San Juan range. They are distinctly b

1

elow the level ?f which t e Cerro till was left, and yet 

farther down the valleys than the depo~its left by the i~e of the la t or. Wisconsin glacial stage. 

The accompanying map (Pl. I) furnishes the essential dlata on the distribution of these Pleistocene glacial deposits within the area ubder considerat on. 

The Cerro morainic deposits, now preserved in scattered r mnants on certain hilltops, 

exhibit many characteristics of great afe. They rest ~pon an erfion surface, most of which 

has b .. een carried away by streams sine~ the ice of. this earliest PI· istocene stage melted . . ·The 

wide distribution of these remnants and the vast stret es of cou try which have been deeply 

dissected since these deposits were ma~e point most mpressive to the great age of these 

moraines. Furthermore, these depositF within the area under consideration are composed 

very largely of volcanic rocks, which onGe capped the mbuntain sufmits to the south but which 

are now almost entirely missing from the range. It is blear that e basins in which the Cerro 

glaciers formed have been obliterated b} subsequent erbsion. Th :. distribution of these glacial 

deposits is .furthermore sugge.stive of a jvalley system iuite di.fferent from that of the present 

day. The present remnants of Cerro tiJl are not oppo ite the mot ths of the modern canyons. 

Many of them are opposite the mount~in spurs, ' at loc lities that ice coming from the mountain canyons of to-day could not reach1

1

. This relatio ship also oints to the great antiquity 

of these deposits. Furthermore, these oldest Pleistoc ne mora· es do not contain an abundance of bowlders from the older formations which hav been unco ered as the dissection of the 

range has gone on. By this. means it ij possible to .di' s inguish th Cerro. mo.rain-es from those 

of the Durango and . Wisconsin stages. The oldest PI istocene d ·posits are much weathered, 

and stream er~sion has nearly oblitera~,d all signs of . rainic top grap_hy in .them. . . 

The morames of the two later Plm~tocene stages ( urango a d W1scons1n) are assomated 

with the moder~ canyons, a~d some o~ them e::ctend ~ut a few iles down the valleys from 

the great amph1theatral basms where lthe glamers fff,med. · In the valley of Uncompahgre 

River, at the east margin of this area, the terminal raines of he Dura:p.go and Wisconsin 

stages have been recognized a short distance down the alley fro the village of Ridgway. In 

the smaller valleys west of Miller Mesa the later glacier were but few miles in length. They 

failed to reach the broad lowland area o, the Dallas Vauky. 

I RLCENT DEPOSIT . -

Since the last Pleistocene glaciers mFlted a.way the freams ha e distributed vast quantities 

of sand and gravel over their flood plailfs, and these lo lands stre ch far off to the northwest, 

beyond the margin of the range. Locai)]y there has b Jen some la1 d sliding, .and a number of 

torrential fans have been developed. 

t Alden, W. C., Pre-Wisconsin glacial drift in the region df Glacier National Par , Mont.: Geol. S c. America Bull., vol. 23, pp. ·687-708, 1912 •. 

Calhoun, F. H. H., The Montana lobe of the Keewatin ices 1eet: U.S. Geol. Surve Prof. Paper 50, 906. 
.16 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

EOCENE TILL. 

TYPE SECTION. 

GENERAL FEATURES. 

The section 1 mile west of Ridgway, at the locality where the Eocene till was discovered, 

has been selected as the type section. The locality is marked as No. 1 on Plate I. It is within 

plain view from the Ridgway station platform, and the term Ridgway till has therefore been 

adopted for this formation. The cliff in which the till is exposed (see Pl. II) contains the Mancos 

shale (a) . at the base. Overlying the shale unconformably is a section (b) 80 to 100 feet thick 

which exhibits all the usual characteristics of glacial till. This section appears from a distance as 

a distinct yellow band. Above the yellow band, resting upon an uneven erosion surface, is a 

dark slate-colored stratum (c), which, on careful examination, has also been found to be a glacial 

till. The .upper till sheet is overlain by a layer of the Telluride conglomerate (d), which varies 

in thickness from 70 feet at the south (left-hand) end of the section shown in Plate II to 10 or 

15 feet at the north end of the section. Above the T~lluride conglomerate, also upon an erosion 

surface, is a remnant of San Juan tuff (e) having a thickness of at least 30 feet. 

The spur in which this type section is exposed is so nearly separated from the main part of 

Miller Mesa that the section above described may be traced around the north end and on the 

west side, so that it is perfectly clear that the formations extend through this spur. The sec-

. tion thus described continues, with some variations in the thickness of the different formations, 

southward along the face of the Miller Mesa escarpment for fully a mile. At the southernmost 

exposure (locality No. 2), which, however, may not be the southern limit of these formations 

in the mesa, the section is somewhat different from that at the type locality and is described 

on -page 18. 

RIDGWAY TILL. 

Lower. or bowlder member.-The lower till sheet 'at the type locality contains an abundance 

of stones which range in size from tiny pebbles to bowlders 15 feet in diameter. .These stones 

vary iri composition, like the rock formations in the core of the San Juan Mountains, beneath 

the great volcanic cap which was added to the mountain area in Tertiary time. They include 

many varieties of porphyries, granites, some volcanic tuff, quartzites, sandstones, limestones, 

and conglomerates. The harder, crystalline bowlders, the pink and purple quartzites, and the 

volcanic rocks came from the core of the range, but the softer sandstones, limestones, light-

·colored. quartzites, and conglomerates were plucked by the ice from the upturned sedimentary 

·strata at the north base of the ·range. These stones have been derived from formations which 

range in age from the pre-Cambrian to and including the Upper Cretaceous. The bowlders of 

tuff came from a ·volcanic series older than any of those which form the present great mass 

of the San Juan Mountains. The San Juan tuff overlies the Telluride conglomerate, which in 

turn overlies the glacial deposits. 

The tuff deposits that contributed material to the Ridgway till seem to have belonged 

to an early and rather local extrusion. These tuffs were largely removed by erosion before 

th~ Telluride conglomerate was laid down, for such material is rarer in the conglomerate, all 

through the western San Juan Mountains, than it is in the known exposures of Ridgway till. The 

San Juan tuff, which reaches a thickness of 3,000 feet near Ouray, is composed mainly of transported material similar in petrographic character to the fragments of tuff in the till. But the 

source of the San Juan tuff was a great volcanic mass erupted, soon after the Telluride conglomerate was laid down, in the area which supplies the Telluride materials. These facts fix the general 

time relations of the Ridgway glacial epoch to the volcanic history of the San Juan Mountains. 

The pink and purple quartzites came from the pre-Cambrian Uncompahgre formation, 

now well exposed near Ouray and up the canyon of Uncompahgre River. Most of the limestones were derived from the Devonian or Carboniferous formations which are also now exposed 

near Ouray. A vast quantity of red sandstone and conglomerate was gathered by the ice 

from the Cutler (late Carboniferous) and Dolores (Triassic) formations. These formations must 
EOCENE GLACIAL DEPOSITS IN SOUTHWESTERN COLORADO. 17 

have been exposed but a few miles south of the localities where the Eocene drift is now located. 

They are well exposed to-day in the Uncompahgr( Valley, between Ridgway and Ouray. The 

white quartzites are from the Dakota sandstone, w· ich is locally a distinct quartzite. 

A great many of these stones show distinct s gns of ,ice action. Small striated stones are 

abundant (see Pl. III, B) and subangular formf: are conspicuously common. Many of the 

huge bowlders are distinctly striated, and in cert in of the great masses of red conglomerate 

from the Cutler formation, the exposed surfaces of the limestone pebbles carry strire. The 

abundance of striated stones found on the expot ed surface of this till may seem somewhat 

surprising, and it should be explained that as th( underlying Mancos shale is rapidly washed 

away, the glacial drift is continually falling, and therefore, at this locality, continuously presenting a fresh exposure. The discovery oi oth« r striated stones in this till may be safely 

expected for mariy years to come. 

The matrix of this lower or yellow till is sand and clay, ~nd it is quite probable that most 

of the clay was derived from the underlying Ma cos shale. The sand may have come from 

various formations. The conspicuous _yellow colo of this lower layer of glacial till is characteristic of every exposure of this member· throughout he area. The yellow coloring is not confined 

to the surface, but evidently continues through01 t the section. It appears to be-due to the 

oxidation of the iron minerals associated with then atrix of this till. If this oxidation proceeded 

from the top throughout the mass, which has a thickness of at least 90 feet, it would seem 

to indicate that this member was at the surface during an exceedingly long period of time. 

Possibly the material as gathered by the ice had t is extreme yellow color. 

In texture this drift is exceedingly firm. It '"'tands with a slope so steep that it is difficult 

to work around the hill on foot at that horizon. In places the large stones, or bowlders, have 

acted as preserving caps, and the rain erosion hat developed great earth pillars 6 to 8 feet in 

height, each with its bowlder cap. Certain of tl ese earth pillars are shown in Plate III, A. 

Throughout the finer matrix of this till there is a abundance of small stones, many of which 

have been beautifully polished and striated. 

Upper or pebble ~ember.-The upper memb(r of the Ridgway till is strikingly different 

from the lower member. When first examined, i was thought to be a layer of clay without 

stones or pebbles. In color it so closely resembl( s the Mancos shale that it was suggested in 

the field that it might be a part of that formation. This hypothesis was, however, very promptly 

. abandoned, for it was somewhat unreasonable to 1 hink that there was a glacial epoch in southwestern Colorado during a period of marine invasic n. Again a lacustrine origin was considered, 

but it is quite certain that this material is unstr. tified. I was for some time in great doubt 

as to what its true origm might be. The absen« e of large stones, ·such as characterized the 

drift sheet just below, seen1ed to exclude the ide of another glacial dep9sit. That the same 

ice which brought the bowlder till could have con ained or carried this upper sheet of material 

seemed improbable. It also seemed unlikely, if ice had brought the upper material, that it 

could have had the same source as the ice which d~posited the bowlder till. It was certainly 

strange that ice co~d have moved over the bowlder till without gathering some large stones. 

However, on very careful search, several small but distinctly striated pebbles were procured 

from. this upper sheet. When a mass of this material could be broken out and carefully examined, it was found that the distribution of the tiny stones and pebbles was similar to that in 

the typical deposit of glacial till, the only difference being that the stones were much smaller. 

These characteristics have been found to persist in the several exposures of this upper member, 

and it is quite clear now that the material is a glacial deposit, and the term "pebble till" is 

suggested as appropriately descriptive of it. This upper member is not so deeply weathered as 

the lower member, but the upper 6 or 8 feet seems to b,e somewhat leached, as if that portion 

had been exposed to the air for some time and affected by the usual processes of weathering. 

The stones in the pebble till are so small that no attempt has yet been made to identify 

them and determine their source, but further studies of these deposits may make it possible 

to procure collections that will help to determine the source of the drift. As the examination 

of the several exposures progressed a few larger stones, the largest 2 or 3 inches in · diameter 
18 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

and one stone 6 inches in diameter, were fourid in the pebble till. The striking .characteristics, 

however., ofthis upper member are an absence of large stones, a firm clay matrix, an unstrati- · 

fied condition, an<:! striated pebbles. 

FORMATIONS OVERLYING THE EOCENE TILL. 

Telluride conglomerate.-The Telluride conglomerate, which overlies the upper till member 

at the type looality, contains many pebbles 4 inches or less in diameter and a few stones as 

much as 10 inches in diameter. The matrix is sandy, with some fine gravel. The formation 

is cemented and stands with a nearly vertical face. It has a general pink color and closely 

resembles other exposures of this well-known formation of the San Juan Mountains. It was, 

however, a notable surprise to find distinctly striated stones in this conglomerate. It is of 

course clear that the formation is of later age than the underlying glacial deposits, and the 

streams which laid down the Telluride conglon1erate must have drawn in part upon glacial 

deposits for their materials. Possibly some of the stones in the conglomerate were not carried 

far, and that would account-for the preservation of glacial strire on stones transported by water. 

San Juan tu.ff.-The San Juan tuff overlies the Telluride conglomerate at the Ridgway 

type section and forms the capping of the spur in which the section occurs. The material is 

quite evidently of volcanic origin, and most of the fragments are angular. They vary from dust 

,to blocks 8 feet in diameter. It is possible to detect distinct lines of bedding in this exposure 

of the tuff. It has a gray, slate-colored appearance, is much broken from weathering, and 

seems at places to have settled irregularly since it was deposited. There is very little rounded 

material in this exposure of the tuff, but otherwise it resembles the San Juan tuff which has 

been fully described by Cross in the geologic folios on the San Juan district. 

OTHER SECTIONS. 

The other exposures about the rim of the mesa 1nay now be described somewhat briefly, 

for the different formations retain their marked characteristics, but at most of the several localities one or another of the formations included in the type 

section is missing. The . numbers used in the descriptions 

represent corresponding numbers on Plate I. 

Locality No. 2 is at the extreme south end of the exposure which contains the type section, but it is fully a mile 

distant from the type section. It is on the west wall of 

Pleasant Valley. At thispoint the Mancos shale is exposed 

at the base. (See fig. 2.) Above it is about 6 feet of the · 

~~- upper or pebble till, in which a nun1ber of striated stones were found. Overlying the pebble till is the San Juan tuff 

fully 200 feet thick. FIGURE 2.-Geologic section at locality No. 2 near b · ·· · · · · 

::':!;";.~:~ t.fi,~:~,~!:~:~:m•m- ~h~~:~~o::; :: ~:~~fi.~t©~ki~;*;:~Jtf;~:'l'$.~ bowlder till and the Telluride conglomerate. The Telluride 

;~:g:.:~r:~e ;~~~t ;,e:t:~fe:t ~~~h~:!: ~~e::e ';":n a J:":~ ;kfiJ.;iif!fil~~~:~fk.::·t?~i~:: tuff, which suggested that possibly a little of the Telluride ::- _- -_ ~- _- =a~ ::._ -. ~ : 

:was left there. The pebble till thickens toward the north ~----=----==-=====~~---=-=== 

and within a few rods in that direction it is underlain by 

bowlder till. FIGURE 3.-Geologic section at locality No.3 near 

Ridgway, Colo. a, Mancos shale; b, bowlder 

member of Ridgway till, 40 feet; c, pebble member of Ridgway till, 15 feet; d, obscured, 15-20 feet; 

e, Pleistocene glacial till of Durango stage. 

Locality No. 3 is nearly east of No. 2, about threequarters of a mile south of Ridgway, · on the west side of 

the Uncompahgre Valley. Here a thickness of 40 feet of 

·the yellow bowlder till overlies the Mancos shale and is in turn o:er~ain by 15 feet of. the 

pebble till. (See fig. 3.) The immediate . covering of the pebble till IS obscured. Possibly 

some San Juan tuff is present at that horizon, but there are no satisfactory outcrops. The 

surface of the hill or ridge is mantled with Pleistocene till of the Durango stage. 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 95 PLATE Il l 

A. DETAILED VIEW OF THE BOWLDER MEMBER OF THE RIDGWAY TILL AT THE TYPE 

LOCALITY, 1 MILE WEST OF RIDGWAY, COLO. 

B. STRIATED STONES FROM THE RIDGWAY TILL AND TELLURIDE CONGLOMERATE. 

Th e seco nd stone from th e ri ght ca me fro m t he Te ll uride conglome rate . 

EOCENE GLACIAL DEPOSITS IN SOUTHWESTERN COLORADO. 19 

Locality No. 4 is about half a mile west of No. 1. The Mancos shale is exposed at the base. 

(See fig. 4.) No bowlder till is present, but there remains at least 30 feet of the pebble till, with 

its usual characteristics. Telluride .conglomerate is lacking, and the pebble till is overlain by the San Juan tuff, 

which reaches to the surface of the mesa. 

Locality No.5 is on the west side of ~ller Mesa, southwest of No. 4. At this point the Mancos shale is overlain 

-•; ~ :' ;' .o ~ ~?? !' ~ .o .o .o.r:: ~ ~ o~o,o.o. o.o:'? ,o-;.o.o. 

~ ~ ~ ?· ~ '? '! ? ~ -~ ~ ~ ~ ~ ? ? ? <!" ~ ? ~ .o ~ ~ ~ ~ c: 

immediately by Telluride conglomerate, and 

that in turn by San 

Juan tuff. (See fig. 5.) 

The section does not 

contain . the glacial formations, but their ab- FrGuRE 4.-Geologic section at locality No. 4 near 

sence at this point is Ridgway, Colo. a, Mancos shale; b, pebble memsignificant and will be ber of Ridgway till; 30 feet; c, San Juan tuff, 90 

feet. discussed . elsewhere 

(p. 23). The Telluride conglomerate at this locality contains striated stones. 

Locality No. 6 is on the west side of Miller Mesa, due 

west of No. 2 and just across a small valley from No. 5. FIGURE 5.-Geologic section at locality No.5 near 

Ridgway, Colo. a, Mancos shale; b; Telluride It is in a small outlier from the main part of the mesa. 

conglomerate, containing striated pebbles; c, san Here the type section is repeated, with the exception of the 

Juan tuff . 

upper capping of San Juan tuff. (See fig. 6.) The M~ncos 

shale is at the base and is succeeded by 30 feet of bowlder till overlain by 20, feet of pebble 

till, and this in turn by 40 feet of the Telluride conglomerate. 

At this locality the bowlder till contains a great deal of 

waterworn material. It is quite possible that the deposit 

represents a local pocket in the older of these Eocene till 

sheets, where water had much to do with the deposition of 

the material. At this locality no striated stones have yet 

been found in this lower member. In the overlying pebble 

till striated stones have been found, and a few such stones 

have also been found in the Telluride conglomerate, which 

0 0 0 o o Q o o e> o o co 0 ° o o- o d o o 0 0 00 

.. . , .. .. . ... ... .. .. ... ...... .... ..... . 

overlies the glacial deposits. 

At locality No. 7, 

at the rim of the mesa, 

------ --·-------

FIGURE 6.-Geologic section at locality No.6 near 

Ridgway, Colo. a, Mancos shale; b, bowlder 

member of Ridgway till, 30 feet; c, pebble member of Ridgway till, 20 feet; d, Telluride conglomerate, with striated pebbles, 40 feet. 

•o ooo o ooo Coo Ooc:>o oo oo o oooo co ~o 

• • • • •••• ••• 0 • • •• •• •• • • 0 ••••••• 

- ... : ~ ~ 00 .0. 

0

: ~ :=>. 0. :=- 0 0 • ~ ~ 7' ~ 0 0. 0. ~ •• •• 0 0 0. ~ : : 

• :='. 0 ••• 0-=- ~ 0. 0 :=- .'"" .0 .. 0 ~ ~ .0 .0".0: •. 0 . 0 . 0::.. c.. ~ 0 .0. c:: c: 0. 

~ ·.· ·: ~.: ·: :.: : : : · .. -·~ ·~ : .... b 0: ·: ~· ~- ~· ~· : :· : ·: • • .-..... :~ ............................ 

•.o~:=~ o.o.-=: c:-: •o• o• ~ ~ c: o.o: ~ ~0.~ .. ~ c:: • •c.•o~~ 

•••••o •• ••.••o ••• v••o•<> <>o ooooo•o ...... "' .......... .. . . ....... . •e>oooooooo•t~Ooooo coo O•ooo 

·.·0 .. ~ .-· ~ :. ~. •. ·._ ~ ~ 0 .. 0. •• ~ . ... ~ > . : .·· .. ··...... _: . -~ _: ~ ._·. •••o•ocoQoo ·oooo ·oooooc..oo•o oooo 

there is an excellent exposure of the San Juan tuff, and 

below that fully 300 feet of Telluride conglomerate, in 

which striated stones have been found. Below the Telluride is · the Mancos shale. (See fig. 7.) This section, 

therefore, resembles that at locality No. 5. It is without 

glacial formations, but very near to the localities where 

the glacial material remains. 

Locality No. 8 is about a mile south of No. 7, just 

south of an east-west dike, which causes the road to make 

a distinct bend toward the west at that point. At this FIGURE 7.-Geologic section at locality No. 7 near 

Ridgway, Colo. a, Mancos shale; b, Telluride con- locality the entire type section is repeated. (See fig. 8.) 

glomerate, with striated pebbles; c, San Juan tuff. The Mancos shale is exposed at the base, followed by 

the bowlder till with its usual characteristics, the pebble till, the Telluride conglomerate, 

and the San Juan tuff. 
20 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

Locality No. 9 is about a mile south of No. 8, in a prominent hill a short distance west of 

Beaver Creek. This hill is not to-day a part of Miller Mesa, and the glacial material forms its 

;I;t:~~Jj;!Ilf:_~i;~i~l~lij] ~-~~.-.-.~.-.~.~-~.:7.~·~.~-~~----~ 

~~rt:~K~:~~-:~:~: 

capping. (See fig. 9.) In this respect this locality differs 

from all others, for the deposit which is interpreted to be 

Eocene till is not buried. Its physical and lithologic characteristics are, however, identical with those of the bowlder 

till in the lower part and the pebble till in the upper 

part. It is with confidence interpreted as a remnant of 

the Eocene till, from· which the covering of Telluride conglomerate and San Juan tuff has somewhat recently been 

removed. It might be 

asserted that this particular hill may have 

been overridden by the 

earliest of the Pleistocene glaciers, but in 

reply it may be urged 

FIGURE 8.-Geologic section at locality No.8 near Ridg- that the composition 

way, Colo. a, Mancos shale; b, bowlder member of 

Ridgway till; c, pebble member of Ridgway till; d, of this till and the fact 

Telluride conglomerate; e, San Juan tuff. that it has two distinct members make it quite distinct from the deposits of FIGURE 9.-Geologic section at locality No.9 near Ridgway, Colo. a, Mancos shale; b, bowlder 

the Cerro stage. The absence of the g.~eat latite bowlders member of Ridgway till; c, pebble member· of 

which characterize the glacial deposits of the Cerro stage Ridgway till. 

would seem to put it in another geologic epoch. If the interpretation is correct, the latite 

flows which contributed so largely to the Pleistocene glaciers were not present in the San 

Juan Mountains at the time the Eocene till was gathered. The intermediate and latest Pleistocene glaciers did not reach the summit of this hill. 

Locality No. 10 is about 4 miles northeast of No. 1. It is on the southeastern rim of Log 

Hill Mesa, a portion of the great Uncompahgre Plateau. The exposure is on the face of the 

g cliff bordering the Uncompahgre Valley, about a mile 

;.;p~:=:::.· . .;..:v··.;~:-;.>·.,. r-.;." v:,..·.p ·,.,:P.... o • .,g north of the Dallas railway station. Upon the surface of r.fl ·v·tl "fl" •·,.)·O. p·~·,.,•p' p·"'·D,.. 0 L Hill M • h" • • • • h" l f 1 • 1 /''c-~-~;."':.p :.e·P:·P.p._,.··~·!.,;,.,~;,:fl.',!:~~.~; og esa m t IS vwmity Is at m manteo g aCia ·"~b·V·J., .,·.P ;l.·.f' .. "!.·P,<t,~'Ji"'P,P'p~• drif h" h d • • ~P·J~·~."',..·v .r.,.._,..;u:;t:,_ -;::~:;·;,.·,.'';·...-::.,.::::. t w IC was eposited durmg the Durango stage of the • ~ t? • &1 fJ P' ~ r . V' • V" V. p P P'. p. 17. y • ""V • P J? . V• • 

~~:~:·~::·(':'·.·~ < <A_:t(c:..:~?~.~:":::c:·-:-.. :~"·~7~: Plmstocene epoch. The formation is in a small hill or knob :::: ~:. ·.:~~-~~~-~~.~_??.~e:~~:"-~".:.:~:·?:? . .;.::-;·?.~~·?·"':o: on the mesa (see fig. 10 and Pl. IV), and the section ex-

_: :~:-~~ ~-:~:~~~~·-.:~:~:yj+?~~ posed ther~ includes, from the top downward, the follow- . o 0 0 · · 0 0o .o . · 0 o. ~ o.. Ing formatiOns· .. · .. · .o · "· ·.· 'a.".o:O.:. :.0

8 :.~p . Section at locality No. 10. 

~~-=-= SanJuantuff ....................................... . 

- . ~:~~e!em::~::::;~d;;.;~-iijj_"_"_-_:::::::::::::::::: . . . . . . . . . . . . . _ . Mancos shale ....................................... . 

. :. : .. · .. · .. ·. : .. · . .' .. ·. · .·. ·. ·. ·. ·: ·: ·: ·:·: ·:.:. :·:·.': Dakota sandstone. 

Feet. 

80-90 

4-10 

40 

70-80 

~·~~~~-~~~~~ McElmo formation. 

· · . . . . . . . · .... ·. ·. ·. ·.'. ·. ·. ·. This exposure is of special interest, for it is the most . . ~· . . . . . . . . . . . . ---- ---~~~.:__::: northern locality -at which the Ridgway till has been found 

~ ·_·_·._· _· ·.:.._·_·_·_·_· ~~.:__· ·_· _· ~~ and is the one locality not immediately associated with 

· · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · Miller Mesa where this older glacial drift is known to ocFIGURE lO.-Geologicsection at locality No.lO near cur. The preservation of the older drift at this place is 

Ridgway, Colo. a, McElmo formation; b, Dakota readily accounted for by the heavy mantle of San Juan tuff 

sandstone; c, Mancos shale; d, bowlder member of and Telluride conglomerate. The exposures on the cliff 

Ridgway till; e, Telluride conglomerate; [, San 

Juan tuff; g, glacial bowlders of the Pleistocene were not altogether satisfactory, and it was found neeDurango till. essary to clear away with pick and shovel the loose material throughout the section described, so as to prove beyond doubt .the presence of a layer of 

till beneath the Telluride conglomerate at this locality. The Ridgway till here, as at many 
U. S. GEOLOGICAL SURVEY PROFESSIONAL PAPER 95 PLATE IV 

A . SECTION IN BOWL DER MEMBER OF RIDGWAY 

TILL AT LOCALITY NO. 10 NEAR RIDGWAY, COLO. 

The t il l immed iat e ly underlies the Te lluri de co nglome rat e. 

Dista nce betwee n wh ite marks is about 8 feet. 

B. SECTION IN BOWLDER MEMBER OF RIDGWAY TILL AT LOCALITY NO. 10 NEAR 

RIDGW AY, COLO. 

All loose material was removed and g laci ated bowlde rs exposed . 

EOCENE GLACIAL DEPOSITS IN SOUTHWESTERN COLORADO. 21 

other localities, differs from the Pleistocene drift of the region in not including any material 

contributed from the later Tertiary volcanic rocks of the San tJuan Mountains. It contains 

representatives from the earlier volcanic rocks and an abundance of bowlders from the sedimentary beds. The section . where the loose surface material was cleared away did not show 

any of the pebble till 

SOURCES OF THE RIDGWA~ TILL. 

Lower or bowlder member.-The composition of the lower or bowlder till member makes it 

perfectly clear that this material was gathered in the San Juan Mountains, south and southeast 

of the locality where it is now exposed. The stones in the drift are just such as are exposed in 

the core of the range. The large tuff bowlders resemble the tuff bowlders in the San Juan tuff, 

· but it is apparent that they came from a volcanic formation which was much earlier than the 

San Juan tuff and which yielded many of the . bowlders contained in that formation. It is 

evident from other studies carried on by Cross that there was a period of volcanism earlier than 

that which began with the formation of the San Juan tuff. 

Upper or pebble member.-The source of the upper or pebble till member is not so clear. 

If it came from the San Juan Mountains, it is very stra,nge that it does not also contain large 

bowlders, or at least an abundance of stony material. If the ice which brought the pebble till 

moved far over a surface mantled by the bowlder till, it would seem that it should have gathered 

a great deal of stony material. The source of the pebble till, as exposed in these localities, 

seems to have been chiefly the Mancos shale. The area in which the Mancos shale was exposed 

at the surface during Eocene time was to the north or northeast of Ridgway, in the direction 

of Vernal Mesa and the West Elk Mountains. If ice had come from that direction to the 

vicinity of Ridgway, it would have traveled the last 30 or 40 miles of its course over a surface 

of Mancos shale, and as most of the glacial drift of any one locality is usually of local origin, that 

would account for the character of the material in the pebble till. The contrast between the 

bowlder till and the pebble till suggests distinct glaciers, either from a single center at distinct times in the history of glaciation or from distinct centers. It appears most likely that 

the earliest Eocene ice which invaded the Ridgway territory must have come from the south, 

and that later, after that ice had retreated, and perhaps disappeared, and after a long period of 

weathering and erosion had elapsed, other ice invaded this same region from another direction. 

Possibly this other direction was the northeast, the ice coming from the region of the West Elk 

Mountains. 

AGE OF THE RIDGWAY TILL. 

The position of the Ridgway till upon the Mancos shale makes it evident that it is late 

Cretaceous or· younger, but the Mesaverde formation overlay the Mancos shale at the time of the 

first uplifting of this mountain area, at the close of the Cretaceous period. The Ridgway till 

was therefore not deposited until after the close of Cretaceous time, and not until the Mesave~de formation had been entirely eroded from the vicinity of Ridgway. 

The Telluride conglomerate, which rests upon the Ridgway till at several localities, has 

been referred by Cross 1 to the Eocene. It therefore appears that the Ridgway till is of early 

or mid-Eocene age. 

EOCENE GLACIERS. 

The facts thus far presented lead to the inference that the earlier Eocene ice may have been 

of an alpine type and descended from an early generation of San Juan Mountains, bringing vast 

quantities of coarse, stony material to the foothills of a range of moll?:tains that has long ago 

disappeared. If the pebble till was brought by ice from the north and northeast, and if that 

ice formed in mountains, it must have come a distance of fully 40 miles and, in early Eocene 

time, probably moved over a surface of slight relief. Such ice could not have been a true 

mountain or alpine glacier. It would suggest the presence of a piedmont glacier, or possibly 

a local ice sheet, in Colorado in early Tertiary time. 

1 Cross, Whitman, U.S. Geol. Survey Geol. Atlas, Telluride folio (No. 57), 1899. 
22 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

PROBABLE FURTHER EXTENSION OF THE EOCENE TILL. 

The distribution of these Eocene glacial deposits at the northwest base of the San Juan 

Mountains suggests the possibility that other remnants of the Ridgway till may be found at 

other localities about the base of the range. Furthermore, if the mountains which existed in 

early Tertiary time in the San Juan region contained glaciers, it is more than likely that other 

mountain areas of early Tertiary time, in Colorado and throughout the Cordilleran province of 

North America, also contained glaciers. If the suggestion of a somewhat distant origin for the 

ice which deposited the pebble till is correct, it would seem that the Eocene glaciers in the 

Cordilleran province may have been more extensive than the Pleistocene glaciers of that region 

and possibly had dimensions commensurate with those of local ice sheets. Certainly many of 

the early Tertiary conglomerates of the western portion of the continent should be examined 

to determine whether they may not be of glacial origin, and the discovery of glacial deposits of 

Eocene age in the western mountain areas may henceforth be expected. It seems reasonable 

to suggest that if there were such extensive glaciers in the Cordilleran province of North America 

in early Eocene time, there may have been glaciers on lower lands within the continent at the 

same time, the conditions thus simulating those of the Pleistocene epoch. 

SUMMARY OF AVAILABLE FACTS PERTAINING TO THE RIDGWAY TILL. 

It is appropriate now to sum up the characteristics of the Eocene glacial deposit as they 

have been determined from the various localities,described and to present a brief statement of 

the available facts pertaining to this formation. 

1. The Ridgway till is divisible into two members. 

2. The lower member is distinctly a bowlder till. 

3. This lower member appears to have been deeply weathered, perhaps weathered throughout its maximum 

known thickness of 90 feet. 

4. The lower member contains an abundance of striated stones. 

5. Wherever it has been found this lower member rests upon Mancos shale. 

6. The stones in the lower or bowlder till have certainly come from the San Juan Mountain area to the south. 

7. The upper member is appropriately described as a pebble till. 

8. It appears to have been subject to a much shorter period of surface weathering. 

9. Most of the upper till sheet is clay, which was probably derived from areas where the Mancos shale was exposed 

_at the smface. 

10. At certain localities the lower member, or bowlder till, is missing and the pebble till is present. 

11. There was probably an erosion period after the deposition of the bowlder till and before the deposition of the 

pebble till. 

12. At certain localities the pebble till is missing and the bowlder till is present. 

13. At most localities the Telluride conglomerate overlies these glacial deposits, but in places it is absent and the 

overlying formation is the San Juan tuff. 

14. At one locality (No. 9) the bowlder and pebble till sheets are present, but not the Telluride conglomerate nor 

the San Juan tuff. At this locality the pebble till is at the summit of the hill. 

15. Striated stones have been found in the pebble till. 

16. Striated stones, presumably from the Ridgway till, have been found in the overlying Telluride conglomerate 

at several localities. · 

17. At some localities within the area bounded by the exposures of the Ridgway till the Telluride conglomerate 

rests upon the Mancos shale. At these localities the Telluride conglomerate has been found to contain striated stones. 

18. The exposures described are within an area 8 miles long in a northeasterly direction and 2! miles wide in a 

northwesterly direction. The areal extent of the region within which the deposits have thus far been found is about 

20 square miles. , 

19. All but one of the exposures are about the margin of the Miller Mesa, and it is quite probable that there may 

be somewhat extensive remnants of the Ridgway till buried within the mesa. · 

20. The surface of the Mancos shale beneath the till is too loose and soft to have received or retained glacial strire. 

GEOGRAPHIC HISTORY SINCE THE CLOSE OF THE _ MESOZOIC ERA. 

With the discovery of evidence of early Eocene glaciation in the San Juan district, it is 

of some interest to review the geographic history of this region. 

At the close of the Mesozoic era and the opening of the Cenozoic era there were mountainmaking movements which affected the entire Rocky Mountain province of North America, 
I 

EOCENE GLACIAL DEPOSITS IT SOUTHWESTER:If COLORADO. 28 

and the great dome which was then formed -l the San Juan area was at once subjected to 

vigorous erosion. As the mountain mass rose rosion began, and as the great dome was more 

and more deeply dissected a mountain topo~f phy must have been f:<1 

produced, and those mountains may be thought of ~s the first g~rera- g ~ · <J> 

tion of the San Juan range. (See fig. 11.) Dunng the perwd of ~ ~ 

mountain growth there was some volcanism. any porphyritic ]ntru- ~ ~ 

I ~· 1:::1 sions and the deposition of the great volcanic tuffs which made con- ~"' ~- 1-0{/Q 

tributions to the Ridgway till date back to t i period. The Epcene o/ ~ 

till indicates that during the dissection of thes 1 early San Jua~ Mj oun- ~ ~ tains ice for:rped in the _ range and descendei !l to the bordermg low- ~ ;· 

lands. Possibly ice formed in neighboring ra ges and approljlched ~ ~ 

the San Juan Mountains, and possibly therre were distinct g[acial 8. ~ 

epoc~s in that period of glaciation. ~ ~ After the retreat and disappearance of he early· Tertiary ice, 2 g 

stream erosion continued, and the western p tion of the San 'Juan ~~· ~­

Mountain area was reduced to a surface of sli~Jit relief which mfy be ~ g 

thought of as a peneplain. 1 This peneplain H~rdered on the west a e:."d 

higher area of mountainous character, whichl supplied the ma~eri~l j. j. 

for the Telluride conglomerate. The depositio~ of gravels upon this g ~ 

peneplain surface was probably due to some uw[

1

.1 t and rejuvenadon of ~[ the streams in the eastern portion of the rang . After the deposition ~ ~ 

of the Telluride conglomerate there was furtH~ 1 erosion in the r

1

ange, ~- ~ 

a~d then ca. me the th~ee. g~eat epochs of volca ~s m, the Sa~ Jual, the l ~ 

Silverton, and the Potosi. Dunng these epoclif, of volcanism a great ~"' g 

-volcanic plateau was developed. By this time I he Miocene epoch had ~ ~ 

been reached and possibly passed, and with tlie quieting down of vol- ~ ~ 

~anic activity began the erosion and dissection 6f the volcanic plateau. · '! § 

During this per~od of dissectio.n another g heration of San !Juan i§. 

Mountains were carved, this time out of volca i debris and great lava ~ ~~ 

flows. (See fig. 11.) ~ f; g 

Recent physiographic studies of the range have led to the recog- ~ g 

nition of a summit peneplain in the San Juan egion (see fig. 11), so ~ 1t 

it would appear that the first dissection stop · d at a plain which is ~ j: 

now represented by many of the summit areas ithin the region. I The- ~ {/Q 

San Juan Mountains that were first carved o t of this great volcanic ~i ~ platea:u should then be thought of as surmo I ting those of td-day. ~ ~ 

Perhaps, if replaced, they would rise 3,000 <Dt 4,000 feet abo~e the ~'! 

presen~ summits. (S~e fig. 11.) . . j § ~-

With the redoming of the area, whiCh m olved the warping or ~~~· 

doming of the summit peneplain, another cyclJ of erosion was begun. ct ~ 

·V aijeys were again formed, and in these vallej~ snows collected ~hich ~ ~ 

in time formed glaciers that advanced to the irwlands bordering the ~- ~ 

range. These e~rliest Pleistoc~ne glaciers re [eated and disappeared. ~ §' 

The range continued to be uphfted, and the ~t\reams were so juve- ~- ~ 

nated that they cut great canyons below the road troughs occupied ~ ~ 

by the Cerro glaciets. Again climatic change fav.ored the forniation ~ [ 

of ice among . the summits, and that ice Ctihe Durango glaciers) a g 

descended thro~gh the main canyons to e foothills and I later ~ ~­

r. etreated and dis. appeared. The canyons we 1 still more deeply cut s· 5"' 

1 a-~ mto the mountain mass, and then climatic onditions favorable for ~- § 

glaciation once more returned and the Wise n-sin or third ~eries of ~ ~ z 

Pleistocene glaciers formed and descended thr :-ili_gh the great canyons, r:~ 

nearly as far as those of the Durango stage. f hese glaciers hat e now disappeared, and there 

1 Cross, Whitman, U.S. Geol. Survey 1eol. Atlas, Silverton folio (:N-o. 120), 1905. -
24 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

is no true glacier ice remaining in the region to-day, but the streams are vigorously dissecting the mountain mass to still greater depths. The vigor of that work is illustrated in many 

a sharp, V-shaped notch cut below the depth of ice action. The debris taken from the mountain 

area is being distributed along the great valleys leading away from the range. 

The ice gouging in the three successive Pleistocene stages and the vigorous stream work 

during the interglacial intervals and since the last melting away of the 'ice suggest somewhat 

continuous mountain growth in this region during late geologic time. 

OTHER GLACIAL EVIDENCE. 

Before closing this report it may be appropriate to refer briefly to other glacial deposits 

made at different times in the history of the earth. The accompanying table presents some 

data as to these deposits and indicates that glacial conditions have existed at many times and 

at very many places on the earth. The records of pre-Cambrian glaciation are found in 

Norway and Canada and perhaps indicate early Huronian or Proterozoic age, though probably 

not exactly the same age in the two countries. The glacial deposits recorded from China are 

known to be either Lower Cambrian or pre-Cambrian, for Cambrian strata immediately overlie 

the glacial till. There is some reason for believing that the early deposits of Australia may be 

pre-Cambrian. Some doubtful evidence of glaciation is afforded by the Devonian of England, 

but the records of Devonian glaciers in South Africa are more certain. The record of late 

Carboniferous (Permian) glaciation comes from South Africa, Australia, Tasmania, South 

America, India, the East Falkland Islands, England, Germany, and the Boston Basin in Massachusetts. In New Zealand there is evidence of glacial ice action during Triassic time. Doubtful indications of Cretaceous and Eocene glaciation come from Italy, and the record herein 

reported is derived from a study of the Eocene till in southwestern Colorado. The Icelandic 

glacial records are of Miocene age, and there are Miocene deposits in Italy which may be of 

glacial origin. The Pleistocene record is found in all the high mountains of the world, in great 

lowland areas in North America and Europe, and in the Arctic and Antarctic regions. Since 

Pleistocene time glacial conditions have continued in many·parts of the world and are still 

present at high altitudes and in high latitudes. 

An inspection of the table should also bring out with some emphasis the occurrence of 

glacial epochs at the opening of each of the grea~ eras of geologic time. Possibly the preCambrian records are separated by periods of time as long as the Paleozoic or Mesozoic eras, 

which intervened between later records of ice action. This distribution of glacial records in the 

history of the earth appears to have some real significance and recalls the profound studies of 

T. C. Chamberlin on the causes of glacial climates. Each great geologic era was closed by a 

great physical revolution, which brought vast areas above the sea and therefore subjected those 

areas to the processes of weathering and erosion. By such revolutions the areal extent of the 

seas was reduced and the distribution of lands and seas changed. These changes have undoubtedly been great factors in determining the climates of the earth. They have affected 

the movements of ocean currents and the distribution of precipitation and of different temperatures. Possibly these great physical changes have had so profound an influence upon the 

climates of the different portions of the earth that they have been fundamental causes of the 

great glacial epochs. The lesser glacial epochs may have been associated with periods in which 

lofty mountains were being made. The glacial records surely suggest a field of most signjficant 

research. 
, 'I 

I 

EOCENE GLACIAL DEPOSITS IN SOUTHWES T'ERN COLORADO. 25 I 

Geologic and geographic distribution of reported evide1 ce of;glacial deposits. 

---------------------------------------------~-----+---~~--------------~------· 

I 

I 

·Cenozoic. 

:Mesozoic. 

.:Paleozoic. 

.:Proterozoic. 

Geologic time scale. 

Recent. 

;... 

~ 

ci 

e 

Q) 

~ & Pleistocene. 

P!iocene. 

Miocene. 

Oligocene. 

- Eocene. 

Cretaceous. 

Jurassic. 

Triassic. 

Permian. • 

Pennsylvanian. 

' 'Mid -Carboniferous.'' 

Mississippian. I 

Devonian. 

Silurian. 

I 

I 

i 

I 

t 

t 

' 

. !I 

. Geographic distribution. 

I 

High la~itud s. High al itudes. 

I 

N orthem North America. 

Northwestern Europe. 

High la itudes. 

High al itud~s. 

Iceland a I Italy.b 

Italy. c 

Southwestenl Colorado.d 

Italy.b I • 

Italy.b I 

I 

New Zealand. e 

I 

South 1 frica.f 

South 1 ustrailia.g 

West A 1stral~a. h 

Tasman·a. i 

South 1 merica. j 

India.k j 

East Fa klan1d Islands. l 

Boston Basin, Mass.m 

Germary.n 

Englan o 

' 

~ 

'II I ------ ------4----~---

0klaho na.P I 

Doubt£ 1 evidence from England.q 

South 1 fricalT 

Canada s 

- ---+---- ---------------·--- -----~----

1 

Ordovician. • I 

---------------------------- ------ -------+---r--------------- ---~--~---------

Cambrian. China. I 

Austral a.u 1 

Canada v 

:Norwa;) .w 

Footnotes on page 26. 

I 

! 

' 

I 
26 SHORTER CONTRIBUTIONS TO GENERAL GEOLOGY, 1915. 

NOTES. 

a Pjetursson, Helgi, Om nogle glaciale og interglaciale Vulkauer paa Island: K. danske vidensk. Selskabs Forh., No. 4, 1904. 

FergUson, H. C., Tertiary and recent glaciation of an Icelandic valley: Jour. Geology, vol. 14, p. 122, 1906. 

b Schardt, Hans, Etudes geologiques sur le pays d'Enhaut vaudois: Soc. vaudoise Bull., vol. 20, No. 90. pp. 1-183, 1884. 

c Mazzuoli, L., Sui modo di formazione dei conglomerate miocenici dell' Appennino ligure: Com. geol. ital. Boll., 2d ser., vol. 9 (vol. 19 on outer 

cover), pp. 9-30, 1888. 

d The present paper. 

eScott, W. B., An introduction to geology, 2d ed., p. 667, 1907. 

/White, C. D., Carboniferous glaciation in the southern and eastern hemispheres: Am. Geologist, vol. 13, pp. 299-332, 1889. 

Rogers, A. W., Geology of Cape Colony; pp. 147-179, 1905. 

Rogers, A. W., and Schwarz, E. H. L., The Orange River ground moraine: Philos. Soc. South Africa Trans., vol. 11, pt. 2, pp. 113-120, 1900. 

Rogers, A. W., On a glacial conglomerate in the Table Mountain sandstone: Philos. Soc. South Africa Trans., vol. 11, pt. 4, pp. 236-242, 1902. 

Corstorphine, G. S., A former ice age in South Africa: Scottish Geog. Mag., vol. 17, pp. 57-74, 1901. 

Molengraaf, G. A. F., Geologie de la Republique Sud-Africaine: Soc. geol. France Bull., 4th ser., No.1, p. 79, 1901. 

u David, T. W. E., Glacial action in Australia in Permo-Carboniferous time: Geol. Soc. London Quart. Jour., vol. 52, pp. 289-301, 1896. 

David, T. W. E., Some problems of Australian glaciations: Australasian Assoc. Adv. Sci. Rept. 11th Meeting, 1907, pp. 457-461, 1908. 

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