Bulletin No. 265 M / A' Economic ^ology, 59 

B, Tescriptive Geology, 69

DEPARTMENT OF THE INTERIOR

UNITED STATES GEOLOGICAL SURVEY

CHARLES D. WALCOTT, DIKECTOK

GEOLOGY OF THE BOULDER DISTRICT,

COLORADO

BY

M.

WASHINGTON

GOVERNMENT PRINTING OFFICE

1905

CONTENTS.

Page

Letter of transmittal .................................................... 7

Introduction ................................................ '............ 9

Acknowledgments -._.-....-.__._..---_--.__.--_.----_.____-.-.--_-_- 10

Physiography -.-'.._.._ _._..__..____._._'___.._._..._._._..-..__..._-..__.- 11

Topography of the area of crystalline rocks -.--------.'---.---.------- 11

Foothills ............................................................ 12

MonoclinaTridges.................................... ---------- 12

Longitudinal valleys.---_-.-.----..--.-.----.--..---.---...--.-.... 13

Mesas................................................................ 13

Slope and height. ---.. ---_.------__----.---..-------------------- 13

Structure and covering ............................................ 14

Origin.......--...--..-.....---...-.. ..-.---..--......--......... 14

Former westward extension of strata...-.--.-.---.--..-......_.-.. 16

Drainage............................................................ 16

Adjustment to structure . .........-..-....-..--..-..-..--........ 17

Examples of stream capture...................................... 17

Natural lake basins.......'...-..--..-..-------.-....-..-.-.--.-..-.... 19

Stratigraphy -...-...........................-.....-..-.'.---.--.-.-..-..- .20

Archean system ................................................. 20

Sedimentary rocks..._...........-...-....---..--.-.-..-- ..--.. ........ 21

Algonkian system .............................. .................. 21

Quartzite of South Boulder Canyon ......................;... 21

Triassic system ................................................... 22

Fountain sandstone............................................ 22

Lithologic character.-..---..-.......---.....,............ 22

Structure........................................ ........ 23

Lyons sandstone.....-.--.--.-.......--......-.--..-..--....- 23

Character and coloring ............................ ...... 23

Cross-bedding ........................................... 24

Lykins formation......-....--:..--..-... . ...1............... 24

Character and thickness ---..-..-.._-.----..-...-..--.... 24

'' Crinkled'' sandstone ....-..--....-....-..-.--..----..._ 25

Jurassic system ................................................. 26

Morrison formation.......................................... 26

Limits ................................................... 26

Generalized section ....... ..^.................... ........ 26

Cretaceous system......................................... ...... 28

Dakota formation...................... ........-....---.---.. 28

Basal conglomerate ....... .----......--.-.----.-...-..----. 28

Character of the sandstone................... ............. 28

Columnar jointing....................................... 29

3 '
4 CONTENTS.

Stratigraphy Continued. - Page. 

Sedimentary rocks Continued. 

Cretaceous system Continued.

Benton formation.-.--.-..-_.___-._.____.._._.._-._._.._.._.. 29

Shales and limestone. _-._-_--...-_.....-_.-.---.--..-._.. 29

Upper beds and Mobrara contact ......................... 29

Niobrara formation............................................ 30

Basal limestone.-..--...----.--.....-.-.-...---..-.-.---.. 30

Shales................................................... 30

Pierre formation ..,.--..-.--....-.........----...-.-......-. 31

Color....................-.......-...-.--....---.---...-. 31

Limestone beds -............--..-..-..--.-..--....... ..... 31

Hygiene sandstone member ............................... 31

Fox Hills formation .......................................... 33

Laramie formation .... ............-..-....-.-..--.-.-..... 33

Basal sandstones..--.---..-----.--.--..-...- ............ 34

Beds associated with the coal............................. 34

Quaternary deposits .............................................. 34

Igneous rocks.... ........................i.............................. 35

Rocks of the Archean area .--.---.-..-..-.....-........--.-...... 35

Intrusives of the foothills .............. .^..............^.......... 36

Sunshine dike ................... ............................ 36

Flagstaff sheet .............................................. 36

Plumely Canyon sheet..........................:............ 38

Altona sheet..-.--..-.....-.-.....-..,.-.....-.---........... 38

Intrusive rocks in the Dakota and Benton ..................... 38

Intrusive rocks in the Niobrara and Pierre.. ...-.--..--.--... 39

Differences in form of intrusions .............................. 39

Valmont dike .................................................... 40

Structure...-..-.-..-.--.--.-.---.-.--.--....--..----_......-....-.-..-.-. 41

Foothill monocline .......-...--.-..-..-.-..-..-.--..-.....-.....-...- 41

Echelon folds ..................'.......-.......-..........--...-.-.... 43

Near South Boulder Canyon....................................... 43

South of Fourmile Canyon ....................................... 43

Section 25, 6 miles north of Boulder .............................. 44

Fold in the Hygiene sandstone opposite Sixrnile Gulch ............ 45

Table Mesa fold ..... ............................................ 45.

Faults and landslips ................................................. 46

South Boulder fault................................................ 46

Green Mountain fault............................................. 46

Fourmile fault...................................... ..j.-.--....... 47

Fault in section 25 ..............-..-.........--.--..--.--..-..-... 47

Dip faults in the foothills .......................................... 48

Pole Canyon slip fault .......................................... 48

Huggins Park .................................................... 49

Foothill overturns -....-..........-.-.....-........-.---...-....- 50

Slip of the Lykins in Pole Canyon ............-............'....... 51

Faults of the plains area ........................................... 53

Geologic history .................................:...................'.... 54

Pre-Fountain topography ............................................ 54

Shore erosion during subsidence...................................... 56

' Deposition of Fountain formation .................. .................. 57
CONTENTS. 5

Geologic history Continued. Page.

Deposition of Lyons sandstone ...-----.......--....-..----..---------- 58

Bays and bars .----.--.......-.--.--.-.-.-.--..-------.--.------.- . 58

Migration of bars ---...-....-.-...-..-.---......-...-------.----. 60

Lykins sedimentation ..................--.'-..-.- ...-.---.-.-..------.. 61

Lykins warping..-..-.-.-......... ..-.---.--........--._-------... .-....' 61

Uplift of the Boulder arch ................................... 61

Morrison sedimentation ...___.________._.___._____----.--_--.---.--._ 62

Dakota sedimentation........_.......................--.........-..... 62

Benton sedimentation and uplift ........... f ......J.................. 63

Niobrara sedimentation .......................... ..---- .---.--.-..-.. 63

Pierre sedimentation and folding .....................---..-.....-.-.-.. 63

Fox Hills sedimentation and Laramie uplift .......................... 64

Summary of history of the Boulder arch .............................. 64

Post-Laramie history ................................................... 65

Economic geology ............-.......-...--.....-....-..........--.-...--.- 67

. Water...........................-..........----...------ ........... 67

Surface zone.. .-----.'----...------------ ------------------------- 67

Deeper zone -..-.----...___.__.______..___.____....-..----------. 67

Flowing wells .........................--...^ .................... 67

Water in the Dakota............................................... ' 68

Building stone......--.............-_--. .............................. 69

Granite ................... .-...........-...............-'----..... 69

Red rock ........................................................ 69

Lyons sandstone...............--'.--.-..----....----.---.--...-.. 69

Red sandstone of the Lykins .....---.-.----......-...-...--.-.-.. 70

White sandstone of the Jurassic. ...............................'.. 71

Building stones from the Cretaceous..___.._,.._.__._...__.__.._.. 71

Grindstone .-....-.....--............---......'..-.- --..----...--.... 71

Lime.................................................................. 72

. Clays...............................................!____.. ..'........ 72

Opportunities for the brick industry................................ 72

Shale supply.-.--..--.-....... ...... .............................. 73

Processes of brick manufacture ...-------.-.._.................... 74

Quality of the brick............................................... 75

Bricks, of light color ............................................... 75

Coal................................................................... 76

Oil and gas ........................................................... 76

Surface indications --------..-----:---..........-.._............. 76

Oil-bearing area ..----'............................................ 77

Drilling in 'the Pierre ....................................... ...... 82

Reports received from well drillers ............................... 82

Oil-bearing strata..'............................... .............. 83

Shooting of wells ............................^.................... 84

Structure of the oil field...............;...... .1 .................. 87

Suggestions as to further development............................ 91

Sources of the oil ................................................. 94

Physical properties .........'...................................... 97

Production ........................................................ 97

Index ----..._...._....._.._._..1....................................I... 99
ILLUSTRATIONS.

Page. 

PLATE I. Topographic map of the Boulder district..--.. ..----.__----_-._ 9

II. Geologic map of the Boulder district _--_--._..._.-.-._...-.--.. 10

III. A, Mesas and foothills south of Boulder; B, View westward from

Green Mountain-.--_.-_.__.._-.._....-.-.____----. ....-..--.. 12

IV. A, Longitudinal valley in the foothills north of Boulder; B. Haystack Mountain, view southward from Table Mesa ........... 14

. V. A, View northward across mouth of Bear Canyon; B, Outcrops

of Niobrara shales --..-.-...-.-...............-..-.._-..----- 30

FIG. 1. Generalized east-west cross section, near Boulder .................. 12

2. Map showing dikes and intrusive sheets west of Boulder ........... 37

3. Map of sees. 25 and 36, T. 2 N., R. 71 W .......................... 44

4. Cross section of east slope of Flagstaff Mountain ....-..........--. 49

5. Lykins formation in Pole Canyon slipping away from the Lyons

sandstone..;............,........................-.......-...... 52

6. Position of cross-bedding of the Lyons sandstone.................. 58

7. Shore line during the deposition of the Lyons sandstone ........... 60

8. Section of a linear embankment retreating landward .............. 60

9. Map showing location of oil wells in the productive area ....'...... 78

10. Cross sections showing the folding of the Hygiene sandstone ...... 90

11. Hypothetical cracking in the formation of the oil-field monocline .. . 95
LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR, 

, UNITED STATES GEOLOGICAL SURVEY,

Washington, D. C., Novevnber IS, 1904-

SIR : I transmit herewith the manuscript of a report on the geology 

of the Boulder district, Colorado, by N. M. Fermeman, and recommend 

its publication as a bulletin. The report has grown out of a study of 

the Boulder oil field, begun by Doctor Fenneman in 1902, when this 

field first came into prominence. Two preliminary reports, giving 

the more important facts of economic interest bearing on the oil 

field, together with records of drilling and production, have been 

published in Bulletins 213 and 225, Contributions to Economic 

Geology, 1902 and 1903. The present report covers the general 

geology of the district, its physiography, stratigraphy, and structure, as well as its economic mineral resources, including water, stone, 

clay, coal, oil, and gas. It is a model discussion of a small but 

exceptionally interesting district, and will prove of both economic 

and scientific value.

Very respectfully,

C. W. HATES,

 Geologist in Charge of Geology. 

Hon., CHARLES D. WALCOTT,

Director United States Geological Survey.

U.S. GEOLOGICAL SURVEY BULLETIN NO. 265 PL. I

R. 70 W.

 :- .^.-.-. ' 1 - 

'mont Bijife /

R. 71 w. R. 7O W.

TOPOGRAPHIC MAP OF THE VICINITY OF BOULDER, COLORADO.

Compiled from U.S. Geological Survey Atlas Sheets.

19O5

01234 

Scale 

mile s
GEOLOGY OF THE BOULDER DISTRICT, COLORADO.

By N. M. FENNEMAN.

INTRODUCTION.

The area described in this report is a rectangle 16 miles north and

south by 9 miles east and west. The city of Boulder is situated

southwest of its center. The immediate occasion of the survey was 

the finding of petroleum, supposedly in paying quantities. The 

chief' exploitation for oil has been near the center of the area, from 

2 to 4 miles northeast of Boulder.

The study was extended southward beyond the village of Marshall, 

where the structure is more apparent than it is farther north. The 

northern limit of the area mapped was determined largely by the 

broad elevation known as " Table Mesa," whose mantle of gravel 

conceals all the older strata for several miles, and which has not 

been pierced by the drill. It was found necessary to extend the 

work to the north side of this mesa, where there are exposures that 

furnish essential data for the structural problems. The work was 

carried east far enough to include the excellent exposures of the 

" White Rocks " at the south end of Gunbarrel Hill; it was extended 

west beyond the stratified-rock foothills, which contain the chief 

evidences of the structure.

It was necessary to make a careful study of areas at considerable 

distances from the oil wells, because the .wells are located in the 

broad central portion of the district on the outcrop of the Pierre 

shales, which are in general so nonresistant and homogeneous that 

exposures of solid rock are extremely rare. Here and there in the 

fields, or on the side of an irrigating ditch, the presence of a thin 

harder bed may be indicated by a few rock fragments and a sandier 

soil, but generally the bed is not strong enough to cause even a small 

ridge, although a little excavation may sometimes reveal its position. 

Such indications and the inferences from the comparison of not very 

satisfactory well records are the only direct means by which the 

structure in the immediate vicinity of the oil wells can be determined.

It is otherwise on the borders of the area. On the Avest are fine 

exposures of the firm rocks of the foothills. On the south, in the

9
10 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

Marshall coal field, are the eroded folds and scarps of the Laramie 

sandstone. On the east the Fox Hills and Laramie sandstones offer 

good exposures. At the north end Haystack Mountain, the only 

remnant of a terrace higher than Table Mesa, affords an excellent 

opportunity to study the Pierre shales. At this end of the field 

even the Pierre contains one or more sandstone strata, which outcrop 

in definite ridges. The structure of these surrounding districts 

affords a good basis from which to infer the probable deformations 

of the strata in the oil territory. It is evident that the structural 

features so clearly revealed in the foothill belt correspond to those of 

the oil field. Although the structural features of the shales of the

011 field can be made out only by wearisome search, their correlatives 

in the foothills are open to the light of day. The practical significance of tfris relation will be evident to any who may be prospecting 

for similar deposits of oil along the front of the range.

The district here described was studied by the Hayden Survey, and 

its general features are discussed in the reports of that organization 

published in 1869 and 1872. The portion of the district south of 

Fourmile Canyon is included in the Denver basin, which is discussed 

in Monograph XXVII of the United States Geological Survey by 

Emmons, Eldridge, and Cross. The southern limit of the area 

mapped by the Fortieth Parallel Survey is a few miles north of this 

district, but the description of the major features of the stratigraphy 

of the east slope of the Front Range as given in the report of that 

organization is applicable to this field.

ACKNOWLEDGMENTS. I 

The preparation of the map accompanying this report was made J

possible by the excellent base map prepared by Mr. Hugh F. Watts,. j

of Boulder, a large part of the geological field work having been 1 

done before the publication of the survey atlas sheets. Mr. Watts' ! 

had also made a geologic map of a part of the region, a generalized > 

reproduction of which is given in this paper as fig. 2 (p. 37). His 

careful observations were useful at various places, and a few of them 

are given specific mention in footnotes. Judge Junius Henderson has { 

systematically collected fossils from the district and his cooperation j 

was found valuable. At th present stage of the study, however, it j 

was not deemed advisable to attempt a systematic treatment of fos- ; 

sils. Mr. William R. Rathvon, who has handled for the United Oil 

Company practically the entire product of the oil field, has generously furnished exact data regarding shipments, and has given much 

general information concerning the industrial phases of the subject. 

The plates used are from photographs by Prof. J. R. Brackett. Over 

half of them were taken expressly for this publication. Prof. R. D. :
en

0

ifg £j CD 

[fi
PENNEMAN.] PHYSIOGRAPHY. . .__ . ^ 11

George, of the University of Colorado, has been consulted on various 

questions; at the request of the writer he has studied several points 

in the field, an.d his conclusions have been received with much confidence. He has read the entire manuscript and his suggestions have 

been most helpful. The manuscript was subsequently submitted to 

Mr. George H. Eldridge, a student of oil fields, and one of the authors 

of the monograph on the Denver basin, who is familiar with much 

of the area and many of the features here described. Mr. Eldridge's 

criticisms were found especially pertinent, and on his suggestion certain changes were made.

PHYSIOGRAPHY.

The area may be divided into three north-south strips. A narrow 

margin on the west belongs to the rough and dissected granite plateau,

commonly considered a part of the Front Range of the Rocky Mountains. East of this is the narrow foothill belt, in which a descent of 

more than 1,000 feet is made to the plains, which form the eastern 

two-thirds of the area.

TOPOGRAPHY OF THE AREA OF CRYSTALLINE ROCKS.

In a broad sense the name Front Range includes all of the Archean 

strip west of the Great Plains and the foothill belt. If this part 

of the range is looked at in more detail, there is seen near the middle 

of the strip a definite north-south ridge, having its crest about 18 

miles west of the upturned strata of the foothills. West of Boulder 

this ridge reaches an elevation of from 12,000 to 14,000 feet, and 

has an average slope of from 500 to 1,000 feet in a mile. At approximately the 9,000-foot contour the slope changes. On the west is a 

relatively abrupt ridge, and on the east a much dissected plateau 

reaching to the foothills. (See PI. Ill, B, opposite p. 12.)

The belt between the restricted mountain ridge on the west and the 

foothills on the east is 10 to 12 miles wide. Its generalized eastward 

slope is about 250 feet per mile, or from one-fourth to one-half that 

of the range proper. This strip is cut by narroAv valleys to a depth 

of over 1,000 fe'jt. The interstream areas are generally steep and 

narrow hills without flat tops, though some ridges continue for 

several miles with approximately level and unbroken crests. From 

any high point almost all the summits appear to be in the same plane. 

The range proper on the west and isolated lumps on the plateau rise 

with conspicuous abruptness, Sugar!oaf Mountain, the top of which 

is 1,000 feet above the general level, being a prominent example. 

The topography at once suggests ai. imperfectly base-leveled surface that has been deeply cut by streams whose erosive power has 

been increased by recent elevation.
12 GEOLOGY OF THE BOULDEE DISTRICT, COLORADO. [BULL. 265.

FOOTHILLS.

' MONOCLINAL RIDGES.

East of the granite are the red sandstones of the Fountain and

Lyons formations, which have an 

average dip of more than 50° and 

form a ridge rising 1,000 feet above 

the plains. The sandstones have 

apparently resisted weathering better than the crystalline rock, as 

streams from the west which have 

cut only narrow canyons through 

the sandstone ridge generally receive tributaries that flow in northsouth valleys in the granite just 

west of the contact^ At places (as 

.at South Boulder Peak) the ridge 

rises to several times its average 

height, and towers high above the 

plateau to the west.

To these red sandstones, from 

I 500 to 1,500 feet thick, is due a 

a i large part of the splendid scenery 

|B si of this exceptionally fine stretch of 

^ 'Z foothills. At some places the ridge

V O

= >,

£ B

:«.C 4)

] 2 has an even crest and resembles a 

. * Avail, while at other places its crags 

3 g run upward in a long line of spires. 

>.2 (See PI. Ill, A.) The bare red 

- *j front of a single crag has Somes' ® times an area of several acres. The 1*1

I outcrop here described may have

i one, tAvo, or even three distinct

1 crests. Thev are the westerly mem- o u \j

} bers of a series of monoclinal ridges

2 known as " the foothills." Alterp . nations of strong and weak strata, 

I upturned at steep angles, offer the 

3 necessary conditions for the carv- 

= ing of -these ridges and valleys.

1 The ridge-making outcrops, taken 

' in order from west to east, are com- 

.; ; posed of the Fountain, the Dakota,

2 and the Niobrara formations, the 

last named appearing as a prominent ridge only in the northern part of the area. The order of
U. S. GEOLOGICAL SURVEY BULLETIN NO. 265 PL.

A. MESAS AND FOOTHiLLS SOUTH OF BOULDER.

B. VIEW WESTWARD FROM GREEN MOUNTAIN.
FENNEMAN.] MESAS. 13

size is the same as that here given, the height decreasing from west to 

east.

 The Dakota " hogback " is second in prominence to the Fountain 

crags. It is absent between Sunshine and Pole" canyons, but elsewhere it rises steeply to heights of from 100 to 500 feet. The lightgray color of its sandstones is frequently in strong contrast with the 

red of the rocks farther west. Its crest is commonly covered with 

pines.

LONGITUDINAL VALLEYS.

Between the red-rock ridge and the Dakota the weaker rocks of 

the Lykins and Morrison have been cut out in a valley from onefourth to one-half mile wide. (See fig. 1; also PL TV, A.) The 

brilliant color of the former, especially north of Boulder, has given

an almost blood-red hue to the soils of the valley. A similar valley,

though lacking the color, marks the outcrop of the Benton, where the 

Niobrara ridge is present. Where the latter forms no ridge, the Benton occupies a long slope from the Dakota hogback to the mesas. 

This slope is characteristically marked by landslides. At frequent 

intervals may be seen the typical hummocky topography due to this 

agency.'

MESAS. 

East of the last foothill ridges the slope to the plains may be continuous or may be broken by a sharp step, leaving a high terrace 

adjacent to the foothills. This terrace is called a mesa, or locally 

; ' the mesa " (PI. Ill, A). It occurs in fragments of unequal height 

abutting against the most easterly foothill ridge or separated from 

it by a trough due to recent erosion. (Compare PI. V, /i, p. 30.) 

These fragments are commonly separated from one another by 

ravines. These mesas have a height of from 100 to 300 feet above 

the lower plains, and a west-east width in this vicinity varying from 

a fraction of a mile to 3 miles.

SLOPE AND HEIGHT.

The eastward slope of the terraces may be as great as 10° on the 

foothill side, but it diminishes rapidly toward the plains. At 1 

mile east from the foothills a slope of 3° to 4° is common, and at 

3 miles the "slope is about 1°. An east-west cross section shows an 

even curve, like the profile of a stream. In addition to the east-

" This small gorge in the outer foothills, 2 miles south of Boulder Creek, is locally 

known as Skunk Canyon. It appears on the maps of the Denver btisin as Pole Cat Canyon. For convenience the abbreviated form is here used. The small stream issuing from 

it is rarely spoken of by any name.
14 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

Avard slope there is generally a distinct nortlrward or southward 

inclination toAvard a loAver banch quite similar to the higher, but 

separated from it by a steep bluff, or more commonly by a ravine. 

In a- general Avay the highest mesas are farthest from the larger 

streams, and the height diminishes as these streams are approached.

STRUCTURE AND COVERING.

These mesas are essentially rock benches covered 10 or 20 feet 

deep Avith unassorted rock Avaste. The rocks, chiefly Pierre shales, 

are steeply upturned, being affected by the mountain uplift. They 

have been smoothly planed off, not to a flat but to an inclined surface. The debris covering may have a fairly uniform thickness of 

20 feet on the highest benches. Generally speaking, the thickness 

is less in proportion as the mesa surface is loAver. This thoroughly 

unassorted debris comprises .fragments of all sizes from sand grains 

to boAvlders more than 10 feet in diameter. The heaviest boAvlders 

are close to the foothills, but fragments a foot or more in diameter 

are found some miles to the east. On the highest mesas these fragments are almost exclusively from the red rock. The loAver benches 

contain varying proportions of crystalline rock mixed Avith the 

debris of red sandstone. The bowlders of the present flood plains 

are almost exclusively from the crystalline rocks.

ORIGIN.

Some similar mesas farther south have been described as river 

terraces; others haA^e been ascribed to the cutting of lake shores.0 

It is believed that the mesas in the area here described are remnants 

of stream terraces,1' and that the underlying rocks Avere planed off

by the streams that deposited the unassorted debris, just as the

present streams are eroding the shales and depositing unassorted 

material on the new surfaces that is, on the present flood plains. 

This conclusion is based on the following facts: (1) The resemblance 

of the eastward slope of the mesas to the stream profile; (2) the 

nortliAvard or southward inclination of the mesa tops; (3) the 

variations in the heights of the mesas, the lower in general being 

nearer .the larger streams; (4) the general resemblance of the escarpments to old river bluffs, the higher and loAver mesas having the 

same relationship as higher and lower terraces, except that the mesa 

remnants are often Avidely separated by erosion. It is not to be

" Emrnons, S. F., Geology of the Denver bnsln : Mon. U. S. Geol. Survey, vol. 27. 

1896,'p. 9.

6 Compare Lee, W. T., The oi'igin of the debris-covered mesas of Boulder, Colo. : 

Jour. Geol., vol. 8, p. 504.
U. 6. GEOLOGICAL SURVEY ;ULLETIN NO. 265 PL. IV

A. LONGITUDINAL VALLEY IN THE FOOTHILLS NORTH OF BOULDER.

Jj. HAYSTACK MOUNTAIN. 

View southward from Table Mesa.
FENNEMAN.] OKIGTN OF MESAS. 15

understood that all the mesas are the terraces of existing streams. 

The general decrease in height as the larger streams are approached 

might suggest such a relation, but this conclusion is not intended. 

In the discussion of drainage the location of several former streams 

concerned in mesa making is indicated.

The hypothesis that these mesas are stream terraces is strengthened 

by the objections to the hypothesis that they are lake terraces. On 

the latter assumption, there existed over the adjacent parts of the 

Great Plains a water bod_y of large but unknown dimensions. The 

western shore of this lake lay, at first, a few miles east of the present 

foothill belt. The rocks of this shore were the upturn6d shales and 

soft sandstones of the higher Cretaceous series; the original offshore 

slope was relatively abrupt. Cliff cutting then caused the shore to 

recede to the present foothill belt. There was left a broad wave-cut

terrace, of which the present mesas are remnants, and this marginal

terrace was covered A\7 ith the sheet of debris Avhich now appears as 

the mesa covering. The facts not readily accounted for on this 

hypothesis are the following: (1) The absolutely unassorted nature 

and the frequent angularity of the debris; (2) the entire absence of 

the outer detrital or built portion of what must in all probability 

have been a cut-and-built terrace; (3) the difference in the height 

of terrace remnants abutting against the foothills; (4) the lateral 

(north or south) slope of the mesas; (5) the dominance of red-rock 

fragments on the higher mesas and of crystalline rock on the lower 

terraces, and (6) the lack of well-attested lacustrine deposits over 

the adjacent plains."

In regard to (1) the fact that the debris is angular and is not 

assorted, it may be stated that detritus on marginal lake terraces is 

always assorted, while the thoroughly heterogeneous nature of the 

mesa gravels agrees to a nicety with that of the deposits of many 

overloaded streams in arid regions where the small streams are 

subject to quick alternations of flood and drought. With reference 

to (2) the absence of the built portion of a cut-and-built terrace, it 

may be noted that the mesas abut against the mountain front in a 

kind of embayment, and that a wave-cut terrace in such a situation 

might be expected to show a lakeward extension of detrital material. 

The absence of such materials is not believed to be adequately 

accounted for by subsequent erosion. In fact, a mesa composed 

entirely of such loose and porous materials as the present surface 

deposit would, in such a climate as this, be extremely unfavorable to 

the development of surface streams on account of the ready percola-

" Compare .lobnson, W. D.,-The High Plains and their utilization: Twenty-first Ann; 

Kept. U. S. Geol. Survey, pt. 4, 1901, p. 653. '
16 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

tion of surface waters, and would therefore endure longer than a 

terrace of shales thinly covered by gravels. , The significance of (5) 

the dominance of red-rock fragments on the higher mesas and of 

crystalline rock on the lower terraces is probably due to the fact 

that the streams were cutting through red rock when they were at the 

level of the highest mesas, but as they eroded deeper channels and 

flowed at lower levels they cut into and carried crystalline rocks. 

This is further discussed under the heading " Former westward 

extension of strata." The other objections enumerated above require 

no discussion.

Wherever the Hygiene sandstone is found it has been planed down 

to the level of the higher terraces. Upon their surfaces it does not 

appear, its edge -being covered'by the mesa gravels. North of Fourmile Canyon, however, wherever the higher terraces have been eroded 

away the Hygiene sandstone forms a bold ridge whose even crest 

has the same height as one of the widest terraces. When the sandstone was planed off to its present height this terrace was continuous 

across the present lowlands.

FORMER WESTWARD EXTENSION OF STRATA.

Whatever streams may have built up any terrace, it is manifest 

that the oldest mesas are the highest and that the age decreases with 

diminishing height. Furthermore, the proportion of fragments of 

red rock to those of crystalline rock decreases as the terraces become 

lower or as the present flood plains are approached. From these 

facts it is fair to infer that when the streams were running at the 

level of the. highest mesas they were cutting through red rock for 

some distance west of the present foothill belt. As a result of the

headward elongation of the valleys and of down cutting through the

stratified rocks to the granite, the streams carried increasing proportions of crystalline rock, until at present the red rock forms a very 

small part of the load and hence of the flood-plain debris. As there 

are no outliers of the red rock on the crystalline area to the west, it is 

impossible to make any but the roughest estimate of the width of the 

belt from which the stratified rocks have been denuded.

DRAINAGE.

The district is drained by eastward-flowing streams, which are 

consequent upon the tilted surface, and is crossed by the two largest 

creeks Lefthand and Boulder. South Boulder Creek, of about the 

same size, unites with Boulder Creek within-the area. Most of the
FKNNEMAN.] DRAINAGE. 17

intermediate smaller streams coming from the foothills join these 

larger creeks within the area mapped.

ADJUSTMENT TO STRUCTURE.

In the almost uniformly strong crystalline rocks to the west all 

the streams are cutting gorges. There is no progressive adjustment 

except that which comes from the more rapid widening of the larger 

valleys at the expense of the smaller. In the almost uniformly weak 

rocks of the plains all the streams have flood plains, though meandering is but little developed. Here also progressive adjustment to 

structure is not prominent.

Within the foothill belt the streams from the mountains cross 

alternately strong and weak strata. Here tributaries are developed 

in the longitudinal valleys, and at some places adjustment has resulted 

in stream capture. Across the outcrop of the Dakota and the Fountain the streams flow in steep gorges and corrade their channels with 

difficulty, while their tributaries in the longitudinal valleys erode 

the softer strata with ease. As a result, stream capture has taken 

place, and in the Fountain and Dakota ridges the abandoned channels now appear as small notches analogous to the wind gaps of the 

Appalachians.

EXAMPLES OF STREAM CAPTURE.

In the Dakota hogback a notch representing a former stream channel appears just north of Bear Canyon. The stream flowing south 

of east from Green Mountain, and now tributary to the stream in 

Pole Canyon, lies in a direct line with this notch, and doubtless once crossed the Dakota at this place. At the same time 

a stream one-third of a mile farther north crossed the Dakota 

where the ridge was- not only much lower and narrower, but was 

broken by the Pole Canyon transverse fault. In this gap the northern stream lowered its channel rapidly and a vigorous tributary 

from the south developed on the weak beds west of the Dakota 

ridge. The development of this tributary gorge was further favored 

by the eastward slipping of the Dakota ledges. (See p. 48.) The 

tributary gorge elongated headward until it captured the headwaters 

of the southern stream and caused its lower course to be abandoned. 

After this capture and sudden acquisition of more water the valley 

west of the Dakota ridge Avas rapidly cut down, and as the support 

of the steep slope on the west side of the hogback was removed landslipping occurred. (See p. 51.) A similar notch cuts the Dakota

Bull. 265 05 M 2
18 GEOLOGY OF THE BOULDEE DISTRICT, COLORADO. [BOLL. 265.

hogback 2 miles farther south and a large one occurs north of Fourmile Gulch. West of the place where Twbniile Gulch breaks through 

the Dakota the ridge of red rock is indented by a notch whose history is similar to that of the notch near Bear Canyon.

The largest stream affected by capture is Lefthand Creek, which 

approaches the foothills from the west in sec. 26, 6 miles north of 

Boulder. When this creek reaches the middle of the belt of red rock 

it turns at right angles and flows north for 1^ miles; it again turns east 

at Altona, crosses the remaining foo'thill outcrops, and emerges upon 

the plains. Almost directly east of its last bend to the north is a 

sag of more than 100 feet in the Dakota ridge to" the east. The drainage channel leading east from this sag, though occupied by a very 

small stream, pursues an independent course for many miles. It is 

locally known as Dry Creek, a name applied also to another small 

stream 5 miles farther south. Along the northern Dry Creek are 

evidences of a former larger stream the lower course of Lefthand 

Creek which once crossed the foothills in a direct easterly course. 

Another stream (the present Lefthand Creek) that crossed the foothills at Altona had a course more favorable to down cutting and was 

able to develop a tributary on its south side, wrhose headwaters worked 

far enough south to tap the former Lefthand Creek. The old valley 

for a short distance east of the " elbow of capture " is now occupied 

by a small westward-flowing stream which joins Lefthand Creek 

where it turns to the north.

This recent shifting of the position of Lefthand Creek explains 

the otherwise anomalous circumstance that Haystack Mountain (PL 

IV, B) , the highest mesa remnant in the region, lies close to one of the 

largest streams, which, as already pointed out (p. 14), are near the 

lowest mesas. The anomalous position of Haystack Mountain is 

due to the fact that previous to the capture described this mesa was 

well removed from lines of active erosion.

The terraces to the east offer further interesting evidence concerning the former character of the streams. These terraces are planation surfaces due to meandering and are not base-levels. Some of 

them are miles in width, indicating that the streams which made 

them must either have worked a very long time at the same level or 

have been of considerable size. Knowledge of either one of these 

conditions, therefore, sheds light upon the other.

As an illustration of the above, much of the surface between Table 

Mesa and the high mesa 4 miles north of Boulder Creek, a stretch 

of 3 miles, is.at one level, about 150 feet below Table Mesa. The 

bottoms of the present valleys are from 50 to 75 feet below this lower 

terrace. Where this terrace has been eroded away by the present
FENNEMAN.] - STKEAM CAPTURE. 19

streams the strong Hygiene sandstone stands out as an even-topped 

ridge, rising to the exact height of this lower terrace. Where the 

lower terrace has not been eroded the Hygiene sandstone does' not 

appear, having been planed off by meandering streams and-covered 

with the coarse gravel that mantles the mesas and terraces. Small 

streams now cross the Hygiene . sandstone in narrow gorges. All 

flood plains and low terraces of present streams are narrow in comparison with "the broad surface of planation, which has the same 

height as the top of the sandstone ridge. If the present small 

streams planed off the broad surface 150 feet below that of Table 

Mesa, they must have worked longer at one level than would be 

consistent with the present rate of down cutting.

On the other hand, it is noteworthy that this broad planation 

surface lies along the course followed by Lefthand Creek before the 

.stream had been shifted northward by the capture described above. 

This stream was approximately as large as Boulder Creek, and its 

planation surface is as broad as that of the latter. The supposition 

that the present small streams meandered at one level long enough 

to make such a broad planation surface would involve conditions of 

level or of climate which are not supported by the other evidence.

NATURAL LAKE BASINS.

Scattered over the plains is a considerable number of ponds, some 

of which have an area of 100 or 200 acres. All of these are used 

for storage of irrigation waters, and have had their capacities 

materially enlarged by dams. Some are due entirely to the damming 

of valleys or "draws," but others are natural basins. Marshall Lake 

and Boulder (Weisenhorns) Lake are good examples of the latter 

type. These natural basins are not confined to the surficial sheet 

of rough gravel, but are often deep in the shales. They originated 

later than the mesas or terraces which surround them. Some of the 

small ones might have originated by small landslips or creep, resulting in the damming of small valleys, but most of them caii not be thus 

explained. Boulder Lake has a nearly flat bottom at a depth of 

approximately 20 feet. The fraction of this depth due to damming, 

either natural or artificial, is insignificant.
20 GEOLOGY OF THE BOULDER DISTRICT; COLORADO. [BULL. 265.

STRATIGRAPHY.

The classification and nomenclature of formations used by Emmons 

and Eldridge for the Denver basin a is in the main followed in this 

paper. For the area here concerned the succession is as follows:

Quaternary ............Alluvium and terrace gravels.

Laramie. 

Fox Hills. 

Pierre. Cretaceous............

Niobrara.

Benton. 

Dakota. 

Jurassic.......... J. ... Morrison.

Lykins.

Triassic (?)&_ Lyons.

Fountain. 

Algonkian .............. Quartzite of South Boulder Canyon.

Archean ...............Granites, etc.

ARCHEAN" SYSTEM.

The sedimentary rocks of this district rest upon a basement of 

granite, granite-porphyry, and granite-gneiss, which are at some 

places gray, but more commonly range from flesh color to red. This 

basement is cut by frequent dikes; several prominent ones have a 

north-northwest direction, others trend south of east. These dikes 

are commonly composed of pegmatite, but there are also several of a 

light-greenish felsitic porphyry, similar to those which intrude the 

sedimentaries of the foothills.

The contact of this Archean foundation with the overlying sedimentaries is not a plane, but, in this district at least, the imdula-

« Mon. U. S. Geol. Surrey, vol. 27, 1S9G.

6 The writer is indebted to Mr. George I-I. Eldridge, oue of the authors of the monograph on the Denver basin, for the following note on the group heretofore called Wyoming 

and tentatively regarded as Triassic:

" Mr. N. I-I. Darton, of the United States Geological Survey, has had, since the publication of the Denver monograph, especial opportunity to study the sedimentary formations 

from the Black Hills in South Dakota to the southern portion of the frontal ranges of the 

Rocky Mountains in New Mexico, and is of the opinion, from evidence accumulated by 

him, that the " Red Beds" [Fountain] and their overlying " Creamy Sandstone " [Lyons], 

designated the Lower Wyoming in Monograph 27 of the United States Geological Survey, 

are Pennsylvania!! in age, and that the series known as Upper Wyoming [Lykins] in that 

monograph is possibly Permian in the lower part and Triassic in the upper.

" The evidence leading to such an inference consists of the presence in the Red Beds, at 

one or more localities, of limestones containing Pennsylvanian fossils and of the presence of Permian fossils in the Black Hills in what he there correlates with the lower 

portion of the Upper Wyoming, in the Denver foothill region that is, with the portion 

here consisting of the pink shales, fine sandstones, and curly limestones. The higher 

members of the Upper Wyoming may still be considered Triassic in the absence of evidence 

to the contrary."
FENXEMAN.] - SEDIMENTARY ROCKS. . 21

tions are very broad and gentle. The question whether this surface 

is due to subaerial or to marine denudation is discussed under the 

heading " Geologic history " (pp. 5^t-57).

SEDIMENTARY ROCKS.

ALGONKIAN SYSTEM. 

QUARTZITE OF SOUTH BOULDER CANYON.

The oldest and lowest sedimentary rock is a quartzite which 

appears only at the extreme southern end of the area, in South 

Boulder Canyon. It is regarded as of pre-Cambrian age. It is 

for the most part white, or stained pink with iron oxide; of medium 

texture, except at what seems to .be the base, where it is a pebble 

conglomerate. One mile south of the southwest corner of the area 

mapped the beds are almost vertical and are cut across by the newly 

constructed grade of the Denver and Northwestern Railroad. The 

stratum is here 550 feet thick.

This formation does not rest upon the Archean surface at the same 

angle as the remaining strata. From a point at the base of the 

Fountain sandstone, which in general rests upon the granite, the 

quartzite strikes south west, then for a space nearly Avest, and then 

S. 27° W. to far beyond the limits of the area mapped. It 

dips throughout to the southeast at angles ranging from 60° to 90°. 

Both north and south of it the same granite complex underlies the 

Fountain. In its relations to the higher stratified rocks the quartzite 

is only a part of the complex, on whose nearly plane surface the 

Fountain is laid down.

The structural relation of this single, steeply upturned stratum, 

outcropping in a narrow belt for many miles between inclosing 

granites, is not clear from observations made within this area. So 

small a part of its entire outcrop is included in this area that the 

attempt to decide its structural relations will not be made here. 

Marvine, in his description of the Front Range,6 regards the granite 

as a mass of metamorphosed sedimentaries. According to this supposition, the quartzite is merely one of many metamorphosed strata, 

this one showing plainly its sandstone origin, while^ its neighbors 

have become entirely structureless. Van Hise, who has studied 

the contacts in South Boulder Canyon, believes that the phenomena 

of contact are those of a sedimentary formation laid clown on a 

granite and gneiss floor. 6: He also thinks it probable that the

"Van Hise, C. R., Correlation papers, Arclican and Algonkian : Bull. TJ. S. Geol. Survey 

No. 80, p. 325.

6 Seventh Ann. Kept. U. S. Geol. and Geog. Surv. Terr., pp. 83-192.

c Van Hise, C. It., Manuscript notes to be used in a revision of pre-Cambrian rocks of 

North America (Bull. U. S. Geol. Survey No. 86).
22 GEOLOGY OF THE BOULDER DISTRICT; COLORADO. [BULL. 205.

quartzite has been folded in, and that the structure is that of an 

isoclinal sync-line.

TRIASSIC *( ?) SYSTEM.

The rocks in this region, hitherto regarded as of Triassic age and 

called " Wyoming," embrace three formations the Fountain, the 

Lyons, and the Lykins. The term " Red Beds " is popularly used 

without definite limitations; but the authors of the monograph on 

the Denver basin have conveniently limited the use of this term to 

the lowest and largest member of the series. The popular use of the 

term " red rock " is similarly restricted.

The subdivision into " Lower " and " Upper " Wyoming, adopted 

for the Denver basin, would also be appropriate to the Boulder 

area. The lower division, however, clearly embraces two lithological 

units which it is desired to distinguish in this report. The lower 

and major part consists chiefly of rather coarse, arkose sandstones 

and conglomerates of reddish color, while the upper and lesser 

part is a finer-grained, quartzose sandstone of white, " creamy," 

or light-reddish color. The- coarse red sandstones were called the 

Fountain formation by Cross in the Pikes Peak folio, a type section being crossed b}^ Fountain Creek, near Manitou. The summit 

of the Fountain formation is not exposed in the Pikes Peak quadrangle, but Darton« has found the character of the Fountain, as 

described by Cross, to continue to a white sandstone corresponding 

with the " creamy sandstone" of Eldridge, occurring in the Garden 

of the Gods, to which the name Lyons sandstone is here given.

FOUNTAIN SANDSTONE.

Lithologic character. In general the Fountain sandstone is a 

rough arkose, the individual feldspars often appearing in large crystals. Locally it passes on the one hand into conglomerate and on 

the other into quartzose sandstone, excellent for building purposes, 

and even into shale. Micas are frequent, especially in the finer 

portions. 6 The conglomerate phase may have pebbles of granite 

and schist or of pure quartz only. The conglomerate of fragments 

of crystalline rocks is found only at the base, and there not universally. .At places it is a beautiful recomposed granite, which at 

casual glance is not different from a coarse, reddish-brown original 

granite. Quartz-pebble conglomerate may be found at any or all 

" horizons.

« Bull. Geol. Soc. Am., vol. 15, p. 22.

6 Prof. K. D. George, of the University of Colorado, has examined many specimens 

and finds that in large part, at least, this mica may be secondary and may have been 

derived from the feldspar.
FENNBMAN.] FOUNTAIN SANDSTONE; 23

The cement of the mass is silica with an admixture of iron oxides; 

hence the quartzose beds weather with extreme slowness. Their 

fragments survive unchanged in the rubble of the mesas, where 

those of the crystalline rocks can be crushed in the hand.

The color of this rock in its coarser phases is due in part to its 

pink or red fragments of feldspar. In a' large way this is more 

true of the lower horizons than of the upper, but the same factor is 

present in nearly all beds to a greater or less degree. In the cement, 

between the grains, crystals, and fragments, however, red iron oxide 

also appears, and in a general way affects the coloring in proportion 

to the fineness of .the grain. Where quartz is more abundant the 

rock sometimes has a lighter color, which locally in loosely aggregated masses may even approach whiteness. Where this is the case 

it is composed almost entirely of large angular quartz grains that 

appear to have been assorted out of disintegrated granite, with 

almost no rounding by wear. ^

Occasional mottling in freakish forms is apparently caused by 

leaching of iron oxide by percolating waters. Where the differences 

in shade are attended by differences in texture, the darker colors 

occur in the denser portions where the water circulation has been 

least. In so far as the differential coloring is due to the action of 

percolating waters, doubtless a concentration of coloring matter in 

the denser portions has accompanied its leaching from the coarser.

Structure. The Fountain sandstone is characterized by very thick 

beds throughout, and cross-bedding may be found at any horizon; 

at places-there is a beautiful flow-and-plunge structure. It is very 

common to find pebble beds a few inches thick alternating with the 

finer sediments. Shale is found in distinct streaks or pockets. The 

thickness of these " Red Beds " is 500 to 600 feet at Boulder Canyon, 

where they are vertical, and increases both to the north and to the 

south, approximating 1,500 feet at Fourmile Canyon, where they dip 

eastward at an angle of 47°. No fossils have been found.

LYONS SANDSTONE.

Character and coloring. Above these massive beds and more or 

less arkose sandstones are other beds of purely quartzose sandstone. 

This upper formation is best developed at Lyons, a few miles north 

of the area mapped. It is quarried there in large amounts. The 

formation is named from that locality. The siliceous cement of this 

sandstone has sufficient iron to produce, shades of pink, but the popular name, "creamy sandstones," conveys a wrong impression as to 

their color. While these beds have a fairly uniform shade they are 

locally almost white, and at some places red. Where these extremes 

occur, as at Fourmile Canyon, leaching and concentration of the 

coloring matter have often produced gaily colored rings and bands.
24 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 2C5.

An extreme phase of this feature is seen in the small red nodules 

which may even be found loose in their sockets in certain layers 0 

and which may involve other causes in addition to those which make 

simple concretions. Many dendrite figures are found, some of marA^elous beauty. They are sometimes obtained in thin flagstones, 

approximately 2 feet square, the entire side of such a slab being 

occupied by one symmetrical fern-like figure of exquisite delicacy. 

No fossils are found.

Cross-bedding. The long washing and complete assortment which' 

these sands underwent on the shore of the early sea was attended by 

cross-bedding of unusual dimensions and perfection. The angle of 

cross-bedding is occasionally as great as 35°. At nearly all of the 

many quarries opened in this stratum, flags or blocks are taken out 

along cross-bedding planes. This is not .universally true outside 

the area represented by the map. At some of the Lyons quarries a 

part of the stone has been taken out along the true bedding, but this 

is not the rule.

The most striking feature' of this cross-bedding, next to its universality and perfection, is its dip. With few exceptions this is at 

a lower angle than that of the true bedding. If, therefore, the strata 

were again horizontal the oblique lamination would clip toward the 

mountains; that is to say, toward the shore that existed at the time, 

the beds were deposited. The significance of this is discussed under 

the heading " Geologic history."

This cross-bedded stratum reaches a maximum thickness of 297 

feet at Fourmile Canyon, and is absent just south of Boulder Creek. 

It passes-by gradations into the Fountain, or " Red Beds," below.

LYKINS FORMATION.

Character and thickness. Lying conformably upon the rocks just 

described, which are prevailingly sandstones and strong ridge makers, 

is a series of sandstones and sandy shales, with a little limestone. 

This series is the Lykins. The name is taken from Lykins Gulch, 

at the north end of the area mapped, the scenery along which 

owes its strange and beautiful character to this formation. It 

is clearly distinguished from the lower formations by its softness 

and by its showy colors, the most striking of which is a rich 

brick red that characterizes its sandy shales and shaly sands. At 

places the color is more brownish, but it is always deep and .rich, 

and the long foothill valley, always present upon these soft rocks 

(fig. 1, p. 12; also PI. IV, 4, p. 14), is at places brilliantly colored.

The entire series is nowhere shown in a single section because it

" Eldriclge (Monograph XXV11, p. 54) limits those at Golden to a horizon about 20 

feet below the summit.
FENNEMAN.] LYKINS FORMATION. 25

is easily weathered and consequently thickly covered by waste. At 

Fourmile Canyon, where the formation is somewhat more than 

800 feet thick, the lowest 230 feet are largely, if not wholly, sandstones of a clear red, a little darker than brick. These are overlain 

by the "crinkled" sandstone (see below), which is here 35 feet 

thick. Above this for 467 feet there are no exposures, but the soil is 

very red, and exposures not far distant indicate that most of this 

thickness is occupied by red arenaceous shales. Above these obscured 

beds are 100 feet more of the familiar red sandstone. Apparently 

any horizon of this formation may be slightly calcareous, but this is 

not general.

The section is essentially the same wherever seen, but the thickness varies greatly. The series is apparently absent just north of 

Gregory Canyon. If present at all it must have a thickness of only 

a few feet and is concealed beneath the sheet of waste which obscures 

almost all the strata between the Fountain and the Niobrara.

As indicated above, for a considerable portion of the column the 

nature of the rocks is for the most part concealed. At and beyond 

the north end of the area mapped there are certain slopes on which 

nearly the entire formation is exposed. Here it does not greatly 

exceed 300 feet, and, except for the " crinkled " sandstone, the whole 

exposure is of argillaceous sandstone, brick red in the main, but more 

brownish near the top. To this, however, must be added at the 

summit about 20 feet of light-pink or dull-yellowish, thin-bedded 

sandstone, slightly calcareous. This forms a kind of transition to 

the well-marked massive gray sandstone, which clearly belongs to 

the Jurassic.

Eldridge has described the Lykins under the name " Upper Wyoming " at localities farther south, where it contains " clayey strata of 

bright colors gray, yellow, green, pink, and lilac." a Throughout 

much of this area there are some hundreds of feet out of the total 

width of the Lykins belt where beds of such colors could easily be 

concealed. However, with the exceptions already noted, it is doubtful if anything but ocherous reds reach the north end of the area 

mapped.

The brownish-red sandstone at or near the top of the series (the 

dull-yellow, thin-bedded sandstone noted above does not appealeverywhere) has a variable thickness. It is more than 100 feet 

thick, at Fourmile Canyon. While primarily a massive sandstone, 

its exposed ledges weather into shingle-like combs.

"Crinkled " sandstone. The persistent band which is 'here referred to as the " crinkled " sandstone is one of the noticeable features of the series. At places it reveals the presence of the Lykins

"Op. cit, p. 56.
26 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

where all other beds are covered by the abundant, talus of the red 

rock and Lyons sandstone. This band, nowhere above 35 feet thick, 

is present wherever the Lykins is found. Its superior hardness 

causes frequent exposures where the ocherous sandstones above and 

below it are buried under waste. Such a line of exposures is everywhere found at a nearly uniform distance east of the Lyons sandstone outcrop. It is in the main a sandstone more or less calcareous. 

Locally it has a bed of dense limestone at its base and sometimes 

other limestone beds above. Some of these have been quarried for 

lime. It is of a light-pink or purplish color, clearly and minutely 

laminated. These laminae are almost universally deformed, are 

closely crumpled, and serve to make prominent the effects of a 

north-south stress which has at places minutely brecciated the rock. 

The evidences of such a stress are all but lost in the softer massive 

rocks which are conformable with this layer above and below.

JURASSIC SYSTEM. 

MORRISON FORMATION.

Limits. At the top of the brownish sandstone above mentioned 

there is an abrupt change to light colors, or at most there may be 

about 20 feet of beds of intermediate character. ' The sudden transition is readily seen at many places and is the convenient indication 

of the division between the Lykins formation and the Morrison, as 

the overlying Jurassic beds are called. The upper limit of the Morrison is quite as clearly marked by the hard sandstones and conglomerates of the Dakota, but the body of the formation shows considerable 

variation from place to place. Its maximum thickness may be taken 

at a little less than"400 feet (as at Bear and Fourmile canyons).

Generalized section. Sections of the Morrison at various places 

in this area differ greatly. In the main, however, the formation contains a large proportion of light-colored clays, some moderately 

indurated and others of flinty hardness, much gray sandstone, often 

calcareous, and at various horizons beds of highly compact limestone. 

A very much generalized section would present the beds in about the 

following order, beginning at the base: Sandstone, clays, limestone, 

clays. The first and last members of the series are persistent, but the 

intervening clays and limestone may show two or three alternations 

and may inclose prominent sandstones.

The thickness of the important sandstone at the base is nowhere less 

than 10- feet, and may be twice that amount. This bed is persistent 

and massive, and yields the white building stone commonly known 

as the " Doctor Bond sandstone." It is slightly calcareous, as are 

the Morrison sandstones in general. In Bear Canyon it forms the
PENNEMAN.] MORRISON FORMATION. 27

western one of two strong ridges. This sandstone is everywhere 

overlain by soft beds which are covered by waste.

The lowest limestone is at places within 15 feet of this basal sandstone. At South Boulder Canyon there are 40 feet of it in one 

stratum, but at Bear Canyon it is separated into two strata by a 

massive sandstone. Near Altona there are three distinct limestones, 

the topmost being a resistant stratum 30 feet thick which may form 

the crest of the ridge where the Dakota ledge is locally lower. This 

is the case for a considerable stretch between Fourmile and Sixmile 

canyons. These limestones are exceedingly compact, and turn brown 

upon the slightest weathering, though they are normally a dark 

bluish gray. As the beds are much fractured or affected with incipient fractures, the brown color is so general, even in fragments broken 

by the hammer, that the normal gray may easily be overlooked. The 

sandstones above the basal bed are (1) the resistant beds between the 

limestones and (2) the Saurian sandstone described by Eldridge for 

the Denver basin.ffi

South of Boulder the intermediate sandstone beds are represented 

by the eastern one of the two ridges in Bear Canyon. The rock of 

this ridge differs little from the basal stratum. North of Boulder 

these beds, whether one or two, have .a characteristic purplish hue, 

an irregular, often closely laminated bedding, a flow-and-plunge 

structure at places, and a wavy or scaly appearance where split 

along bedding planes. Where the sandstone shows this phase it may 

be very hard. Occasionally the crest line of the bold Dakota hogback jogs to the west and follows one of these strong ledges of the 

Morrisort. This occurs at several places'south of Altona.

The next 75 or 100 feet are everywhere obscured in this district. 

The space is doubtless occupied by the "Atlantosaurus clays" 

described by Eldridge, but their character can only be inferred from 

exposures farther south beyond this field. Above these softer beds, 

which do not outcrop, are limestones generally interbedded with 

clays.

The horizon above the zone of intermittent limestones is marked 

by a rather persistent sandstone. It is generally calcareous and 

stained with iron, the coloring usually taking the form of round 

dots, in general about the size of shot. When so marked it strongly 

resembles certain strata of the Dakota, but is distinguished from 

the latter by the fact that it effervesces with acid. The stain is not 

always distributed in this way, but may give an irregular rusty 

color to the whole rock. This is the " Saurian sandstone " described 

by Eldridge, but no animal remains have been found in this field. 

Its greatest measured thickness here is less than 15 feet.

" Eldridge, G. H., Geology of the Denver basin: Mon. U. S. Geol. Survey, vol. 27," 1896, 

p. 61.
28 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BUM,. 265.

The remaining rocks at the top of the Morrison are dense, hard 

clays and argillaceous sandstones. They are green, yellow, and purple, and occur in beds that are from a few inches to a few feet thick. 

The fine-grained indurated rocks are full of fractures and tend to 

crumble into minute angular fragments. Some beds are calcareous. 

The thickness of these clays is about 30 feet at South Boulder Can}^on 

and nearly three times that amount at Fourmile. They are well 

exposed on the south side of Bear Canyon, where they are 75 feet 

thick.

CRETACEOUS SYSTEM. 

DAKOTA FORMATION.

Above the Jurassic comes the prominent Dakota formation, easily 

recognized and well known in the great hogback which parallels the 

Front Range for hundreds of miles. It is a firm sandstone that is 

often quartzitic, is generally thick bedded, and is characterized by 

frequent cross-bedding and ripple marks. Its more resistant, heavy 

beds may give to the hogback one, two, or even three constituent 

ridges or combs.

Basal conglomerate. At is base, though not everywhere present, 

is a pebble conglomerate. The constituent pebbles include " abundant limestones, quartzites, clays, flints, jaspers, and rocks of granitic composition, together with the separate mineral constituents 

of the last." 0 The whole is so firmly cemented that, where unweathered, the rock fractures in broad planes which pass through 

the pebbles. Frequently a series of thin pebble beds in a mass of 

sandstone takes the place of the continuous conglomerate. This 

alternation of beds makes the thickness of the basal zone indefinite, 

but there are few pebble beds above the first 20 or 30 feet.

Character of the sandstone. Considerable variation may be discerned in the sandstone, but with the exception of the basal conglomerate no one bed has a constant position in the column. Generally 

the sandstone is composed of quartz grains, with a siliceous cement, 

and is gray or yellowish gray. With increase of iron oxide the 

sandstone exhibits striking features of differential coloring. The 

most common of these is due to the concentration of the iron into 

evenly distributed spherical specks, which are of uniform size in a 

particular bed, but which are as small as small shot in some beds and 

as large as peas in others. The color of these specks is light brown, 

but in a weathered crust of such a rock it is often bright red, due, no 

doubt, to the loss of water from the iron oxide. This dotted coloring 

is exceedingly common, perhaps more so in the upper portion than 

in the lower. Frequently, also, the weathered surface of a Dakota

0 Eldridge, op. cit., p. 63.
FENNEMAN.] BEjSTTOF FORMATION. 29

block or ledge is blackened by a smooth or even shining crust of 

manganese and iron oxide.

At intervals between the stronger ridge-making ledges occur layers 

of thinly laminated shaly sands, aggregating in thickness only a few 

feet. These crumble out in small angular fragments and their 

wasting accentuates the relief of the strong ledges. Fire clay is not 

found in this area. Fragments or impressions of wood are not 

infrequent, but good fossils were not obsen^ed. 

; Columnar jointing. Some of the upper beds have a distinct 

columnar jointing. At Sixmile Canyon certain thin ledges Avhen 

viewed from a short distance look like sheets of basalt. The bedding 

faces of these same strata are exposed and resemble a rough pavement 

of polygonal blocks. This jointing produces irregular columns from 

2 to 8 inches in diameter. These beds are not to be confused with the 

igneous rocks, which are intruded.

The line between the Dakota and the overlying Benton shales, 

while very distinct in a large way, is not so easily located in detail. 

Alternations of sandstone and shale occur over a zone of 50 feet or 

more. The thickness of the Dakota at Bear Canyon is about 320 feet, 

and at Fourmile Canyon a little greater.

BENTON FORMATION.

Shales and limestone. The Benton shales have a thickness of 

more than 500 feet at the north end of the field. They are somewhat 

thinner at the south end and taper uniformly from both ends toward 

Boulder Creek, where the formation almost disappears. The great 

body of the shale is dark. There are frequent layers a few inches 

thick which are strongly impregnated with iron. The formation 

is calcareous in varying degree. At many places some beds are composed of a black limestone showing a crystalline surface when broken 

and having a strong bituminous odor. Six miles north of Boulder 

several hundred feet of the upper strata have' this nature. Elsewhere 

this phenomenon seems to be confined to isolated beds.

Upper beds and Niobrara contact. As the summit is approached 

the blackness disappears and the-last 75 feet (observed near the 

north end of the field) show light-colored limestone, shale, 'and sandstone. North of Sixmile Canyon these beds, which may be regarded 

as transitional to the Niobrara, show the following section:

Section of upper part of Benton formation north of Sixmile Canyon.

Feet. 

Greenish sandstone, calcareous in upper third_________________ 15

Shales, blue to yellow, lioncalcareous______-_____________ 15

Sandstone, noncalcareous, firm, rather purplish_l__ _______.._..___ 10

Shale, iron stained at intervals___ _ 10 

Shale, calcareous (or calcareous at top).
80 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

Below these beds is a limestone of variable thickness strongly 

resembling the basal Niobrara. It occurs in lenses only. 

. The greenish sandstone at the top is found below the basal Niobrara wherever the latter is-exposed in this district. ' It is generally 

10 or 15 feet thick, and is not calcareous except in the upper portion. 

As it is often very much fractured and the cracks are abundantly 

filled with carbonate of lime, this calcareous character may be due to 

infiltration from the limestone above. The blue and yellow shales 

below are also seen wherever the base of the greenish sandstone is 

exposed.

NIOBRARA FORMATION.

The Benton is succeeded by the Niobrara, whose prominent characteristic is its calcareous nature. It is comp'osed in small part of 

true limestones, but the greater mass is made up of calcareous shales, 

while considerable portions are of intermediate character. Its thickness at Fourmile Canyon is a little more than 400 feet.

The basal stratum of compact limestone rests upon the greenish 

sandstone which is at the top of the Benton. Occasionally a foot 

or more of light-colored marl intervenes. Below the Niobrara, as 

pointed out above, are occasional beds of similar limestone and calcareous shales interbedded with noncalcareous sandstones. In lithological character these transitional beds resemble the Benton more 

than the Niobrara: The faunal relations are not decisive.. It is 

therefore convenient from a field standpoint to adopt the abovenamed horizon as the dividing line between the Benton and the Niobrara because of the sharp change at this horizon from a sandstone to 

a compact limestone.

Basal limestone. This is generally about 15 feet thick, reaching 

a probable maximum of 20 feet; it-is light gray, dolomitic, and very 

compact; it breaks with a conchoidal fracture. Its individual beds 

may be 2 or 3 feet thick and are sometimes separated by thin layers 

of clay. The bedding is irregular and a 3-foot bed has been seen 

to pinch out within 10 feet. It abounds in shells of Inoeeramus and 

has also some dendritic markings. On its upper side it passes into 

the calcareous shales through an alternation of thinner limestone 

and shale beds. These thinner limestone beds are characteristically 

jointed into small blocks whose edges are rounded, and when exposed 

in the quarry the bed has the appearance of a pavement.

The basal limestone has been frequently overturned (see p. 50) and 

has been much crushed by lateral stress. In outcrops it is by far the 

most prominent stratum between the Dakota and the Laramie, except 

where the Hygiene sandstone makes its, prominent ridge.

Shales. Most of the Niobrara shales are dark brown, but their outcropping edges weather light gray, sometimes almost white. The top-
U. S. GEOLOGICAL SURVEY BULLETIN NO. 265 PL. V

A. VIEW NORTHWARD ACROSS MOUTH OF BEAR CANYON,

B. OUTCROPS OF NIOBRARA SHALES.
FENNEMAN.] PIERRE FORMATION. 31

most beds, which are 10 or 20 feet thick, are of a much lighter brown, 

and weathering gives to them a characteristic yellow color. These 

are highly calcareous, and pass at places into impure limestones, 5 or 

C inches thick. This horizon is frequently marked by a ridge. 

Generally, too, about midway between base and summit, the dark 

shales which weather white contain harder and more calcareous beds, 

whose outcrops make low ridges (PI. V, B}. Locally these also are 

impure limestones. In this zone are beds several inches or even a foot 

thick composed entirely of Ostrea congesta shells. More than one of 

such zones occur at places. As in the basal limestone, the rock has 

a strongly bituminous odor when struck, especially where fossils are 

present.

PIERRE FORMATION.

\

Color. Above the Niobrara are the Pierre shales, which are more 

than 5,000 feet thick. They are slate colored, leaden gray, dark 

brown, and sometimes nearly black. Weathering gives to them a 

greenish-drab hue, which, at any considerable distance from the foothills, is their color to a depth of 30 feet, more or less. It is therefore 

the one 'ordinarily seen. Near the base of the formation, however, 

just in front of the hogbacks, erosion is more active and the shales are 

often seen with their original dark colors.

Limestone beds. While in general noncalcareous, the Pierre has 

local limy beds. At places these form continuous strata, as, for 

example, 4 miles north of Boulder, one-half mile east of the contact 

with the Niobrara. Here, for a thickness of nearly 40 feet, strong 

limestone beds are so closely grouped as to give the outcrop the appearance of the basal Niobrara. At other places the limestone beds 

are smaller and more isolated, or are divided into concretionary 

masses often containing fossils. Less prominent calcareous masses 

may be found at any horizon either in beds or in more or less perfect 

concretions.

Concentrations of iron occur in similar but less massive forms, 

ranging from clear-cut beds to well-formed nodules. The lime and 

the iron may or may not occur in the same concretionary mass. 

Many of the calcareous nodules mentioned contain much iron carbonate, which, in progressive oxidation toAvard the center, gives rise 

to sharply marked concentric shells differing in color.

Hygiene sandstone member. Sandy beds may occur, at any place 

in the section. The most prominent and persistent of these is about 

one-third way up from the base, or a little higher. Its outcrop 

is chiefly at the north end of the field, where it forms a consid-' : 

erable ridge, though disappearing under Table Mesa. Where it 

reappears north of this broad -mesa its outcrop again forms a strong 

ridge, which is almost continuous for many miles. The ridge passes
32 GEOLOGY OP THE BOULDER DISTRICT, COLORADO. [BULL. 265.

within a mile and a half of the village of Hygiene, where the sandstone is typically developed. The name Hygiene sandstone has 

therefore been adopted for this member of the Pierre formation. 

To the public it has become well known on the Culver ranch, 6 miles 

southwest of Berthoud, Avhere it crosses Little Thompson Creek and 

where a seepage of oil has long been known. At its outcrop Avest of 

Berthoud it has a thickness of several hundred feet. At the north 

edge of the area described in this report it has a thickness of 250 

feet. At Bear Canyon is an isolated outcrop of an equally prominent sandstone, 150 feet .thick, which is probably of Hygiene age.

The Hygiene sandstone is thick bedded and frequently crossbedded. Much of it is dark greenish gray and gritty. The remainder is light gray. The whole is calcareous where fresh. It loses its 

lime in weathering, takes a paler-greenish tint, and becomes friable. 

It frequently contains carbonaceous matter resembling small sticks 

of wood turned to coal. It also contains fossils of invertebrates, but 

its fauna is not yet known to be distinctive of this horizon.

.Near Boulder the Hygiene sandstone is within less than 1,000 feet 

of the base of the Pierre and is itself a very thin stratum. It thickens greatly toward the north, as do also the shales which lie below 

it. Five miles north of Boulder the underlying shales, which are 

1,000 feet thick near. Boulder, have a thickness of about 2,700 feet. 

Here also the sandstone begins to separate into two strata by the 

coming in of a shale parting. In the next 2 miles this intercalated 

shale becomes 200 or 300 feet thick. In so far as the upper sandstone can be identified north of Table Mesa, the indications are that 

this intervening stratum goes on thickening to the north, in common 

with other strata.

From these pure sands at one extreme to pure clay shales.at the 

other, the Pierre shows all gradations in composition. The sandy 

layers are generally firm and gritty, almost as dark colored as the 

shales themselves, and not very porous. In rare instances lightcolored, friable sands are encountered in drilling oil wells.

The thickness of the sandy beds is as variable as their.constitution, 

while the lateral extent of such beds, as indicated by occasional outcrops and by the records of wells, is, in a large majority of cases, a 

small fraction of a mile. Some of these beds may be lenses, but 

doubtless the more common mode of lateral limitation of the sands 

is by a gradual change in composition of the beds into the shales. 

Such beds are found at irregular depths. As some of them disappear and others come in, their depths vary greatly from place to 

place.

The upper division of the Hygiene sandstone is characterized by 

large calcareous concretions several feet in diameter. Where the outcrop appears as a low ridge or only a stony band, these concretions,
FENNEMAN.] FOX HILLS AND LARAMIE FORMATIONS. 33 \

crumbling, form isolated rocky patches of angular fragments, frequently showing fossils. The exposures of beds of this description 

in or near the area described in this report are insufficient for the 

correlation of their outcrops, or even for. the determination of their 

number. It seems necessary to assume, however, that there is more 

than one bed, and it is probable that the beds occur at any horizon 

and are of very limited lateral extent. The upper division of the 

Hygiene sandstone is here given as a typical case, except that it has 

far greater continuity than others of its kind. Whatever, may be 

true of these calcareous beds, the sandy beds, except the main stratum 

described above, are distributed through the column of shales at 

irregular intervals and are generally of small extent.

FOX HILLS FORMATION.

Within the limits of this area the great body of the Fox Hills is

but indefinitely distinguished from the Pierre. In mild contrast 

with the latter its shales are yellowish instead of slate colored, and 

are also more arenaceous. This latter quality is plainly shown in the 

soil produced. At some places the transition zone spoken of by 

Eldridge* as marked by " limestones and small ferruginous nodules " 

is plainly seen. No attempt has been made to trace on the accompanying map the contact between these two formations, as definite 

points of contact are infrequent and separation of the areas of the two 

terranes in a detailed way must in fact rest largely on the character 

of the soils. This distinction is frequently masked by outwash from 

the foothills.

The topmost stratum of the Fox Hills is, however, a very definite 

feature in the stratigraphy. For many feet below it there are occasional sandstone beds, and the intervening shales are highly sandy, 

but at the top is a continuous bed of sandstone 40 feet thick. 

It is best exemplified just east of the area mapped, where a fine fault 

brings it up to view alongside the basal Laramie. The sandstone is 

here fine grained and yellow, very slightly calcareous, and incloses 

great calcareous, iron-stained concretions. The entire thickness of 

the Fox Hills near Niwot is about 1,300 feet.

LARAMIE FORMATION.

The Laramie outcrops in the southeast corner of the area. Within 

the limits here defined this formation consists essentially of (1) 

sandstones at the base, (2) sandstones alternating with shales and 

coal overlying, (3) sandstones above the coal, and (4) clays containing isolated sandstone beds and lignitic streaks.

« Mon. U. S. Geol. Survey, vol. 27, p. 71. 

Bull. 265 05 M 3
34 GEOLOGY OF THE BOULDEE DISTRICT, COLORADO. [BULL. 265.

Basal sandstones, The basal sandstones are exposed at various 

places in.the Marshall coal field, but best of all at White Rocks, on 

the eastern border of the area mapped. . Here the bold sandstone 

escarpment comprises two distinct and massive beds, whose topographic effect, however, is that of a single stratum continuous with 

the Fox Hills sandstone beneath. The lower Laramie sandstone 

.is 40 feet thick, equaling the underlying Fox Hills sandstone, while 

between the two are a few feet of thinly laminated sandstones, 

varying to a lignitic shale. The second and thicker stratum of 

Laramie is separated from the first by gray, shaly sands, inclosing 

10 inches of lignitic shale and above it 10 inches of coal.

Both Laramie beds are clearly distinguished from the Fox Hills 

by their gray or white color and coarser grain. They are composed 

of coarse quartz sand, and in addition always show some black 

specks of an undetermined mineral. The two sandstones differ in 

the following respects: The upper is distinctly whiter and, where 

exposed along bedding planes (and to a less degree on fracture 

planes), the surface is marked by weathered-out cracks, making 

polygonal patterns a few feet across. Both strata are weakly 

cemented and rioncalcareous, but contain giant concretions which 

are very calcareous, hard, and, on weathered surfaces, are stained 

brown with iron oxide.

Beds associated with the coal. The 75 feet of thinner-bedded sandstones above, containing some shale and coal, are best seen at Marshall. 

The shales occur at frequent intervals, and are often lignitic. The 

one workable coal seam is sometimes at the top of this division, but 

again is overlain by 5 or 10 feet of shales and thin sands; all, however, lie below the more persistent sandstone referred to under (3) 

above.0 Aside from the coal seam, no horizon above the basal sandstones is more definite than the Ostrea glabm zone, which OCCUTS 15 

or 20 feet from the base of the series.

The sandstone above the coal is not unlike the second massive bed 

above the bottom. It weathers into the same angular patterns, and 

has the same appearance in the hand specimen. Its thickness is 

from 8 to 15 feet. The overlying clays, as shown.in this area, are 

yellow and purple, and have frequent lignitic. bands but no coal, at 

least within the limits of the area covered by the map.

QUATERNARY DEPOSITS.

There are no deposits younger than Laramie except the uncompacted mantle rock. This is the coarse mesa gravel (see p. 14), or 

finer alluvium, similar to the deposit formed by the present streams.

"The sandstone "C" of Eldridge, op. cit., p. 74; also p. 346.
FENNEMAN.] IGNEOUS ROCKS. 35

In the eastern part of the area, especially in the northeastern 

corner, are patches of loam which plainly belong to the western 

margin of the great sheet which covers the surface a little farther 

east. This is the " loess " of Hayden and Aughey a and of Emmons.fi 

Just what relation these patches bear to the " Plains marl " of Hay,c 

or the great silt body of the Plains Tertiary,d was not determined on 

account of the narrowness of the area studied.

The whole surface east of the foothills is, in a large way, a patchwork of stream terraces. The oldest surficial deposits are highest 

and farthest from the larger streams, and there is a decrease in age 

from those at the top of Haystack. Mountain to the recent alluvium. 

No other time scale is suggested by the phenomena of this portion 

of the plains than that based on relative altitude and distance from 

streams. In large part, at least, these deposits are of Pleistocene age, 

though the time of their beginning is not definitely fixed.

IGNEOUS ROCKS.

The igneous rocks are of interest chiefly with reference to their 

manner of occurrence and relations to the deformation of the sedimentaries, rather than to their character. Constituent minerals and 

rock textures are therefore given very brief and summary mention.

ROCKS OF THE ARCHEAN AREA.

The Archean rocks are in general granite and granite-gneiss. The 

texture is generally coarse and the color very frequently pink on 

account of the pink orthoclase contained. These rocks are cut by 

numerous dikes, which are of all sizes up to a maximum of many 

miles in length and 50 or more feet wide. Their texture is as varied 

as the dimensions of the dikes. In direction the larger dikes show 

a tendency to approximate parallelism with the foothill outcrops 

that is, with the axis of the foothill monocline. .They would, however, if prolonged to the south, meet the foothill belt at a small 

angle. Their most prominent direction, therefore, is more nearly 

that of the axes of the echelon folds which branch out from the foothills. This comparison of the general directions of two sets of features of different age is used as a mere convenience in description; 

it is not intended to affirm a genetic relation.

One of these dikes passes through Orodell, 2 mile? west of Boulder. 

It passes up the valley of Fourmile Creek for a mile north of that

« Eighth Ann. Kept. U. S. Geol. and Geog. Surv. Terr., 1874, pp. 245-255.

"Mon. U. S. Geol.,Survey, vol. 27, 1896, p. 261.

"Water resources of a portion of the Great Plains; Sixteenth Ann. Rept. U. S. Geol. 

Survey, pt. 2, 1895, p. 570.

''Johnson, W. D., The High Plains and their utilization: Twenty-first Ann. Rept. U. S. 

Geol. Survey, pt. 4, 1901, p. 653.
36 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

point and goes far to the north-northwest. ! Its southern end is 

somewhat interrupted, but its general course is in line with that of 

the Green Mountain fault (p. 46). The " Hoosier dike," of the same 

kind, lying 2 miles farther west, is even larger and more continuous.

INTRUSIVES OF THE FOOTHILLS.

In the sedimentaries the intrusion is commonly- in the form of 

sheets. There are also series of apparently disconnected bosses, but 

these are ranged in lines following the strike of the strata, three or 

four appearing at about the same horizon within a mile.

There are here no intrusives in the foothills south of Gregory Canyon. Within the narrow limits of the area mapped, at least, such 

intrusion is limited to the areas of complex folding that is, it shows 

a marked association with echelon folding. The greatest sheets 

occur at the north end, where such folding is shown on the largest 

scale. In view of the association of certain intrusive rocks with 

small deformations, the presence of similar rocks beyond the limits 

of the area here described may well be noted and taken tentatively 

as a suggestion of the presence of folds. The large importance of 

these minor folds in the structure of the adjacent oil field gives 

significance to such associated features. (Compare p. 93.)

SUNSHINE DIKE.

A large dike, 1 mile long and from 15 to 40 feet wide, follows the 

south side of Sunshine Canyon. A short distance beyond its Avestern 

end is a similar dike that runs north-northwest parallel to the Oroclell 

pegmatite dike for a mile and then trends northwest in an irregular 

course to the Orodell dike. The small interruption may be disregarded, and both dikes are here called the Sunshine dike. This 

dike is crooked and branching, and at places there are two or three 

parallel dikes, as if the main one were subdivided. The rock of this 

dike has a dense greenish-gray felsitic groundmass. It contains 

phenocrysts of quartz, feldspar, and mica, any of which may be onefourth of an inch in diameter. The proportion of the volume of 

phenocrysts to that of the groundmass varies greatly. Locally the 

phenocrysts may constitute half the rock.

FLAGSTAFF SHEET.

On .the east side of Flagstaff Hill and near the base of the red 

rock is an intrusive sheet of the same character as the Sunshine dike. 

It extends north from Gregory Canyon to within half a mile of 

Boulder Creek. North of that stream the sheet reappears at the same 

horizon, and continues a mile to the north. Its greatest width is at 

least 40 feet. The exact contact on its east side is seen in the Silver
PENNEMAN.] FLAGSTAFF SHEET. 37

Lake ditch, one-fourth mile north of Boulder Creek. The red rock 

has here been greatly altered by the heat and both rocks have been 

much weathered by spring water following the contact.

i i \w Archean 

FIG. 2. Map showing dikes and intrusive sheets west of Boulder. By Hugh P. Watts.

Pegmatite Rhyolite Jurassic and Cretaceous Archean Boss of basic rock 

Tdassic

At the north end of Huggins Park is a small offshoot from the 

main Flagstaff intrusion, or, it may be, merely a dislocated portion 

of the larger mass involved in the eastward slipping of the red rock. 

Just below the Huggins Park intrusion is an exposure of the Dakota
38 GEOLOGY OP THE BOULDER DISTRICT, COLORADO. [BULL. 265.

sandstones and shaly sands, apparently altered by heat. On this 

account it is presumable that this small intrusion followed a separate 

fissure which conducted it to a higher horizon than that of the larger 

sheet on the west.

PLUMELY CANYON SHEET.

A large sheet of rock similar to that at Flagstaff is intruded in the 

red rock in Plumely Canyon, a western tributary to Lykins Canyon, 

at the north end of the area mapped. This is fully 30 feet thick 

and has thoroughly baked the including strata of arkose sandstone, 

giving them a black color and a siliceous appearance. Minor beds 

and pockets of shale in the sandstone have been much more affected 

by the heat than the latter. Pockets of shale are baked and blackened where the including feldspathic sandstone is unchanged.

ALTONA SHEET.

A larger sheet of the same nature, having at places a width of 100 

feet, occurs at or near the base of the Lykins, above the Lyons sandstone and below the ocherous shales. The exposure of this sheet 

begins at a point west of Altona and extends 2 miles to the north 

without interruption. For much of its length it forms a prominent 

ridge on the floor of the large longitudinal valley which has been 

cut out on the Lykins shales.

South of Altona a 20-foot sheet of the same rock is intrusive near 

the top of the Lykins. The sandy shales have been greatly altered 

by heat. Their color has changed from red to black, and some of 

the shales so strongly resemble lavas that they might be mistaken 

for such if gradations were not apparent. The contacts of this 

sheet are far from regular, the magma having run out in small 

branches, sometimes surrounding blocks of the country rock.

All of these sheets are similar in character to the Sunshine dike 

described above, though varying greatly in details of texture. All 

are distinctly porphyritic, though the proportion of phenocrysts to 

the entire mass is subject to much variation. The very fine-textured 

groundmass is characteristically green or greenish gray, giving color 

to the whole rock.

INTRUSIVE BOCKS IN THE DAKOTA AND BENTON.

The Dakota contains intrusives at various places in the northern 

part of the area. In Sixmile Canyon there are three sheets with an 

average thickness of 10 feet, and the same rock appears at intervals 

for 2 miles farther north. It appears in sheets at various places in 

sec. 25, T. 2 N., K. 71 W. On the north side of sec. 36 of the same 

township it occurs as a boss which has greatly altered the Dakota
FENNEMAN.] INTRUSIVES OF THE FOOTHILLS. 39

rocks which it intrudes. The intrusives in the Dakota and those in 

the Benton, which are very similar, differ little, except in color, from 

those in the older strata. Those in the Dakota and Benton are a dull 

gray.

A line of bosses occurs near the base of the Benton shales. The 

most northerly and largest one is in sec. 25. This irregular mass, 

together with the dike which runs eastward from it, traverses almost 

the entire width of the Benton strip, which is nearly one-half mile 

wide at this place. This fissure crosses one of the most prominent 

of the echelon folds of the foothills. (See fig. 3, p. 44.) The largest 

boss is centrally located in a group of such folds.

INTRUSIVE ROCKS IN THE NIOBRARA AND PIERRE.

Near the contact of the Niobrara and the Pierre and within a mile 

north of Boulder is a line of small volcanic knolls. The igneous

rock which forms these knolls may be seen at four places. It occurs 

at the Colorado Sanitarium, near the mouth of Sunshine Canyon, 

at the base of the Niobrara. At this place it was thrown out from a 

small excavation and does not cause any topographic feature. Onefourth of a mile to the north it makes a low knoll 500 feet in diameter extending across the Niobrara and into the Pierre. A second 

and similar knoll occurs 600 'feet farther north, this one being 

entirely within the Pierre. A little north of the middle of sec. 24, 

T. 1 N., R. 71 W., is another knoll, similar in every way to the other 

two.

No sheet connecting these four masses has been found, but as they 

are ranged along the strike of the strata and are all composed of the 

same rock, they belong undoubtedly to a common magma, which at 

this horizon became separated along at least four lines of slight 

resistance. The rock at this highest horizon contains comparatively 

few crystals. Large masses of it are nearly white, purely felsitic 

lava.

DIFFERENCES IN FORM OF INTRUSIONS.

From the Fountain sandstone to the shales of the Pierre the 

sheets become progressively more irregular in continuity and thickness. The thickness of the sheets is almost uniform throughout the 

Fountain sandstone, but varies in the Lykins. In the Dakota the 

intrusive sheets have but limited extent and there is a strong tendency 

for the igneous masses to be concentrated into bosses. In'the Benton 

such localization is the rule. All the intrusives in this formation 

appear now as isolated knolls, except in the one instance described 

above, where a dike runs eastward from the largest of such knolls. 

In the Niobrara and Pierre all igneous rocks appear as completely 

isolated and well-spaced knolls with no evidence of connecting
40 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 2G5.

sheets. The association of all these igneous rocks with the echelon 

folds suggests that both the intrusion and the folding occurred 

during the same process that is, during the formation of the great 

foothill monocline.

VALMONT DIKE.

The basic igneous rocks of this area are represented by only one 

prominent formation the Valmont dike.0 This is a great mass of 

dolerite. Both its occurrence and its petrographic character have 

been described in detail by Whitman Cross.6 It has a maximum 

width of 45 feet and an extreme length of nearly 3 miles, though it 

is not uninterrupted. From the village of Valmont it extends east 

by north uninterruptedly 1 mile and as a series of knolls for another 

mile. For 1 mile west of Valmont it does not appear, but at that 

distance, in an almost straight line with its course, is a knob of the 

same rock. The main portion forms a ridge, which rises at Valmont 

more than 200 feet above Boulder Creek, but which is now being cut 

away for road material.

This dike is of an: even-grained dolerite, divided into large blocks 

by joints which run parallel and transverse to its length. Within a 

few inches or a foot of its walls the rock may be divided into thin 

plates by numerous joints parallel to the bounding surface. In a 

very narroAv zone next to the contact the rock may have a lava-like 

appearance, but at a distance not greater than a very few inches the 

characteristic uniform texture of the dike appears.

The stratified rocks containing this dike are (except in case of the 

isolated knoll to the west) the Fox Hills. shales. There is nothing 

more striking in the whole occurrence than the small degree of 

alteration which these shales have undergone. Beyond a very little

hardening it can not be said that they have been changed at all. 

In this inference, however, the subsequent weathering is not considered. The vicinity of the contact has doubtless been subject to 

the work of percolating water. Along the vertical joints of the 

dike near the contact there have been much weathering and some 

deposition of calcite.

It is inferred by Cross c that this dike " represents a channel 

through which the basaltic magma rose to the surface; " that this 

eruption occurred during the Denver epoch; and that strata, since

0 One-half mile north of Green Mountain there is, surrounded by the granite, a small 

Isolated mass of a peculiar nearly black igneous rock, with large phenocrysts of several 

kinds. This knob was located and specimens were collected from it by Mr. Hugh P. 

Watts, to whom the writer is Indebted for information and specimen. The rock is unlike 

any other known within the area.

6 Bull. U. S. Geol. Survey No. 150, 1898, p. 261; also Geology of the Denver basiu: 

Mon. U. S. Geol. Survey, vol. 27, 1896, pp. 280 and 298.

" Bull, cit, p. 26i.
FKNNEMAN.] STRUCTURE. 41

denuded, made the surface of the ground about 2,000 feet higher 

then than now. The nearly complete uniformity of texture from 

wall to wall is taken to indicate that the magma cooled at a very 

uniform rate and that the adjacent shales had been thoroughly 

heated by long-continued passage of lava.

At Burnt Knoll, 3 miles northeast of Marshall, are rocks which 

bear a remarkable resemblance to lavas. The zone characterized by 

this appearance is horizontal and confined to the top of the hill, 

though much of the material has rolled down and now clothes the 

sides. Apparent injection among fragments of baked shale is the 

most common phenomenon, but nothing of undoubted volcanic origin 

has been found. The coal beneath the hill is nowhere affected by 

heat. Gradations from the most lava-like appearance to ordinary 

shale indicate that the phenomena are due to heat alone and not to

injection. Doubtless the heat came from the burning of associated

coal beds, probably ignited by prairie fires.0

STRUCTURE.

FOOTHILL MONOCLINE.

The master structural feature of this region is the great upturn of 

the strata against the mountain range. The form and magnitude of 

this fold appear most striking when the Archean surface alone is 

considered. As shown under the heading " Physiography," the 

first Archean belt west of the foothills is a dissected plateau 10 to 12 

miles wide, which slopes east and abuts against the mountain ridge 

proper on the west. It is from 6,500 to 7,000 .feet above sea level at 

its eastern edge, where it ends abruptly and is flanked by the Fountain sandstone, which has an average dip of about 50°. The height 

of tliis sloping face above the plains is nearly 1,000 feet, but this is 

a small fraction of the true height of the monocline. Five miles 

east of the base of the foothills the Archean surface is at least 9,500 

feet below the surface or 4,200 feet below the level of the sea. The 

real face of the granite plateau is therefore about 2 miles high, and 

this enormous rise is accomplished in 6 miles. These figures represent merely the measured thicknesses of strata outcropping in the 

first 5 miles from the foothills. They do not take into account the 

possible thinning or thickening within that distance, nor the probable presence of other beds overlapped by those exposed.

The granite plateau on the west shows no evidence of any deformation since its elevation above base-level. The uplift therefore resulted 

in the formation of an almost simple monocline not less than 2 miles 

high and 6 miles wide. But it does hot follow that this flexing

a See also Eldridge, op. cit., p. 75
42 GEOLOGY OP THE BOULDER DISTRICT, COLORADO. [BULL. 265.

was brought about entirely by the mountain-making forces. In 

fact, the greater part of it was apparently accomplished while the 

thick formations were being deposited to the east and land continued 

to the west. When the topmost marine stratum, the Fox Hills, was 

laid down in the shallow Avater, the elevation of its surface probably 

differed little from that of the granite terrane to the west. The same 

statement would be fairly correct for the lowest stratum of the Fountain sandstone during its deposition, and for any intervening bed. 

While at any time during the long process of sedimentation the lastlaid stratum was approximately horizontal and close to the sea level, 

the originally (nearly) horizontal granite base was getting constantly 

lower, and its eastward or seaward, dip near shore was correspondingly increased.

It follows from the above that before the present Rocky Mountains 

were formed, and while the area now included in them was close to 

the sea level, there had been produced a monoclinal fold whose height 

was but little less than that which would now appear in the granite 

surface if the sediments were removed. This conclusion involves no 

hypothetical premise except the generally accepted continuance of 

land conditions on the site of the Front Range of the Rocky 

Mountains.

In determining the height of the monocline it must be remembered 

that the land surface on. the west was constantly being lowered by 

erosion. Occasional or continuous uplift was necessary in order to 

furnish the material for the sedimentary formations. The amount 

of flexing is, therefore, the sum 'of the subsidence of 9,500 feet on the 

east and an unknown elevation on the west. This rising on the 

west might even be equal to the sinking on the east, hence the actual 

height of the flexure is but partially represented by the 2-mile 

vertical interval between the exposed Surface of the Archean W6St of 

the foothills and its buried surface under the plains. The total 

amount of flexing is equal to the offset that would appear if the 

denudation west of the hinged shore line had not taken place and the 

sedimentary formations east of that line were removed.

If the above statements are correct the influence of actual mountain making in increasing the height of this monocline was comparatively small. It is, however, conceivable that an east-west compressive force may have accentuated the fold along the steepest part of 

its slope, thus tending to narrow the belt of dipping strata, while 

their dip was at the same time steepened. The slope of the monocline 

previous to the mountain uplift can not be determined unless the 

shore line is definitely located, which is probably impossible. No 

reason appears in this area, however, for departing from the common assumption that the sea did not cover the line which is now 

the axis of the Front Range and which lies a few miles to the west.
FENNEMAN.] ECHELON FOLDS. 43

It is not conceivable that mere sinking under an increasing load of 

sediments should have folded the rocks so sharply or along a belt so 

narrow as the present monocline; yet the height of the flexure is 

mainly due to that cause. It is only in association with the last and 

smaller part of the deformation that lateral compressive force is 

evident. The process of mountain making gave to the granite plateau 

west of the foothills a comparatively small relative uplift above the 

plains.

ECHELON FOLDS.

Other definite effects of lateral compression are seen in a special 

type of folding known as " echelon," which at places accompanies the 

foothill uplift. Such folding indicates that there was a north-south 

pressure in addition to the dominant east-west pressure. The 

resultant stress was in lines a little north of east and south of west, 

causing folds whose axes diverge slightly from the foothill belt, and 

trend south by east. Within this area such folds are all small. 

In general, however, such folds are larger in the younger and easternmost strata. They are largest in the Pierre, become less and less 

marked to the west, and almost die out by the time the Fountain 

sandstone is reached. This general rule is not without its exceptions, 

because the size of such folds is also in a measure dependent on the 

strength of the strata affected. They are therefore larger and rather 

more prominent in the weaker Jurassic than in the stronger Dakota 

above it. Wherever a stratum of some strength is thus folded the 

end of the canoe-shaped synclinal trough shows a distinct eminence. 

All the echelon folds are shown on the geologic map (PI. II).

FOLD NEAR SOUTH BOULDER CANYON.

The echelon fold affecting the lowest strata occurs a mile north of 

South Boulder Canyon. Here the top of the red rock is offset 1,000 

feet to the west on the south side of a small stream as compared with 

the north side. The younger strata to the east are less and less 

affected, so that the Dakota is offset but 500 feet and the Niobrara is 

not folded at all. The location of the fold is significant, being, as 

it is, at the extremity of the South Boulder fault. The two are but 

different phases of the same feature and are due to the same act of 

deformation, only the style of deformation differs.

FOLD SOUTH OF FOURMILE CANYON.

One-half mile south of Fourmile Gulch a mild tendency to this type 

of folding shows itself in a strong southwest trend of the Dakota 

hogback, which makes a jog of some 400 feet from its otherwise 

uniform course. In diminishing degree this fold can be traced
44 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

north-northwest and south-southeast to the Jurassic and Benton, 

respectively.

FOLD IN SEC. 25, 6 MILES NORTH OF BOULDER.

A large number of small, well-developed folds of this kind appear 

in sees. 36 and 25, 6 miles north of Boulder. In the west half of

sec. 25 a prominent, bald, 

rounded 

hundred

hill rises several 

feet above the 

crest of the Dakota ridge, 

which at this place is broad 

and less distinctly marked 

than usual, though not 

lower. Here, for a short 

distance, the ridge-making 

rock is not the Dakota, but 

the strong, purplish sandstone in the Morrison. The 

rounded hill mentioned is 

at the end of a canoe-shaped 

structural trough in this 

gandstone. (See fig. 3. 1) 

East of it, in the same 

stratum, is a well-delineated anticline, with an axis 

pointing south by east, 

thus diverging slightly 

from the main trend of the 

outcrop. The exposed edge 

of this stratum first runs

north, turns around the 

synclinal hill, loops back to 

the south for a third of a 

mile, and then turns north 

again around the anticline. 

All the beds of the Jurassic 

and those adjacent are 

strongly affected in similar

manner. In a direction north by west the fold affects the Fountain 

sandstone for a number of miles, flattening its eastward dip of nearly 

30° to 7° or 8° along the line of the fold. It is largely due to this 

flattening of dip that the outcrop of the Fountain sandstone is so 

greatly broadened at the north end of the area mapped. The effect 

on this formation is also seen in the northeast trend of its eastern 

boundary, west of Altona.

*"->. Faults $881$ Igneous rocks 

v"^' Boundaries of members within formations

Scale -. 

o '/* . -'imile

FIG. 3. Map of sees. 25 and 36, T. 2 N., R. 71 W.
FENNEMAN.] ECHELON FOLDS. 45

Near the northern line of sec. 25 the upper beds of the Dakota are 

distorted by an abrupt fold of the same kind. The outcrop, as traced 

southward, turns backward to the northwest, but in this instance less 

than a fourth of a mile. The north-northwest continuation of this 

fold and the fold above mentioned together produce the flattening 

in the dip of the Fountain sandstone. The southward continuation 

of this fold is beautifully revealed in the basal limestone of the Niobrara. The. higher, shaly limestones of this series are somewhat 

folded along the same axis, but seem in the main to have slipped one 

over another at the place where corresponding folds might have been 

expected. By such means the site of their folding was shifted nearly 

a, mile to the south. The folds show clearly near the east side of 

sec. 36.

FOLD IN THE. HYGIENE SANDSTONE OPPOSITE SIXMILE GULCH.

Just as the north-northwest continuation of this group of folds in 

sees. 25 and 36 causes a flattening of the dip of the red rock, its 

continuation in the opposite direction has like effects in the Hygiene 

sandstone of the Pierre. If the line of the northwestern continuation be extended in the opposite direction, it crosses the Hygiene 

sandstone in sec, 6, T. 1 N., R. 70 W. At this point the outcrop of 

the sandstone deviates from its general northerly course and trends 

straight northeast, making an offset of an entire mile within the section. (See map, PL II, p. 10.) This is one expression of the tendency to echelon folding. Another is seen in the reduced dip, 18°, as 

compared with 55° on the south and 25° on the north. If the line of 

these folds is extended still farther south, it passes a little west of 

the producing oil wells. The significance of this in the structure of 

the oil field is mentioned below (p. 87).

TABLE MESA FOLD.

Two miles north of Altona all strata younger than the red rock 

strike approximately northeast for several miles, but to the north 

again following their usual northward trends. In this deformation 

no such sharply marked echelon folds were developed in the foothill 

outcrops as are seen farther south. The deformation represented 

by this incipient fold is, however, large. The effect of the same 

stress on the Hygiene sandstone was to make a definite anticline 

whose western limb dips 18° toward the mountains. The sandstone has been eroded from the crest of this fold for fully 3 miles, 

while it remains in the canoe-shaped, syncline west of it. The edge 

of the Hygiene sandstone is not everywhere exposed, but if laid 

bare it would make the long loop indicated on the map, west of
46 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

Haystack Mountain. (See map, PL II. A detailed description is 

given on p. 88.) The axis of this anticline, if extended southward, 

would pass a little east of the producing oil wells. Its significance 

is discussed below.

FAULTS AND LANDSLIPS.

In the foothills of this area there are three large faults and at 

least one small one whose directions differ little from the strike. 

The three large ones which affect the western margin of the Fountain outcrop are shown plainly on the map.

SOUTH BOULDER FAULT.

The fault between the two South Boulder peaks strikes south by 

east from the vicinity of the peaks, but its southerly extension 

curves more to the east, and it dies out in the incipient echelon fold 

mentioned .above. The downthrow, more than 2,000 feet, is on the 

west side, repeating the outcrop of the Fountain sandstone and causing two peaks instead of one. (See fig. 1, p. 12.) The two South 

Boulder peaks will here be spoken of as outer and inner. The outer 

is made by the main outcrop of the Fountain sandstone. The inner 

and higher peak is on the fault block of the same formation. It is 

half a mile southwest of the outer peak.

The fault can be distinctly seen at a point halfway up the valley of 

the small stream which runs south from the peaks. Here its hade is 

to the west. As all other features indicate east-west compression in 

this region, rather than stretching, it is safe to assume that this is a 

thrust and not a gravity fault. This implies that the hade was 

toward the upthrow or east side when the strata were faulted, and 

that it was rotated to its present position by the further tilting of 

the strata. If made since the strata attained their present position 

it must be a normal fault, for the present hade is toward the downthrow side. This would indicate stretching, a supposition which is 

unsupported.

It is difficult to determine how far this fault extends north of the 

peaks, because its farther extension is in the granite. The block of 

red rock is not more than TOO feet deep, being limited below by its 

stratigraphic base, which dips E. 52°, and by the fault plane, which 

dips west. (See fig. 1, p. 12.), As the gorge leading west from the 

outer and more northerly peak is much deeper than this, the fault 

block terminates here and the red rock is not seen farther north.

GREEN MOU:N7 TAIN FAULT.

, Another fault, similar in every respect to the one just described, 

bears the same relation to Green Mountain as the South Boulder
 ENNEMAN.] FAULTS AND LANDSLIPS. 47

:ault does to the inner South Boulder Peak. The Green Mountain 

:ault extends south by east from Green Mountain and dies out in 

the vicinity of South Boulder Peak. Its extension north of Green 

Mountain is in the granite and is not apparent. The width of the 

red rock whose outcrop is thus duplicated west of the main exposure 

is fully 400 feet; the depth of the block is about the same. The 

block is terminated by the deep valley just north o'f Green Mountain. The maximum throw of this fault is probably in Bear Canyon, 

and is apparently less than that of the South Boulder fault. The 

granite strip between the duplicated outcrops of the red rock is 

occupied by streams, as in both the other cases of similar faulting in 

this area.

FOURMILE FAULT.

A third great strike fault is encountered west of Fourmile Gulch. 

For 2 miles north of this point it is made evident on the map by the 

appearance of the red rock in .a narrow belt on its west or downthrow side. North of this it extends an unknown distance in the 

crystalline rock. Its southward continuation runs into the main outcrop of the red rock, south of Fourmile Canyon. The map indicates 

that at this point a block of red rock is offset to the west. This same 

block was at first continuous with the isolated fragments to the north, 

which have subsequently been separated by erosion. The fault has 

therefore the same general direction and the same relation to the 

Fountain sandstone as the two faults south of Boulder.

The fault plane itself is not seen, hence the hade is unknown and 

the depth of the block of sandstone can not be accurately computed. 

The dip here is but 38°, and the width of the sandstone belt west of 

the fault is not over 400 feet. If the fault is vertical, the depth of 

the red-rock block would be but little more than 300 feet. The 

gorges which cross the fault have cut entirely through the block, and 

indeed several hundred feet deeper, dividing it into two widely separated parts and isolating both patches from the main outcrop of the 

Fountain sandstone. Its maximum throw is about 1,100 feet.

FAULT IN SEC. 25, T. 2 N., R. 71 W. ' 

It is significant of the thrust character of these faults that their 

directions agree with those of the axes of the echelon folds. This 

relation is emphasized in the case of a small strike fault 6 miles 

north of Boulder, in the west half of sec. 25. This fault is on the 

west slope of the prominent bald hill referred to above (p. 44) as 

formed of a hard Jurassic sandstone and as marking: the end of the o

syncline where the sandstone is characterized by echelon folding. 

For perhaps half a mile there is a repetition of less than 150 feet
48 GEOLOGY OF THE BOULDER DISTRICT, COLOEADO. [BULL. 265.

of Jurassic strata. This intimate association of fold and fault at 

once suggests that the two were caused by the same force; and that 

each supplemented the -other in relieving the east-west stress. This 

would imply a thrust fault, a supposition which is harmonious with 

all other observations in the district.

DIP FAULTS IN THE FOOTHILLS.

Dip faults in this area are small. In the next section north of 

Sixmile Canyon the basal Niobrara shows at least eight dip faults, 

offsetting the line of outcrop from 10 to 150 feet each sometimes to 

the east, sometimes to the west. The belt occupied by the interfault 

blocks is 300 feet wide. (See fig. 3, p. 44.) The close relation with 

the folds of the locality leaves little doubt that the two types of deformation are due to the same cause and were produced at the same 

time. Similar repeated faulting, on a very small scale, also occurs in 

the Niobrara limestone half a mile south of Gregory Creek. Minute 

faulting and brecciation at places accompanied the crumpling of the 

" crinkled " sandstone of the Lykins. It was intense for some distance north and south of Fourmile Gulch.

POLE CANYON SLIP FAULT.

The apparent dip fault in Pole Canyon is the most prominent 

feature of this nature and is also of great interest. VieAved casually, 

the feature especially noticeable is a jog of 400 feet in the Dakota 

ridge, which on the north side of the stream is small, and disappears 

altogether within a few hundred feet. This jog, which would seem 

to be sufficient evidence of a fault, does not appear in any formations 

older than the Jurassic. Of the younger formations, the Benton and 

Niobrara show no natural exposures on the south side of the stream. 

On the north side the Dakota, Benton, Niobrara, and Pierre appear 

in regular succession. In excavations under the end of the Dakota 

ridge, on the south side, the Niobrara has been encountered in its 

normal position, unfaulted. The Pierre and Benton are also 

unbroken; the upturned edges of the Dakota and Jurassic have 

slipped, forward over the unmoved higher strata. There are two 

fault planes here a vertical one in an east-west position and a horizontal one which is considerably higher than the present bed of the 

adjacent stream. This structure must be called a "fault," though its 

structural value is quite different from that of a fault, as the word 

is generally used. For want of a specific designation it is here 

referred to as a " slip fault." It grades into the landslip, but where 

characteristically represented, as it is here, its true nature must be 

taken into account in mapping. It is characteristic of such faults
1'ENNEMAN.] HUGGINS PARK. 49

that they are (1) superficial and (2) younger than the upturning of 

the rocks in the foothills.

HUGGINS PARK.

A feature intermediate between the slip fault and the more ordinary landslip is'seen at the middle of sec. 36, at the foot of Flagstaff Hill, west of Boulder. About 300 feet above the plain a bench 

or terrace a fourth of a mile wide east and west, and of somewhat 

greater length north and south, is composed of massive beds of the 

Fountain red rock. Near the front of the bench these rocks are 

essentially horizontal, but on the west they curve gently upward 

and become almost vertical. The higher strata from Jurassic to 

basal Niobrara, inclusive, come up to this bench on the north,and on

Fountain Dakota Benton M/obrara Pierre

PIG. 4. Cross section of east slope of Flagstaff Mountain.

the south, and there disappear as if passing beneath the red rock. 

The bench, known as Huggins Park, is a slide of great dimensions, 

whose constituent beds ao*e so massive and so unbroken that on the 

map the red rock is represented as occurring here instead of the 

higher and younger strata which are covered by the slide. In this 

case the basal plane, along which slipping has occurred, is in the 

main a bedding plane, and the phenomenon differs in that respect 

from the Pole Canyon slip fault.

The recency of this event is shown by a study of the outcrops of 

the younger strata which were overridden by the red rock in its 

eastward slipping. The edges of the Dakota, Benton, and Niobrara 

had been eroded back to a position not differing much from that "of 

the corresponding outcrops which have not been thus covered. When

Bull. 265 05 M 4
50 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

the slip occurred these outcropping edges were dragged forward and 

overturned. Under the eastward-facing scarp of Huggins Park one 

of these overturned beds of the upper Niobrara has been found in an 

almost horizontal position 0 (fig. 4, p. 49).

FOOTHILL OVERTURNS.

Just north of Huggins Park the basal Niobrara and its neighbors 

are overturned in a manner similar to that just described, though 

not at the present time covered by the red rock. It is fair to assume 

that at least a part of these beds were overturned by the original movement and their edges dragged eastward. By a careful examination of 

the contacts of a dike at the north end of the park, also, Prof. R. D. 

George has shown clearly that there has been vigorous movement 

along the planes which bound the dike. The relative motion of the 

dike rock has been forward (east) with reference to its hanging wall 

(on its w.est), and backward with reference to its foot wall. The 

drawing out of the micas into long streaks plainly indicates this.

As similar overturning is common in the Niobrara of this area, 

and is sometimes alluded to as " fan structure," implying an origin 

connected with lateral pressure, the significant features of this occurrence north of Huggins Park are here enumerated. They are as 

follows: (1) The actual overturn is very superficial, its depth being 

no greater than the height of the ridge due to the strong stratum of 

the basal Niobrara above the surface of the neighboring shales, 10 to 

20 feet in all. (2) With the exception of this narrow crest there is 

small difference in dip between the nearly vertical red rock on the 

west and the slightly overturned Niobrara shales on the east of the 

basal limestone. (3) The overturn of the limestone increases to the

south, where the outcrop swings off to the east toward Huggins Park.

(4) The overturn is seen half a mile still farther south, under the red 

rock in the eastward-facing escarpment of the park.

As pointed out above, this feature resembles fan structure, and it 

should be so described if interpreted as the result of the profounder 

processes of mountain making. Recourse to these processes is, however, unnecessary to account for any phenomena within the Niobrara 

of this area; on the contrary, many of the most pronounced examples 

of overturn, or fan structure, are clearly the result of mere superficial slipping, aided, no doubt, by frost.

Fan structure occurs most frequently in the Niobrara, but the cause 

lies in the Benton shale west of the Niobrara outcrop. This shale 

occupies the steep eastward slope of the Dakota hogback, and its 

surficial parts may be regarded as having a slow eastward movea The first recognition of this relation was by Judge .Tunius Henderson and H. F. Watts. 

It was first described by Henderson in an article entitled The overturns of the Denver 

basin: Jour. Geol., vol. 11, 1903, p. 584. Fig. 4, drawn by Watts, is taken from this 

article.
FENNKMAN.] FOOTHILL OVERTURNS. 51

ment, due to gravity and frost. This movement has involved many 

landslips, but it has been chiefly effected by creep. The ocular demonstrations of these processes are patches of hummocky landslip 

topography and superficial overturning of beds. Overturns .in the 

Niobrara may therefore be regarded as due mainly to l^lie eastward 

creep of its outcropping, edges, which were pushed forward by the 

Benton from the higher slope. In exceptional cases the outcrops 

have been dragged eastward by landslips.

It is not quite so apparent that the Dakota should have a steeper 

dip than the Fountain sandstone from, causes similar to the ones here 

defined. It may be shown, however, that the phenomenon is, at some 

places, a superficial one. The dip of the block affected by slip faulting in Pole Canyon is 20° steeper than the normal dip of the Dakota 

in that vicinity. Throughout the area the Dakota; hogback is in general a high, narrow ridge, rising much higher above the plains on its 

east side than above the valley on the west. Against the west side, 

therefore, there is constant pressure of the Morrison and Lykins shales 

and sands, whose surface has a general eastward slope and thus a 

tendency to creep eastward, causing pressure against the Dakota 

ridge. Under these conditions it is difficult to see how the strata of 

the latter could entirely withstand the tendency to rotation. Where 

the strata are thickest and the ridge is broadest, and best supported 

on the east, the dip is least steepened.

An example in the Fountain sandstone further illustrates the shallowness of the zone in which the dips may be affirmed to agree with 

those at the surface. West of Boulder, and for a short distance north 

of Sunshine Canyon, the " Red Beds " are vertical. Going north, the 

dip decreases with comparative abruptness to less than 60°. The 

strike continues in a uniform direction, and there is no jog in the outcrop. It follows from this that if these diverse surface dips continue 

to any great depth the Fountain strata are dropped down sharply, 

and form.a deep east-west trough under the city of Boulder a supposition which is not supported. There has certainly been, since the 

middle of the Pierre epoch, a great flattening of the Boulder arch, 

but it is difficult even to conceive of a structural trough in the Fountain sandstone so profound that its dips should be continuous with 

the present dips of the crags around Sunshine Canyon. It is far 

more probable that the steep dips of these exposed crags do not continue far beneath the surface.

SLIP OF THE LYKINS IN POLE CANYON.

The direct effect of gravity on foothill topography may be seen 

at various places along the contact of the Lyons sandstones and the 

Lykins sandy shales. It is well exemplified in sec. 12, just east of
52 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BITLL. 205.

Green Mountain. (See fig. 5, below.) Here the Lyons sandstone is 

covered by a long talus slope hundreds of feet high, composed 

of large blocks derived from the sandstone. The talus slope ends 

where the Lykins begins. Along this line is a trough, or, more 

properly, a ^eries of pits, often 20 feet wide and 8 feet deep. Into 

this irregular trough the lower margin of the talus has fallen. The 

steeper slope thus made at the foot of the talus has not yet had, time 

to be reduced, and many blocks are in an unstable position. Some 

have been broken or turned over, and expose fresh surfaces, which 

contrast with the older weathered or lichen-covered surfaces. SimiFIQ. 5. Lykins formation in Pole Canyon slipping away from the Lyons sandstone.

lar occurrences are found at various points between Pole and South 

Boulder canyons.

In explanation of this phenomenon, it is not necessary to assume 

any other force than gravity. The slope eastward from the contact 

of the Lyons sandstone and the Lykins is steep and is underlain by 

shale or sandy shale. At the foot of this steep slope, a fourth of a 

mile away, a young, deep valley has been cut behind that portion of 

the Dakota hogback which has been swung forward by slip faulting. 

(See p. 48.) Similar opportunities for the slipping eastward of the 

Lykins exist at other points, and are accompanied by the peculiar 

evidences of slipping described above. The tendency would, of 

course, be greatly aided by any continuance of the mountain uplift.
I.'ENNKMAN.] FAULTS AND LANDSLIPS. 53 

FAULTS OF THE PLAINS AREA.

The generally horizontal rocks of the plains are broken by faults 

much longer than those in the foothills. The throws are, however, 

very much smaller, rarely exceeding 300 feet. Most of the plains 

faults within this area converge at Marshall and have their chief 

interest in connection with coal mining. Any description of them 

must be little more than a repetition of the excellent account given 

by Eldridge.0 They have been traced again in the field and plotted 

with some detail on the large-scale base map used in this survey. 

All are far from straight, and their courses are necessarily represented in a generalized way. The actual line of a fault can rarely be 

seen. Commonly its location is fixed only within several hundred 

feet by outcropping rocks, whose relations indicate a fault somewhere 

within the limits named. Fragments of crushed and slickensided 

rock may define the location more narrowly. In two cases faults can 

be seen in mines. The faults enumerated in Eldridge's " Marshall 

subsystem " and two of his " North Boulder faults " are here briefly 

described under the names given by him.

The main or northern fracture of the Marshall subsystem trends 

southwest along the northern base of Davidson Mesa to its west end, 

south west of which its course.is more southerly to beyond the middle 

of sec. 21, where it is lost. Its northeastern extension also runs in 

the same direction as its southwest end. It passes half a mile west 

of Burnt Knoll, northeast along the line of Dry Creek, and is lost. 

The fault plane, where observed by Eldridge, in a mine now abandoned, di-ps S. 28° E., at an angle of 45°. As the upthrow is 

on the southeast side, this is a thrust fault. The maximum throw 

along its middle course is about 350 feet.

The middle fracture branches off from the main fault near the 

railroad station at Marshall and follows a northeasterly course, 

though at its southwestern end the trend is more east than north. 

Its course is curved similar to that of the northern fault, but the 

curve is less sharp. The line of the fault crosses the highest part of 

the mesa near its west end, but farther northeast it follows the foot 

of the mesa. The fault itself can be seen in a cut of the railroad 

switch, which encircles the mesa at its western base. The maximum 

stratigraphic throw of this fault is near its eastern end, and amounts 

to 180 feet. The downthrow is on the south, so that the fault block 

on the north has been relatively lifted up between its neighbors.

The southern fracture is approximately parallel to the middle one 

and distant from it one-fourth to one-third of a mile. It follows 

a gently sinuous course along the south side of the valley, and south

0 Mon. U. S. Geol. Survey, vol. 27, 1896, pp. 128-136.
54 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BOLL. 265.

of the Colorado and Southern Railroad, but crosses the valley about 

the middle of the south side of sec. 15, ascends the mesa to the northeast, and is lost to view in about half a mile. Its southwest end 

approaches the main or northern fracture near the middle of sec. 21, 

but here a strong monoclinal fold appears and probably takes the 

place of the southern fault at its westerly end. The downthrow is 

on the north and reaches 260 feet. Nearly midway between the 

middle and southern fractures is another fault of similar direction 

with downthrow of a few feet on the north."

The block between the middle and southern fractures is cut by the 

" terminal cross fault," and its west end dropped down about 30 feet. 

This fault lies a little, west of the middle of sec. 15. It trends nearly 

southeast, but turns more nearly to the south at its southern end, 

where it seems to cross the southern fracture. It is no doubt the 

same fault which is encountered in the Gorham mine of the Northern 

Coal and Coke Company, where the drop on -the west side is 10 feet.

The canal fault (so named by Eldridge) is well seen in the deep 

railroad cut in sec. 13. It runs almost east and west, and has a 

vertical downthrow on the north of about' 50 feet. The massive 

sandstone on both sides is basal Laramie.

Two of the group of faults appearing on the north side of Boulder 

Creek lie partly or wholly within the area discussed in this report. 

No. 1, with upthrow of 180 feet on the east side, brings up the Fox 

Hills shale 1 in the east halves of sees. 12 and 13 at the eastern 

edge of the area mapped. The opposing stratum on the west is 

basal Laramie. No. 2 has 60 feet of upthrow on the east, the opposing strata being Laramie on both sides. All trace of these faults 

is lost under the alluvium of the valley on the south and also under 

the surface of Gunbarrel Hill on the north.

GEOLOGIC HISTORY.

PRE-FOUNTAIN TOPOGRAPHY.

The geologic history of the Fountain sandstone is considered with 

especial reference to the topography of the preexisting land surface. 

The uneven contact between the Archean and Fountain rocks is 

understood to represent the subaerial topography of Fountain time 

as modified by shore erosion during gradual submergence beneath 

the sea. The principles by which the character of. the early topography and of the marine denudation is inferred from the present 

features can not here be discussed .in detail. 6 The observations

"The writer's attention was called to this smadl fault by Mr. H. F. Watts. 

6 See the writer, Effect of cliff erosion on fornji of contact surfaces : Bull. Geol. Soc. 

Am., vol. 16, pp. 205-214.
FKNNKMAN.] GEOLOGIC HISTORY. 55

given below, however, point .to the conclusion that during submergence the form of the old land surface was much changed by 

wave erosion.

The Archean-Fountain contact differs from a plane only when 

broad areas are considered. The undulations of the contact line 

shown on the map give an exaggerated idea of the unevenness of 

the Archean surface, but, ignoring the effects of faults, .most of the 

deviations from straightness are due to the irregularities of the 

present surface.

In the main the proof of the inequalities of the Archean floor is 

the varying thickness of the red rock. A casual look at the map 

may be somewhat misleading as to the amount of such variation, 

for at Boulder, where the band is narrowest, the beds are vertical; 

toward the north the dip decreases, and where the outcrop is very 

wide, the dip is but from 7° to 30°. The thickness of the LyonsFountain sandstones varies from 500 feet at Boulder to pOSSlbly 

1,500 feet at Fourmile Canyon and at Bear Canyon, 3 miles south 

of Boulder. At Lefthand Creek, 3 miles north of Fourmile -Canyon, 

it does not exceed 1,000 feet.

If the eastward tilting due to mountain-making forces were undone 

and the red rock removed, the exposed Archean surface would show 

only very smooth, broad undulations. In this area the slopes would 

not be less than 3 miles long, would have a grade, except at one place, 

not exceeding 150 feet to the mile, and would be entirely unbroken by 

valleys. The one exceptional feature of the relief would be a hill, 

or east-west ridge, at Boulder, rising possibly 1,000 feet above the 

valleys north and south of it, but having a base 6 miles wide and 

perfectly smooth slopes. This hill or ridge, however, marks a wellknown line of weakness, along which there has been frequent uplifting; it does not, therefore, represent the relief of the floor upon 

which the Fountain sandstone was laid; it was more probably 

being elevated during the deposition of the latter, or, rather, it was 

subsiding less rapidly than the areas north and south. (Compare 

p. 64.)

The Archean rocks below the Fountain are unweathered. Those 

which are in immediate contact with the red rock do not differ in 

this respect from those which occur at a greater distance back in the 

mountains. In like manner, the felspathic constituents of the stratified rocks are essentially unchanged. The sediments also contain 

many unweathered rock fragments. The material composing much 

of the red rock is not unlike the disintegrated granite which now 

covers many of the steepest hillsides, with this exception, that it has. 

been to some small extent transported by water. Nevertheless, these 

sediments are in the main but little waterworn and poorly assorted. 

They are such as should be found near a land which is being actively
56 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

eroded by streams as well as by waves that is, where erosion is relatively active as compared with weathering; the more arid the 

climate the less active need erosion be in order to yield such waste. 

Such sediments belong to a shore where trituration is not prolonged, 

and where the bottom subsides so rapidly that the particles are carried downward into quiet water before they have been rounded or 

broken up.

The above features of form and material suggest that the configuration of the present contact is the result of a considerable 

change in the topography of the old land. , The absence of smaller 

irregularities on the broad swells that slope 150 feet to the mile and 

an unweathered rock bed would not both characterize an area subjected to subaerial erosion. If the former land surface was a peneplain its deeply weathered surface could not have supplied the large 

proportion of feldspar fragments in the red rock, and its slowly 

flowing streams could not have transported them from the higher 

land on the west. On the other hand if the surface had its present 

relief its long slopes would have been cut into sharp hills and valleys. 

The invading sea, therefore, must have shaved from the surface 

enough material to obliterate all the relief except the major features, 

which have been left in a very generalized form.

SHORE EROSION DURING SUBSIDENCE.

The form of the shore line of an advancing sea is indicative of 

the form of the sea bottom near the shore. This is apparent from 

the fact that the shore line at any stage of subsidence is the highest 

contour of the sea bottom and is similar to the other contours, each 

of which was, in its .turn, the shore line. When the. submerged surface, therefore, is a plane, whose contour lines are straight, the shifting shore line is known to have been without bays. When the sea 

bottom is undulating, the shore line of the advancing sea was characterized by bays and headlands.

During the invasion of the Fountain sea, therefore, the shore line 

was not straight (since the surface of denudation is not a plane), 

but there has been found no trace of bay heads where filling was 

in progress. The universal freshness of the underlying granite 

is evidence that cliff cutting occurred everywhere. The bays that 

existed did not extend so far inland as to preclude erosion at their 

heads. While this universal cliff cutting was taking place the surface rocks were being removed from both hills and valleys, though 

the erosion was greater on the hills than in the valleys. Between 

headlands 6 or 8 miles apart the water would have been possibly 500 

feet deep. These dimensions, which follow from measurements 

given above, are perfectly consistent with the assumption that the 

bays were so wide open as to allow wave erosion at their very heads.
PBNNHMAN.] SHORE EROSION DURING SUBSIDENCE. 57;

The supposition is a long way within the possibilities of shore 

erosion.

From the above-stated relation of the shore line to the contours 

of a submerged surface it follows that if a retreating shore line is 

a. sinuous cliff the sea bottom will have a fluted surface. There will 

be ridges along the lines of retreat of the headlands and furrows 

where the bay heads receded. If there is suitable offshore slope and 

rate of subsidence, even the coarse waste may be laid down in beds, 

which are continuous over these ridges, instead of abutting against 

them. All these conditions occur along the lower contact of the 

Fountain sandstone. Fine flow-and-plunge structure is seen where 

it should be expected that is, in the thickest part of the formation 

or within the bays of the Fountain sea.

All the undulations of the Archean floor might be due to gentle 

crustal folding during Fountain time, This hypothesis also would

provide for the continuity of the " Red Beds " over the eminences. It 

would make unnecessary the assumption that hills stood out as promontories in the advancing sea and that the shore line was crenulated. 

It does not, .however, account so well for the general cross-bedding, 

and for the flow-and-plunge structure that so plainly characterizes 

the places which, on account of the greater thickness of the " Red 

Beds," are regarded as bays. Except in the vicinity of Boulder there 

is also a lack of collateral evidence of such crustal deformation. The 

hypothesis of warping during sedimentation might be used to support the theory of a pre-Fountain peneplain, but it has already been 

pointed out that the character of the Fountain sediments does not 

admit of the assumption that they were derived from a peneplain. 

The Boulder arch itself was probably slowly bulging upward while 

the " Red Beds " were being deposited. The evidence on this point 

is given under the discussion of the next higher formation. (See 

p. 61.)

DEPOSITION OF FOUNTAIN FORMATION.

Generally speaking, the local thinning of the red rock is due 

to thinning of all the beds rather than to a disappearance of a part 

of them. Except in the vicinity of Boulder the topmost strata of 

the formation can be traced continuously from point to point along 

the line of towering crags, which form the most conspicuous feature 

of the foothills. The Lyons sandstone shows the same .continuity 

at a higher horizon, and the strata at the base of the red rock are 

almost or quite as continuous as those at the top. If the contact is 

followed from canyon to canyon over the intervening spurs, the basal 

ledge of the red rock,may frequently be seen for a mile or more. In 

almost all such cases the continuity of the stratum at the base is 

noticeable, although this is not quite universal, as there seem to be
58 GEOLOGY OP THE BOULDER DISTRICT, COLORADO. [BOLL. 265.

a few places where the beds at the base abut against a low rise in the 

granite surface. At such places the contact appears to shift to successively higher beds of the red rock.

DEPOSITION OF LYONS SANDSTONE.

The change which brought about the deposition of the Lyons sandstone was comparatively abrupt. Only a short time after the deposition of rough arkose, the sediments were composed of thoroughly 

assorted clean-washed quartz sand. The details of structure also 

changed to a neat cross lamination, producing'flags of a uniform 

thickness and large area. Most of these flags which are now exposed 

dip east at a smaller angle than the true bedding; that is to say,

PIG. 6. Position of cross-bedding of the Lyons sandstone. 1. Lykins formation. 2. 

Lyons sandstone. 8. Fountain sandstone.

if the true beds were again horizontal the cross-bedding would dip 

west, or toward the old shore. (See fig. 6.)

BAYS AND BARS.

The nature and peculiar bedding of these sediments indicate that 

the shore line was indented on the west by a bay, and that this bay 

was spanned by a bar of clean sand the formation known as the 

Lyons sandstone. Most of the sandstone now exposed was deposited 

on the landward side of the bar and dips toward the land. The area 

here discussed embraces parts of two such bays, separated by a 

salient point between Boulder and Gregory creeks. (See fig. 7, p. 60.) 

In front of this headland the peculiar sediments ascribed to bar 

building are absent.

Not only are the bar sands absent from this locality, but the headland seems to have been eroded and to have furnished a part of the 

material for the bars. It can not be stated beyond doubt that there 

was such erosion, because, at the place concerned, the upper beds of 

the red rock are poorly exposed. It is certain that just west of
FEJJNEMAN.] DEPOSITION OF LYONS SANDSTONE. . 59

Boulder the red rock is much thinner than it is a short distance 

to the north or south. From appearances it is probable that this 

decrease in the thickness of the formation is due in part at least 

to thinning of the individual beds, but it is not plain that the whole 

difference can be thus accounted for. In so far as the upper beds 

fail to cross the arch," it is most consistent with the physical history 

to assume that they were eroded from the headland which, without 

doubt, projected at this point when the bars of the Lyons sandstone 

were being deposited. Cliff erosion on the salient would certainly 

be expected from the relative position of the headland and bar.

These bays could not have resulted from the continued submergence 

of an uneven land surface. The headland at Boulder was plainly due 

to warping resulting from north-south stress. It corresponded in 

position to the Boulder arch, which has been subjected to repeated 

upward warping. As the sediments became thinner at this place and

as a headland appeared immediately after their deposition, while 

sedimentation continued to the north and south, the cause of all these 

phenomena was no doubt crustal deformation.

The conclusion that the Lyons sandstone is the sediment of bars 

rests finally upon the position of its cross lamination, which in general 

dips toward the shore. This necessitates either the above-stated 

hypothesis of bars or that there was land farther east from which 

sediments were derived. The supposition that there was land farther 

east is entirely without support. The hypothesis that the sands were 

laid down in the form of bars is supported by the nature of the 

sands and by their absence at the most probable headlands, from 

which, in part, the shore drift should have been derived.

It is to be remarked that the dip of the cross-bedding at Gregory 

and Boulder canyons, where the formation abuts against the supposed headland, is away from the mountains. The Lyons sandstones 

exposed at Boulder and Gregory canyons are in immediate contact 

with the old headland, and therefore represent the seaward side of 

the bars. From the nature of the contact of .a bar with the salient 

from which it springs, it is the seaward front and not the inside or 

landward slope of the bar that is tangent to the land. (See fig. 7, p. 

00.) Whatever cross-bedding existed on this seaward front must, of 

necessity, have dipped seaward. Therefore the relative eastward 

dip of the cross-bedding at these places is in exact accord with the 

theory of the deposition of these sands in the form of bars.

The supposition that two bays were separated by a salient between 

Boulder and Gregory creeks rests upon (1) the absence of bar sediments at that point, (2) the probable erosion of the topmost red

" See Eldi-idge, G. H., Geology of the Denver basin: Mon. U. S. Geol. Survey, vol. 27, 

1896, p. 110.
60 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BIJLI, 265.

rock, (3) the eastward dip of the cross-bedding of the beach sands 

nearest to this point.

Bay

-> 

Head/and at Bou/c/er

Fro. 7. Shore line during the deposition of the Lyons sandstone.

From the above reasoning it follows that the Lyons sandstone 

should be intermittent in its distribution alongshore after the 

manner of bars. This is true. They should also form a relatively 

narrow reef-like zone, and, if followed outward from the old land, 

the direction of dip of the cross lamination should be reversed. 

Along this line no direct evidence is at hand.

MIGRATION OF BARS.

A bar extending across a bay retreats landward with the recession 

of the cliff from which it springs. This retreat is accompanied (1) 

by progressive deposition on the landward side of a series of layers 

which dip toward the land, and (2) by erosion on the seaward side, 

which results in the removal of the upper part of layers previously 

deposited.0 (See fig. 8.) The first effect is exemplified here 

in the prevailing dip. The second is beautifully shown wherever 

there is a good contact between the Lyons sandstone and the next 

higher formation. The bottom of the Lykins is occasionally exposed 

as a clearly defined surface against which the clean-cut flags abut 

at an angle which may reach 36°. (See fig. 6, p. '58.)

B'IG. 8. Section of a linear embankment retreating landward. The dotted line shows 

the original position of the crest. (After Gilbert.)

This process of building bars of clean-washed sand, which are 

shifted gradually landward and truncated, implies a considerable 

stability of the land with reference to the sea level, though it is not 

prevented by moderate sinking. In the epoch here considered, not 

only the well-assorted, clean-washed, creamy sands, but also the shaly 

sands which follow, indicate quiet conditions and freedom from 

crustal deformation.

0 Compare Gilbert, G. K., Lake Bonneville : Mon. U. S. Geol. Survey, vol. 1, 1890, p. 53. 

The conditions here described are discussed by Gilbert, as exemplified In the mole Inclosing Toronto Harbor. Fig. 8 is taken from this discussion, p. 56.
FENNEMAN.] GEOLOGIC HISTORY. 61 

LYKINS SEDIMENTATION.

While the bars of Lyons sands Avere being truncated sediments 

could not be deposited on the truncated base unless they Avere .coarser 

than the sand Avhich had been removed. Later, however, this surface 

subsided to such a depth that it Avas buried by a sandy and highly 

ferruginous mud.

The deposition of the other sediments of the Lykins formation 

required similar depth, and during this time there was nothing 

peculiar in the shore conditions. Probably at this time while the 

sea bottom Avas depressed so that mud Avas deposited in places from 

Avhich sand had been removed, the relief of the land was reduced, 

and thus the supply of coarse sediment Avas checked. During much 

of Lykins time the sea Avas so deep that ripple marking and crossbedding did not take place, but toward the close both are apparent.

LYKINS WARPING.

At Twomile Canyon the cross-bedding near the top of the series 

again indicates deposition on the landAvard side of a bar. It may 

be inferred from this that headlands Avere again being eroded, and 

this assumption is borne out by the nature of the sediments, which 

are coarser near the top. The headlands Avere probably caused in 

part by a gentle Avarping, Avhich harmonizes Avith indications of 

crustal movement at the close of the Lykins, Avhich may have 

caused a slight unconformity between the rocks of this epoch and 

those of the Morrison. The broAvn sandstone at the top of the Lykins 

is not of uniform thickness. (See p. 25.) The exceptional abruptness, also, Avith Avhich at many places the highly ocherous rocks give 

place to the gray above, at least raises the question Avhether there 

might not have been a transitional stage whose record is lost. As 

pointed- out in the description of the stratigraphy, such beds, transitional in color and texture, appear in a number of places.

Uplift of the Boulder arch. While clear and definite indications of a general unconformity at the top of the Lykins are wanting, there is decisive evidence that the Boulder arch was uplifted 

and eroded. The " crinkled " sandstone approaches to within a few 

hundred feet of the south side of Gregory Canyon, maintaining about 

its usual distance above the Lyons. The same conditions are found 

on the north side of Boulder Creek, but for more than half a mile 

north of Gregory there is no trace AvhateA7er of the Lykins. The 

uplift of the fold north of Gregory Creek Avas almost equal to the 

combined thicknesses of the Lykins and the Morrison. The former 

is normally from 600 to 800 feet thick and the latter is not less than 

150 feet. Only the topmost beds of the Morrison probably covered 

the arch and are concealed beneath the talus of red rock.
62 GEOLOGY OF THE BOULDEE DISTRICT, COLORADO. [BULL. 265. 

MORRISON SEDIMENTATION.

The Jurassic in this vicinity is represented by but one formation, 

the Morrison. The physical history of the period is not well indicated within this area. Fossils have not been found here, but those 

collected at other places show that the water of the Morrison sea was 

fresh. On the north and south, therefore, the ocean was shut out by 

land barriers extending. east from the mountains to other land at 

an unknown distance.0 At first the Boulder arch stood less than 

100 feet above the water, and its height above the sea was gradually 

reduced by subsidence or by a change of the lake level. The arch was 

probably covered by the upper beds of the Morrison.

At a distance from the vicinity of the Boulder arch, the clays at the 

summit of the Jurassic have a nearly but not altogether constant 

thickness. If present at all between "Gregory and Boulder canyons, 

they are very thin. Erosion is plainly indicated. The fresh-water 

lake gave way to land. Paleontologic evidence "taken from broader 

areas indicates that this area was land through the whole of Lower 

Cretaceous time. The relief of this land surface was low. This is 

inferred not only from the fact that no valleys appear in its present 

buried surface, but also from the nature of the sediments when it was 

again submerged.

DAKOTA SEDIMENTATION.

At the beginning of Upper Cretaceous time the sea again covered 

this region. The first bed of sediments laid doAvn contains much 

sand, but consists largely of very resistant and generally well-rounded 

pebbles of quartz, quartzite, jasper, and chert. The selection and separation of such materials from the waste of the previous land surface 

are consistent with low relief and deep weathering.

These facts are best explained by assuming that these residual pebbles of very, resistant minerals were embedded in the surficial deposits 

of the submerging land. The soil sheet was worked over and 

assorted by w.ave action and these large pebbles deposited near shore. 

An unknown quantity of such deposits, brought down by streams from 

the land to the west, is doubtless mingled with the material derived 

from the submerging mantle rock. The finer and much more abundant products of decomposition were necessarily deposited farther 

seaward. According to this explanation this conglomerate is a shore 

phase of the lowest Dakota beds.

. Mingled with the hard pebbles are occasional fragments of rocks, 

such as limestone, clay, and feldspathic rocks, that would readily 

yield to weathering and wear. These may have come in part from

a Etnmons (op. cit, p. 24) suggests that the Morrison lake may have extended from 

near the northern boundary of Colorado to the lutHu.dt? of the Katon Hills.
PENNBMAN.] GEOLOGIC HISTORY. 63

cliffs that were cut from low hills of the old land surface. They also 

suggest what they alone would not be sufficient to prove, namely, that 

the oncoming of the sea was accompanied by some warping and steepening of the land; but of this there is other evidence, as follows:

Before the close of the Jurassic the Boulder arch was covered by 

sediments, which were not entirely eroded away during the succeeding land epoch, showing that the land must have been very near the 

sea level. The absence of the lowest Dakota near Boulder, however, 

shows that just before the Dakota sediments were laid down elsewhere there was a differential subsidence amounting to a renewal of 

deformation along the old line.

BENTON SEDIMENTATION AND UPLIFT.

As a result of deepening of the sea, decrease in the relief of the land, 

and gradual removal of the shore, the Dakota sands were followed by 

the Benton shales, not suddenly, but after some alternation of sediments. The arching was again strongly renewed, this time between 

Sunshine and Boulder canyons. Erosion cut away the entire Dakota, 

but there is nothing to show whether the Jurassic was further wasted. 

The uplift remained above water until very near the close of the Benton epoch. Only the very latest beds of the Benton are continuous 

over it. The amount of uplifting at this time was therefore approximately equal to the combined thickness of the Dakota and Benton. 

The average thickness of these formations north and south of Boulder 

is not less than 700 feet.

NIOBRARA SEDIMENTATION.

The deposition of the Niobrara was not accompanied by noteworthy physical conditions, except those attributable to diminishing 

sediment, removal of shore, or deepening water. There is no evidence 

in this area of any disturbance at its close.

PIERRE SEDIMENTATION AND FOLDING.

As a single mass of shale, the Pierre is extraordinary on account 

of its great extent and thickness. A. discussion of the factors adequate to produce these results would not be possible from observations 

in this small area. It is safe to assume that during Pierre time there 

was a steadily sinking sea bottom and a steadily rising land that 

had a relief sufficiently low to allow thorough decomposition. The 

sinking area on the east and the rising area on the west were necessarily separated by a monoclinal fold of increasing height. It is 

highly probable that this fold lay along the line of the present foothill belt and that although its dip was more gentle than that of the 

present great monocline it determined the line along which the forces
64 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BOLL. 265.

of mountain.making later gave form to the steep monocline which 

now follows the eastern border of the Archean belt.

The record of changes on the Boulder arch continues legible during a considerable fraction of Pierre time. The arch continued to 

rise relatively that is, it sank less rapidly than the adjacent areas. 

This is shown by the position of the Hygiene sandstone. At Boulder 

this stratum is separated from the Niobrara by barely 1,000 feet of 

Pierre shale. Five miles farther north the same shales are 3,000 

feet thick, showing a relative rise of the Boulder arch amounting to 

2,000 feet during the first half of the Pierre epoch. The record of 

the latter half of Pierre time is not clear, because there are not 

enough outcrops to allow satisfactory determinations of thickness 

over the Boulder arch. The shale-making conditions were not uniform throughout the epoch. The deposit of so thick and continuous 

a sandy stratum as the Hygiene sandstone probably indicates a temporary dominance of the uplifting forces over subsidence.

FOX HILLS SEDIMENTATION AND LARAMIE UPLIFT.

There were no noteworthy physical changes at the beginning of or 

during the Fox Hills epoch, except variations similar to those which 

occasioned the sandy layers in the'Pierre. The Laramie indicates a 

general uplift, both by the coarser character of its sediments and byits beds of coal.

SUMMARY OF HISTORY OF THE BOULDER ARCH.

With the exception of the growing monocline which separated the 

sinking sea bottom on the east from the rising land on the west, the 

Boulder arch is the index of all the crustal deformation definitely 

recognized as occurring in this area before the Rocky Mountains were 

formed. It is noteworthy that the forces which caused it acted in 

a north-south direction. In the later deformations east-west forces 

were completely dominant.

From the evidence given above, the successive rises of this east-west 

anticline may be summarized as follows: During the Boulder and 

Lyons epochs there was a rise which may have reached the maximum 

limit.of 1,000 feet. (See p. 55.) The axis of this movement was south 

of Boulder Creek. At the close of the Lykins there was a further elevation, approximating 850 feet. (See p. 61.) In the interval between 

Jurassic and Dakota there was a relative elevation whose minimum 

may be placed at 25 feet. The uplift at this time may have been broad, 

extending from south of Gregory Creek to a mile north of Sunshine 

Canyon. At the close of the Dakota the minimum uplift was TOO 

feet along an axis between Sunshine and Boulder creeks. In the 

first half of the Pierre epoch the further rise amounted to 2,000 feet.
B-ENNHMAN.] » GEOLOGIC HISTORY. 65

Maxima are given in some cases, and minima in others, bit the sum 

of all the above amounts, which is 4,575 feet, may ba taken as an 

approximation to the total elevation of this anticline.

An examination of the geologic map shows that there has been 

a great depression of the Boulder arch since the formation of the 

Hygiene sandstone. If the strata of the foothills and the plains 

were now deprived of their eastward dip, the upper beds would 

show a pronounced east-west syncline on the site of the former anticline. Even in the lower beds little of the former east-west anticline 

remains. Since the Hygiene sandstone is the youngest formation that 

gives evidence concerning the age of Boulder arch, the date of this 

flattening can not be fixed within narrow limits. A similar arch, 

stretching eastward from Golden, about 17 miles to the south, can be 

studied in more detail, because of the. presence of younger strata.

Emmons 0 and Eld ridge 6 held that the flattening of the Golden arch 

may have occurred in Denver time, and perhaps was due in part to 

the extravasation of basalt along the line of the old anticline. The 

two arches had almost the same history, and the sites of both are 

marked by volcanic extrusions. Cross assigns the eruptions to the 

same epoch. Whatever causal relation exists between the extrusions 

near Golden and the 'flattening of the Golden arch may also be 

assumed to exist between the Valmont dike and the depression of the 

Boulder arch.

POST-LARAMIE HISTORY.

Post-Laramie history is recorded in the deformations incident to 

mountain making and in the forms due to erosion. The time of most 

of the events can not be accurately determined by local evidences.

Topographic features due to erosion are presumably in the main 

Quaternary. They indicate continuous degradation that has resulted in a series of river terraces. With the exception of the small 

fragments of the loess sheet (see p. 35) and a few isolated alluvial 

fans, there are no fluviatile deposits except those left by degrading 

streams. There is no broad sheet, such as that of the High Plains,0 

implying an epoch of stream aggradation. Everywhere the stratified 

rocks of this margin of the plains have been neatly pared off by meandering streams. The sheets of gravel which cover the planed surfaces 

are such as down-cutting streams should be expected to leave.

 " Mon. U. S. Geol. Survey, vol. 27, p. 50. 

" Id., p. TOO.

"Johnson, W. D., The High Plains and their utilization: Twenty-first Ann. Kept. TJ. S, 

Geol. Survey, 1901, pp. 651-656.

Bull. 265 05 M 5
66 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

No features in this area require for their formation streams larger 

than the present ones. The present Boulder Creek is a degrading 

stream, with moderate meanders, and a broad flood plain sloping 

toward the stream. Where it reaches the bluffs which front its terraces the river is cutting into the bed rock and planing off a surface 

on a level with the bottom of its channel. On this planed surface it is 

depositing a sheet of heavy gravel and finer alluvium, similar both in 

character and thickness to the sheets which everywhere cover the 

mesas and other terraces. (For a more explicit comparison see p. 

14.) That the streams which caused the present plains topography 

were degrading their channels is plainly shown by two facts: (1.) 

The debris sheets have a uniformly limited thickness, and (2) the terraces always slope toward the streams to which they were flood plains. 

This continuity in the cutting by the streams, gives to.the erosional 

history of the district a unity which makes subdivision impracticable.

This uniformity of erosional history near the foot of the mountains is in contrast with the more complex history of the plains farther east," where the denuding processes have plainly been interrupted by aggradation one or more times. The obvious inference is 

that the entire recorded erosional history of this narrow belt must be 

matched with the last epoch recorded on the plains, which is one of 

stream degradations.''

It is entirely natural that when the character of streams is changed, 

whether from aggrading to degrading, or the reverse, the belt nearest 

the mountains should be the first to suffer complete obliteration of the 

records of former conditions. The rivers may recently have acquired 

increased cutting power by an uplift on the west that increased their 

grade, or by an increase of rainfall, or, as Johnson suggests, by a 

mere cooling of the climate, which would decrease evaporation. In' 

any case the upper course must first be cut down and in this the 

records of former aggradation will first be erased. If the change 

of stream habit be the reverse, the prompt burial of features near 

the mountains is equally well understood. If the climate is moderately arid, as in the area under consideration, there is an additional 

reason for the prompt erasure of aggradation records near the mountains, as running water is more abundant j:here, many streams actually 

losing volume as they proceed.

« Johnson, W. D., op. cit, p. 628; also Etntnons, S. F., op. cit, D. 255.

6 As to the geologic data of this epoch Johnson says (op. cit., p. 630) : "The beginning 

of the final and present degradation, during which the smoothness of the Great Plains 

has been in large part destroyed and their surface lowered, with the exception of the sodcovered plateaus of the central zone, doubtless dates from the opening of that period of 

climatic oscillations in the Pleistocene which, in the Great Basin region of Utah and 

Nevada, gave rise to repeated floodings of large areas and the creation of lakes."
FKXXKMA.X.] GEOLOGY OF THE BOULDEE DISTRICT, COLORADO.. 67

ECONOMIC GEOLOGY.

WATER. 

SURFACE ZONE.

Ground water in the area under consideration is found at moderate 

depths. Over the well-drained parts of the plains it is commonly 

encountered at depths of from 10 to 20 feet. Since the bed rock is in 

large part relatively impervious shale, the slope of the ground-water 

surface is very similar to the slope of the ground.' One bluff 40 feet 

high is quite dry at the base, but on the terrace above, less than 200 

feet from the bluff, water is found at a depth of 12 feet. The surface 

shales are everywhere more permeable than the unweathered rock 

below, and the movement of water in-them is freer than at greater 

depths. In drilling oil wells, therefore, it is found necessary to 

case off this surface water.

DEEPER ZONE.

Below this superficial zone, Avhich is 25 to 50 feet thick, the shale 

is so dense and the consequent movement of the water so slow that 

wells are spoken of as " dry." At irregular intervals within the 

shales, as deep as the formation has been explored in drilling for oil, 

are sandier beds of limited lateral extent. In these the movement 

of the water is more rapid, and after drilling through them it is 

sometimes necessary to case off the water from these deeper sources. 

When the character of the water is suitable, these beds might afford 

supplies for deep wells, but the water is not infrequently slightly 

saline, and it may be a brine. Flowing wells are not obtained from 

these sources in the Pierre shales, nor are the}7 expected, on account 

of the limited lateral extent of the sands and their close texture where 

unweathered.

FLOWING WELLS.

South of Burnt Knoll, about 0 miles southeast of Boulder, ti faint 

flow of water was obtained from the Laramie at a depth of SO feet. 

A similar flow was obtained at Lafayette, 11 miles east of the foothills, at various depths between 400 and 700 feet. It was encountered while drilling for oil.

At the north end of the field the Stuart well, drilled for oil, 

encountered abundant water in the Morrison formation. At last 

accounts (April, 1904) the well was still flowing. It is located on 

the Pierre shales, one-half mile east of the Dakota outcrop (SW. £ 

of sec. 5, T. 2 N., R, 70 W.). The dip of the beds at the surface is 

here 26°. The succession of strata passed through leaves no doubt
68 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

that the Morrison was encountered at a depth of 1,336 feet. If all 

the beds passed through dip at an angle of 26° and have the thickness 

given above, under the heading "Stratigraphy" (pp. 20 et seq.), 

the Morrison should have been found at a somewhat greater depth 

than 1,336 feet. Probably at a short distance below, the surface 

the dip is less than at the surface*, where it has been steepened 

by the process described above as tending to produce a fan structure. The water is from a horizon about 60 feet below the top 

of the Morrison; it is therefore from one of the upper beds, probably from the " Saurian " sandstone. This supposition is supported 

by the fact that the water comes from yellow sand. Presumably 

the lower and larger Morrison sandstones, and especially the basal 

stratum, would yield water still more copiously. This assumption 

is supported by. observations on the small stream flowing east over 

the Lykins and Morrison outcrops in Lykins Canyon. The channel 

when examined was dry as far as the outcrop of the Morrison. At 

this point water appeared within a few yards and there was a flowing stream. No well, however, has reached these lower beds, the 

Stuart being the only one which is known to have entered the 

Dakota. The high dip of the strata makes it impracticable to obtain 

water from even the best beds of the Morrison except within»a narrow strip east of the foothills.

WATER IN THE DAKOTA.

Water was obtained in the Stuart well from the Dakota, but not 

.in amount to be compared with that from the Morrison. The relations of the Dakota are not favorable to any great water supply for 

northern Colorado. The hardness of the formation causes it to 

outcrop in a single rocky ridge, which quickly sheds the precipitation and is unfavorable to percolation. True, the outcrop is crossed 

by streams flowing from the mountains, but even this intake must 

yield a small supply on account of its extreme^ restricted area. 

Moreover, the Dakota sandstone is practically sealed beloAv by relatively impervious shales and clays, and therefore can not receive 

much of the water that penetrates the lower strata which- outcrop 

west of the Dakota hogback. If the Dakota of northern Colorado 

be compared in these various respects with other well-known waterbearing strata for example, the Potsdam of central Wisconsin, or 

even the Dakota sandstone of the Black Kills its area of intake 

will be seen to be very small and topographically unfavorable to 

percolation. The slow inflow into the Stuart well from this formation also indicates that it is denser than the sandstones of the 

Morrison. -The Morrison sandstones, though much thinner than 

the Dakota, outcrop in a valley, or on the western slope of the great 

ridge of which the Dakota forms the crest. In either case their
FKNNRMAN.] BUILDING STONE. - "69

edges are generally covered with a sheet of loose waste, favorable to 

percolation.

BUILDING STONE.

In the matter of building stone the area here described is typical 

of a much longer stretch of the foothill belt of which it forms a 

part. Most of the formations yield valuable economic products, 

the building stone coming mainly from strata not younger than 

Jurassic.

Within the area mapped granite has not yet been quarried or 

even studied with reference to its use as building stone. Apparently, 

however, the area may furnish granite for architectural purposes 

of suitable grain, color, strength, and size of block.

THE RED ROCK.

The great mass of the Fountain red rock is not appropriate to 

building purposes. Locally, however,, there are, near its summit, 

beds of the finest quality of structural stone. One-fourth mile 

north of Fourmile Canyon a quarry has recently been opened at this 

horizon to supply material for the new Episcopal Church at Boulder. 

In contrast with the prevailing arkose character of the Fountain, 

these exceptional beds are even-grained, quartz sands, containing 

only here and there minute grains of feldspar. Biotite scales are 

rather plentiful, and are arranged parallel to the bedding, so that 

transverse faces show only very minute black lines, which are noi; 

sufficiently prominent to indicate the position of the bedding except 

by very close examination. The stone is of a light pink color, which 

runs uniform so far as the quarry referred to has been worked. 

Except in the biotite, all the contained iron is supposably in the 

peroxide form, and the stone contains nothing to cause a stain upon 

exposure to the weather. The stone is massive and must be dressed 

on all sides of the block. Blocks which dress 2 or 3 cubic feet are 

frequent. The strength has not been tested, but there is no doubt 

an ample margin beyond the needs for all ordinary building purposes.

LYONS SANDSTONE.

This is by far the most abundant rock of economic importance 

within the area described, and probably within the foothill belt north 

of Denver. The name " creamy " sandstone, which is often applied 

to it, is unfortunate, for it is distinctly pink. It varies somewhat in
70 GEOLOGY OF THE BOULDER DISTRICT,, COLORADO. [BOLL. 200.

color, however, and at one extreme it may reach an almost creamy 

tint, while at the other it may be red. Still, the great mass is so 

uniform that stone from many quarries might easily be used in one 

building. The standard color of this stone has more of the creamy 

and less of the vermilion tint than that of the rock quarried fromthe Fountain sandstone. It does not develop any discoloration in 

the wall.

In composition the stone is of the purest even-grained quart/ 

sand, its siliceous cement deriving its color from ferric iron. The 

cement is so abundant as to make the stone quartzitic, and it is known 

in the trade as a crystalline sandstone.

The distinguishing feature of the structure of this stone is its 

delicate lamination and perfect partings in the quarry. Fifty laminations to the inch may sometimes be readily distinguished, but at 

a distance of a few feet the lines are invisible and the stone has a 

uniform tint. Occasional stronger lines may make the lamination 

more prominent.

In the quarry the stone parts perfectly into thicknesses varying 

from a fraction of an inch to more than a foot. Flagstones of convenient thickness and having as much as 60 or even 80 square feet 

of surface may be obtained. The flagstone industry has its center 

at Lyons, about 3 miles north of this area. Near Boulder the thickness of the beds is generally greater, and the stone is commonly quar-. 

ried for building. The blocks for this purpose may be as thick as 

15 inches, but the average is probably not above 8 inches. As the 

bedding planes are perfectly smooth and parallel, the blocks require 

dressing on four sides only. Both the convenience and the strength 

of this stone cause its general use in foundations in the near-by 

towns and cities. It is also used for the superstructure in some 

large and handsome buildings. The stone is likewise in common use 

for sills and ctirbings. As pointed out on page 24, the bedding planes 

observed in nearly all quarries of the Lyons sandstone are those of 

the cross-bedding, having had a dip toward the shore on the west 

when deposited.

The amount of this stone in northern Colorado is enormous. If its 

workable thickness in this area averages 50 feet and if quarries be 

worked 100 feet down the dip of the stratum, each linear mile of the 

foothill belt would yield 1,000,000 cubic yards of stone.

KED SANDSTONE OF THE LYKINS.

Stone from this series of beds is not quarried in this area. At a 

few places the Lykins yields a very fine-grained and close-textured 

red sandstone, easily worked and readily obtained in large blocks.
FENXEMAN.] BUILDING STONE. 1

The same stone occurs abundantly within this area, but, whether from 

the repeated warping of the Boulder arch or from the diagonal compression resulting in the echelon folds, it is too much fractured to be 

obtained in large blocks. This is the case at each of the few places 

where it has been prospected.

WHITE SANDSTONE OF THE JURASSIC.

<o

The chief quarry opened in the lowest Morrison sandstone stratum 

is known by the name of Doctor Bond. The product of this quariy 

is a slightly calcareous sandstone of moderate strength, of a gray 

color, inclining toward yellow rather than blue, and having no lamination or subordinate bedding planes within a 10-foot stratum. It is 

not always free from iron stains, which may appear in the quarry or 

may develop after building into the wall. Large dimension stones 

are readily taken out and easily worked. The bed in which this one 

quarry is opened has a fairly uniform thickness and character for 

many miles. It is probable that in the future this stone will be quarried much more extensively.

BUILDING STONES FROM THE CRETACEOUS.

But little stone is quarried from strata younger than Jurassic. The 

Dakota is very strong, but difficult to work, the blocks generally 

requiring dressing on all sides. Within this area it need scarcely be 

taken into account, as it is accompanied b}^ other formations containing such vast stores of more desirable material.

The massive summit sandstone of the Fox Hills is quarried in 

considerable quantit}' farther north. It is but little exposed within 

this area, and where found it is commonly too weak for structural 

purposes. Beyond the eastern border of the area it is not improbable 

that this stone will be found locally of fair quality.

The white sandstone of the Laramie can scarcely be called a commercial product. It .is present in great quantities and easily taken 

out and worked in blocks of large dimensions, and a few houses have 

been built of it. Its strength, however, is small, and it is much stained 

with iron.

GRINDSTONE.

The white sandstone at the base of the Morrison, which is used 

for building purposes as described above, is locally used for grindstone and enjoys considerable favor. About a mile north of Altona 

a quarry, known as the " Grindstone quarry," has been opened by 

Peter Haldi, of Altona. The few grindstones from this source which
72 GEOLOGY OF THE BOULDER DISTRICT,, COLORADO. [BULL. 205.

are in regular use have been dressed by hand. To all appearances 

they are doing their work in a highly satisfactory manner. The 

grain is fairly sharp, the cement is very slightly calcareous, and the 

stone is easily dressed. Several much-used surfaces seemed to be 

clogged with the muddy products of the abrasion of the stone itself, 

but the users maintain that such an occurrence is the result of 

improper use. The crude methods of quarrying have produced 

rather small and ill-shaped blocks, but, judging from* the same 

stratum where quarried for building purposes, this does not indicate 

any fault in the stone itself. Suitable shapes and sizes can no doubt 

be easily obtained.

LIME.

The production of quicklime is carried on to a limited extent. 

The stone used is mainly from the basal stratum of the Niobrara. 

At various places also the limestone lenses of the " crinkled " sandstone in the Lykins have been utilized; occasionally also the beds in 

the Morrison. The limestone of the Niobrara is highly magnesian a 

and very dense. On account of its density it does not calcine very 

readily and the large percentage of magnesia makes a " cool " lime.

CLAYS. 

OPPORTUNITIES FOR THE BRICK INDUSTRY.

The brick industry at Boulder is already large and there is no 

necessary limit to its expansion, except the size of the market, the 

raw material being presumably inexhaustible. It appears, however, 

that the Pierre shale, Avhich is the chief resource for red brick, is not 

entirely uniform in its brick-making properties. Yards within the 

limits of Boulder produce bricks differing in shrinkage, density, and 

strength. As the surficial aspect of the Pierre shales is highly 

uniform for long distances, it is very probable that laboratory tests 

will come into general use- in the locating of yards as the industry 

expands.

Ked brick must always constitute the chief product of the clay 

industry. Three plants, each with a capacity of 20,000 bricks a day, 

are operating steadily at substantially full capacity. These are all 

using the Pierre shale and are practically within the city of Boulder. 

The Benton has not been used. At present all the pits are on mesa 

edges, the mesa in each case being from 30 to 50 feet higher than the 

adjacent lowland. While such a situation may be desirable from

"An analysis of Niobrara limestone Is given by Bldridge, op. cit., p. 67. It shows 

18.03 per cent MgO.
PKNNEMA-N.] CLAYS. 73

the standpoint of moisture conditions, it is mainly a matter of convenience, and must not be understood to be necessary. There is 

nothing to bar from future use any part of the shales of the Pierre 

belt as shown on the map.

SHALE SUPPLY.

At each of the three pits the shale is covered with the rough mesa 

gravel, which at one place has a thickness of 15 feet, and which is 

removed. Where the mesa gravel is almost absent, the clay resulting 

from decomposition of the shale may be as thick as 2^ feet, but this 

is of no significance in the manufacture of brick. Where the mesa 

gravels are thick, the clay is wanting. The shale is almost free from 

lime, though some highly calcareous nodules occur," sometimes containing fossils. Occasionally, also, a thin plate of shale is dark and 

hard with impregnated iron, but the quantity of such is almost 

negligible.

If the shale is not reduced to clay, it is not to be understood that 

it has been entirely unaffected by weathering. ' Its bedding remains 

perfect and it is apparently unweathered, but where excavation is 

deep the color is always observed to change from a greenish drab 

above to' something darker (commonly called " black") below. 

Throughout the belt underlain by the Pierre, though the normal 

color varies, the surficial zone shows almost the same greenish-drab 

color. This ends abruptly at a depth of 30 feet, more or less. In 

the black shale (or " slate ") below lamination is less apparent, and 

large masses break with a conchoidal fracture. Naturally the concretions are not observed within this fresh mass, as they are in the 

partly weathered zone, from which they roll.out upon blasting. The 

shales of the upper zone have little grit when tried between the 

teeth.

A partial analysis of the shale from the Austin yard, made by 

frof. C. S. Palmer, is given by Mr. Austin, as follows:

Analysis of shale from Aiistin yard. Boulder.

SiO2 _ ___. ______.

A12 0,~- -

Fe.0, _ - _ - _ -

MgO _ - - -

Per cent. 

. - __ . . ___ ._ 60.0

_____________________ __ _ 18. 0

- _ _ ._ . . _ '. _ _ 4. 75

._ . _ .___ _ ________ 2.0
74 GEOLOGY O.F THE BOULDER DISTRICT., COLORADO. [HULL. 265.

An analysis of a single selected lump (not a mixed sample) of the 

shale from Mr. Lee's yard, made by Mr. W. B. Stoddard, of Boulder, 

and furnished by Mr. Lee, is as follows:

Analysis of shale from Lee yard, Boulder.

Per cent. 

SiO2 _____________________________________ 03.309

ALO,___________________________________________ M.38Q

FeA____________________________________ 0. 270

FeO__________________________________ .859 

CaO__________________________________________ 1.810

. MgO___________________________________ 2.570 

Na,O____________________________________ 2. 190 

K.O__________________________________________ 1.280

Loss by drying____________________________ 2.053 

Loss by ignition_______________._______ 5. 223

99. 944 

PROCESSES OF BRICK MANUFACTURE.

The material is first loosened by blasting. Concretions are thro'wn 

out, chiefly to save wear in the crusher. They are said to be harmless 

in the brick if crushed sufficiently fine. In one of the three pits the 

black shale below is not reached. In another it is reached at the 

bottom, but not used. At the Austin works, on the north side of the 

city of Boulder, it is taken out and used along with the shale of the 

higher zone. It is harder, drier, and less plastic, and is therefore 

mixed with the partly weathered material. 'When thus mixed, no 

difference in the resulting brick can be detected, and there is no inconvenience in the manufacture. If properly ground, the unweathered 

black shale may be mixed with the weathered shale in equal parts. 

The blasting of this fresher shale in the autumn and its weathering 

or slacking through the winter has been tried, but no notable difference was produced.

The shale is crushed, screened, and pressed dry. Generally the 

moisture of the bank is sufficient for .the pressing process. When the 

shale is too dry it is moistened in the crusher. No mud bricks are" 

made.

The burning is in up-draft kilns, which generally have permanent 

walls for two-thirds or three-fourths of the height. One-half to t\yothirds ton of local (Marshall or Louisville) coal per 1,000 bricks is 

used in the burning. Mr. Austin has installed a gas plant, and the 

project of burning with gas may be said to be on trial.

The shrinkage in both drying and burning of the best and strongest 

samples tested was but one-sixteenth inch in the height of the brick 

(4 inches), or less than 3 inches in the height of the kiln. Either by 

reason of difference in the clay or in the burning, this shrinkage 

varies at other yards to a maximum of three-sixteenths of an inch per 

brick, or 7-| inches in a kiln 40 bricks high.
FEXXKMAX.] BKICK INDUSTRY. 75 

QUALITY OF THE BRICK.

Standard tests made upon the red bricks by Prof. L. Duncan, of the 

University of Colorado, show an average crushing strength for the 

best brick equal to 4,100 pounds per square inch." Supported at the 

ends (supports 6 inches apart)- and loaded at the center, the same 

bricks showed a maximum fiber stress of 498 pounds per square inch. 

The same bricks absorbed water equal to 11.2 per cent of their weight 

in fifteen minutes, 11.6 per cent in thirty minutes, and 11.9 per cent 

in twenty-four hours. The promptness with which these bricks 

absorbed nearly the whole amount of water which they were capable 

of containing indicates largeness of the individual pores. The more 

rapidly water is absorbed, the more rapidly it is lost, and the probability that a sudden freeze should find the pores of these bricks 

filled with water is very small. This statement does not apply to 

bricks laid below the water line.''

No attempt has been made to make vitrified brick on a commercial scale at the Boulder yards. Experiments have been carried 

on in this line, and a very good brick resulted, but the shrinkage was 

great. In two tests it was seven-sixteenths inch and one-half inch 

in the height of the brick (4 inches).

BRICKS OF LIGHT COLOR.

For several years past small quantities of light brick of a prevailing creamy color have been burned at the Boulder yards from clay 

brought by wagon from the coal-mining districts to the southeast. 

The first clay for this purpose was brought from Coal Creek. The 

largest amount has come from an abandoned coal mine at Marshall, 

from 3 to 4 feet of the clay being taken from the floor of the mine. 

More recently a number of samples from near Burnt Knoll have been 

tested, and encouraging results are reported.

Tests of light bricks from the Marshall clay gave a crushing 

strength about equal to that of the best red brick (4,090 pounds per 

square inch), a maximum fiber stress about two-thirds as great as 

that of red brick (332 pounds per square inch), and absorption 

slightly greater than that of the red bricks (12.5 per cent in tw'enty- 

.four hours).

Red brick sell in Boulder at $7 to $9, the last-named price being 

for facing brick. The common price f. o. b. for shipment is $10

" This may be compared with the strength of one of the best-known building stones of 

the United States, the Bedford limestone of Indiana, which showed, in 17 tests, an average crushing strength of 4,32(17 pounds per square inch. Twenty-first Ann. Rept. Indiana Dept. Geol. Nat. Resources, p. 317.

b See Btickley, E. R., Building and ornamental stones: Bull. Wisconsin Geol. and'Nat. 

Hist. Survey No. 4, 1898, p. 20.
76 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 2G5.

per thousand. Buff brick have not been exported. They sell in 

Boulder at $20 per thousand.

Just beyond the western border of the area, near Sunshine, is a 

deposit of kaolin, much of which is to all appearances very pure. 

Probably the'deposit is due to the alteration of feldspars in a pegmatite dike. The clay is white and without grit. Some of it is 

stony and contains kaolinite crystals. Upon grinding and wetting 

it becomes thoroughly plastic.

COAL.

A small part of the northern Colorado coal field lies within the 

area here described. The whole field was carefully studied by 

Eldridge," who has described the structure, discussed the quality of 

the coal, and correlated the seams of the different districts. Since 

the publication of his work, in 1896, several new mines have been 

opened, but these have revealed no structural relation not known and 

discussed in Monograph XXVII. The reader will find in that publication substantially all that is known in regard to the geologic relations of the coal.

OIL AND GAS.

SURFACE INDICATIONS.

The development of oil and gas in the Boulder area began in 1001, 

though their presence was suspected and rumored as early as 1867. 7' 

The early beliefs were based upon grounds which might now be 

regarded as far from demonstrative. The subsequent finding of oil 

must therefore be regarded largely as a piece of good fortune rather 

than the assured outcome of a safe business venture. The evidence 

consisted largely in the strongly bituminous odor of the Benton and 

Niobrara limestones and shales. It was'therefore assumed that the 

strong-smelling rocks were themselves the reservoirs of petroleum, 

and in the first developments there was no thought of finding oil 

above these beds. Oil was found several thousand feet higher than 

these two formations, which have never produced anything of (his 

nature. They have the same characteristic odor near the oil fields 

and far away. It was reasoned out with equal certainty that the 

Benton and Pierre shales must produce coal on account of their black 

color. In fact, it was while prospecting for coal in the Benton that 

the project of drilling for oil was first conceived.

A second class of surface indications, not found nearer than about 

10 miles from Boulder, consists of seepages and " oil springs." The

a Op. cit, p. 345.

b A brief historical account of this industry is given by the writer in Bull. U. S. Geol. 

Survey No. 213, p. 322.
FBNNKMAN.] OIL AND GAS. 77

fact that several such, most persistently reported, are on the Archean 

granite west of the foothill outcrops of stratified rocks shows that

0 72 R.70W.

 Wells which have 

produced oil

Dry wells

o yz __ _____1 mile 

PIG. 9. Map showing location of oil wells in the productive area.

the distinction between the various kinds of oily substances was 

poorly made. Some of the springs which issue from the Fountain
78 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

red rocks are abundantly coated with oily matter at no great distance from the point of issuance, and these springs have been taken 

very seriously by oil prospectors, but in none of those examined Avas 

the odor of petroleum detected. The \vell-lmoAvn oil spring on the 

bank of the Little Thompson, on the Culver ranch, 17 miles north of 

Boulder, issues from the heavy sandstone member of the Pierre, 

Avhich has been called in this report the Hygiene sandstone. This is 

a seepage of petroleum.

OIL-BEARING AREA.

On the evidence of these phenomena, corroborated by the positive 

behavior of " bobbers,"" several Avells Avere drilled. In January, 

1901, the McKenzie well, 3 miles northeast of Boulder, struck oil. 

Since that time there has been more or less drilling from Fort Collins on the north to Golden on the south. Sheet XII of Hay den's 

atlas has been freely used as " Hay den's oil map," and land underlain 

by his Colorado group (Avhich includes the Pierre) has been AA^ell 

advertised as lying Avithin " Hayden's oil belt." Development has 

been successful only Avithin a limited area, the center of Avhich is 

from 3 to 4 miles northeast of the city of Boulder and a little less 

than 3 miles east of the foothills. About 100 Avells from-300 to over 

3,400 feet in depth haAre been drilled Avithin 5 miles of the McKenzie 

and about 20 more are scattered over the area betAA^een Fort Coll ins 

and Golden. Most of the Avells (about 70) are Avithin a rectangle 2 

miles east and Avest by 3 miles north and south, comprising sees. 

5, 9, 16, 17, 20, and 21, T. 1 N., R. 70 W. of the sixth principal 

meridian. Within this rectangle are all the producing AAT ells except 

the Boulder Illuminating, a A^ery small producer in sec. 10 only a. 

feAv feet from the eastern boundary of the rectangle named, and the 

Buffalo gas well, Avhich lies 1 mile south of the rectangle.

The folloAving list gives the location of AA'ells, the total depth, and 

the depth at which oil was found. The accompanying sketch map 

(fig. 9, p. 77) shows the locations of all Avells Avithin the productive 

area. Some of the dry Avells are comparatively shalloAA7 ; therefore 

in determining their significance their depths, as shoAvn in the table, 

should be taken into account.

"The exact location of a large proportion of the wells of this field has been fixed by 

this means. The- principle on which the use of the bobber rests is the same as that by 

which the proper site of a water well is determined by the involuntary turning of a 

witch-hazel sprout when held in the hand. In two instances, completed derricks have 

been taken down and removed to distances not exceeding 40 feet, because of the behavior 

of the bobber after their erection.
FENNKMAN.] OIL-BEARING AREA. 79

Well* in the Boulder district.® » .

WELLS THAT HAVE AT SOME TIME PRODUCED OIL.

No.

1- 

2 

3 

4 

5 

6 

7 

8 

9 

10 

11 

12 

13 

14 

15 

16 

17 

18 

20 

21 

22 

23 

 24 

25 

26 

27 

28

Name.

Bradford No. 1....... ......

Boulder Valley. ........ ...

Boulder Valmont No. 2. ....

OteroNo. 1 ................

OteroNo. 3 ................

Otero No. 4 _ ......___....._

Otero No. 5 ................

Savanna No. 3 .......:.....

Savanna No. 4 ........ ......

Savanna No. 5 (Younglove). 

Disniuke .-.-.-....---.-....

Orcini .....................

McAfee No. 1 ..............

McKenzie .....-.-.-.- .---..

Wellington ................

Bingham ..... ............

Arnold. .....................

Wisconsin No. 2 ...........

Crawf ord ..................

Central .....................

Two-ten ...................

OteroNo. 2 ......... .......

St. George .. .-.-... ...----.

Boulder Illuminating .......

Bradford No. 2 .............

Depth.

Feet. 

2,404 

2, 050 T. 

1,960

1,718 

1,905+

1,763

1,475T. 

2, 005+ 

2,070+

2, 585 T. 

961 

2,461

2,820 

2,900 

2,735T. 

2,621 

2,141 

1,761

2,700 

1,720 

2,300

Depth to oil.

v Feet. 

1,810 

2, 010 

1,650 

1,639 

1,807?

1,584 

.1,468 

1,980T. 

2,064

2,105T. 

2,537T.

885 

2,423 

2,845 

2,720

2,683 

2,551 

2,111 

1,700

2,130

Location.

Section.

8 

9 

9 

8 

8 

8 

8 

. 9 

9 

9 

16 

9 

17 

21 

20 

20 

16 

16 

16 

 17 

17 

9 

8 

16 

10 

8 

8

Township.

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

1 iN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN, 

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN.

Range.

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

'70W.

DRY OR NEARLY, DRY WELLS.

No.

29 

30 

31 

32

Name.

McAfee No. 2___. ...... .......:........

Eagle. -.-.---.--..-.-..--....-.....---..

Great Western .........................

Depth.

Feet. 

2, 700 

2,740 

2, 825 

1.600

* Location.

Section.

17 

20 

21' 

20

Township.

IN. 

1 N. 

IN.

IN.

Range.

70 W. 

70 W.

70 W. 

70 W.

 ° Numbers in the left-hand column refer to -corresponding numbers on the sketch map 

(fig. 9). Wells not numbered are outside the mapped area. Depths marked T were.determined by steel tape measure; most of the others by cable measure.
80 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

Wells in the Boulder district Continued. , 

DRY OR NEARLY DRY WELLS Continued.

No.-

83

84

85

86

87

88

89

40

41

4?.

48

44

45

46

47

48

49

50

51

59,

58

54

55

56

57

58

59

60

61

69

68

64

65

66

67

68

69

70

Name.

Planet No. 2 ............................

Searchlight No. 2 .......................

Home .-.---.--.-.-...-....-.-..........

Hayden No. 1 ............................

Hayden No. 2. ..........................

Phoenix No. 2 ........... ..............

North Star ..............................

North Bend ............................

Republic No. 2 .........................

Republic No. 3 ....................... .

Homestake ..............................

Vindicator .............................

Savanna No. 1 .................... ......

Boulder Petroleum ................. ...

Pioneer No. 2. ............ ...............

St. Julian ...............................

Canfield ................................

Wood ..................................

Otero No. 6. now Rathvon No. 2 ........

Wisconsin No. 1...... ............ ....

Cleveland ..................... .......

Savanna No. 6. ..........................

Savanna No. 7. .........................

McAf ee No. 3 ....................:......

Chicago Banner ......_...____ __.____. .

Langridge ...............................

Sale Boulder ............................

Me Edma. ................................

Keystone ...............................

Blue Jacket ..-.. ^. ......................

Boulder Basin ..........................

Independence. .........................

Palm ...................................

Empire No. 2. ..........................

La Ford. . ...............................

Craig & Bartlett. . ......................

Savanna No. 8 ..........................

Rathvon No. 1 ...........................

Depth.

Feet. 

2, 025

2,480

1,650

2,670

2 145

2,400

1,160

787

1,008

1,100

1,300

700

1,630

2,160

2, 525

«. 3, 575

2.000

2,492

9 580

2,600

2,500

3,300

2,550

2, 500

2,700

Section.

9

9

9

17

17

9

20

20

20

20

20

20

20

8

8

8

9

16

8

8

20

9

8

16

17

16

20

20

21

21

21

8

 17

9

8

9

8

8

Locatio

Townsbip.

1N.

IN.

IN.

1 N.

1 N.

IN.

1 N.

IN.

IN.

IN.

IN.

IN.

1 N.

1 N.

IN.

IN.

IN.

IN.

IN.

IN.

1N

IN.

1N

IN.

IN.

IN.

IN.

IN.

IN.

IN.

IN.

IN.

IN.

IN.

IN.

IN.

IN.

IN.

n.

Range.

TOW.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70. W.

70 W.

70 W,

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W.

70 W,

« Prilling deeper.
FEN NEMAN'.] OIL-BEARING ABE A.

Walla in the Boulder d'tstfnci Continued. 

DRY OR NEARLY DRY WELLS Continued.

No.

71 

72 

73 

 74 

75 

76 

(a) 

(a) 

(a) 

(a) 

(a) 

(a)

(«) 

(«)

(«)' 

(a)

(«) 

(«)

(°) 

(a)

(a) 

(a)

(«)

(«)

(a)

(«) 

(a)

<«) 

(a)

 («) 

( a ) 

<«) 

<«) 

'(«)

'(«)

(a) 

(a)

Name.

Hayden No. 3. .._...__.__...._...-.--.-

Citizens No. 2 ......... ...... .........

Empire No. 1. ...........I.. ___«__ ......

Anderson ...............................

Peoria .................................

Phoenix No. 1 ................... ....^...

Citizens No. 1 ..........................

Maxwell No. 1 ...........................

Maxwell No. 2....'. .'............... ......

Pioneer No. 1 ............................

King...................................

Lafayette, Colo .........................

Planet No. 1. ................... ........

Searchlight No. \. ................'......

Monarch ...... .........................

Moore.--. .---.:....-..-.-.-.-..-..-----

Balkan .................................

Boulder Valmont No. 1 ............... .

Rose Crude. .........................

Jasper ................:....... .........

Carnahan ..............................

Hoover. ................................

Martin .................................

Olean ..................................

TTtrcripnp

Phenomenal ............................

Silver Lake (Stuart well) ...............

Sternberg ......................... ....

Pioneer No. ^.. .........................

Prince No. 1 ............................

Prince No. 2.... ................ ........

Buffalo (gas well) ....... 1 ....... ......

Depth.

Feet.

1,850

3,684 

2,800 .

2, 500

2,268T.

2,7004- 

2,870 

2, 325

700 

1,185 

1,500 

1,500 

1,000 

2,740 

2,500

3,006

3,105

. 3,216 

2,995 

2,993 

2, 300 ? 

2,150 

3,055 

1,396

1,800

Location.

Section.

8 

5 

16 

16 

15 

29 

25 

24 

24 

29 

22

27 

23 

29 

22 

~ 29 

10 

10 

15 

18 

35 

5 

14 

30 

36 

2 

6 

19 

5 

33 

34 

27 

27 

29

Township.

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN. 

2N. 

IN. 

IN. 

IN. 

IN. 

IN. 

IN, 

IN. 

IN. 

IN. 

IN. ^ IN. ' 

IN. 

IN. 

IN. 1 S.' 

IN. 

3N. 

3N. 

2N. 

1 S. 

2N. 

2N. 

IN. ^ 

IN. 

2N. 

2N. 

IN.

Range.

70 W.

70 F70 W. 

70 W. 

70 W. 

. 70 W. 

71 W. 

71 W. 

71 W. 

70 W. 

70 W.

70 W:

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

70 W. 

69 W. 

70 W. 

70 W. 

70 W. 

69 W. 

70 W. 

70 W. 

70 W. 

69 W. 

70 W. 

70 W. 

70 W. 

69 W. 

69 W. 

70 W.

"Wells not numbered are outside the area, of sketch map (fig, 9). 

Bull. 265 05 u 6
82 GEOLOGY. OF THE BOULDER .DISTRICT, COLORADO. [HULL. 265.

The Savanna No. 9, in sec. 9, has reached at the present writing 

(October, 1904) a depth of 1,84-0 feet, where a good showing of oil 

has been encountered.

The Rathvon No. 2 (No. 51 on the map, fig. 9), already the deepest 

well in the field, is being drilled deeper.

DRILLING IN THE- PIERRE.

'With few exceptions, the wells thus far put down have been drilled 

entirely in the Pierre. The exceptions are among the scattered wells 

to the east, which are on the very similar Fox Hills, or even on the 

Laramie, but these also traverse the Pierre for the greater part of 

their depths.

Drilling in this formation is comparatively easy and rapid. It 

is not uncommon to make 100 feet a day at considerable depths, and 

this is sometimes done without change of bits. On the other hand, 

slow progress is made in some beds. The average expense of drilling 

a well under contract with responsible parties is, at the present time, 

about $1.65 per foot for the first 2,000 feet. Below that depth the 

cost is greater. Under such contracts the owner of the well furnishes casing. * i

Surface water is usually encountered at a depth of about 15 feet 

and may be found in the first 100 or even the first 200 feet. Below 

this the wells are commonly " dry " or the seepage into the wells 

from the dense shales is so slow as to be of no significance in drilling. 

In isolated cases deep water-bearing strata are encountered, and such 

water is sometimes salt. At Lafayette, 11 miles east of the foothills, 

a well that was drilled for oil yielded a vigorous flow of water from 

several horizons between 400 and TOO feet deep. The oil from another 

well is mixed with about 10 per cent of salt water, which comes from 

just below the oil, and a slight admixture of salt water is found in at 

least one other well.

Many wells are cased only to the depth of the surface water. 

Others require 1,000 or even 2,000 feet of casing. In general, deep 

casing is not for the purpose of shutting off water so much as to 

avoid caving, which is common though not general. It has necessitated the abandonment of several wells.

REPORTS RECEIVED FROM WELL DRILLERS.

'By far the most common material reported as passed through is 

shale or " slate," occasionally varied as " clay" or " soapstone." 

Sand or " sand rock " is often reported. If it shows oil it is often 

called " the oil sand," with no particular reference to correlation with 

the sands of other wells. These sands are described on pages 31-33.
FENNEMAN.] REPORTS FROM WELL DRILLERS. 83

Their composition is generally far from that which the name would 

suggest. They range from sand to clay, vary in thickness up to a 

maximum of over 100 feet, have a limited lateral extent, pass by 

gradations into the normal shale of the Pierre, and occur at any 

depth. One Avell may pass through five or six such beds, while 

another near by may penetrate nothing but shale to a depth of 2,000 

feet. In drilling, the sandy beds are distinguished from the shales 

by their greater hardness, by the' more rapid wear of the tools (the 

driller's chief criterion), and by the smaller amount of suspended 

matter in the water of the bailer. The washed-out samples commonly appear upon close examination as a collection of gritty shale 

granules, whose edges have been rounded under the drill. The 

better grades show an admixture of quartz grains, and exceptional 

beds are pure sand.

" Streaks " and " shells " are other terms used to indicate more than 

usual hardness at certain horizons. In the outcrops of these shales 

may frequently be detected hard beds a few inches or even several 

feet in thickness. They are either very calcareous or stained with 

iron oxide, or both, and owe their superior hardness to concentration 

of these substances. Such induration may affect the entire bed 

equally or may be concentrated into ellipsoidal concretions, which 

may be more or less separated, so that there are all gradations 

between the continuous hard plate and the isolated concretions: The 

word " shell " is suggestive of the concretions, as the word " streak " 

is of the plates, but in the reports the two words may be taken to 

be synonymous and alike indefinite. The comminuted fragments 

of these hard masses are not easily suspended in the water of the 

bailer, and probably for this reason, as. well as on account of their 

superior hardness, are not infrequently reported as sand. On the 

other hand many of the so-called hard streaks are siliceous. The 

reports of " lime rock " are probably sometimes to be traced to the 

same occurrences, though, as seen on page 31, there are some true 

limestone beds in the Pierre.

OIL-BEARING STRATA.

The beds from which the oil is obtained are the highly variable 

sands or sandstones, described above as varying between clay shale 

and silica sand. Such beds may be met with at any depth, but there 

is no depth at which such rock is certain to be found. Reports 

might indicate that it is slightly more abundant at a depth approaching 2,000 feet, but this may be due to the sharper lookout for sand 

as the well gets deeper. Not all of these strata contain oil or gas; 

some of the most porous sands give no indication of either.

While the thickness of such beds may be anything up to 100 feet
84 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

or more, oil or gas may be obtained from only a part of their thickness. 

Such strata are plainly not so homogeneous as would appear from 

the bailings and, except locally, are far from being porous sandstones. 

Except in an oil well they would probably not be called sands at all. 

It is not uncommon to drill many feet into such strata and then strike 

a showing of oil or gas with no attendant change in the texture of 

the rock that can be detected in the bailings.

These small reservoirs of oil or gas seem mutually independent, 

being separated by the impervious shales. This isolation is well 

illustrated by the occurrence of deep veins of water in some wells 

and the absence of water from near-by sands in the same well or in 

adjacent wells. This mutual isolation of the several pockets is not 

accompanied by any great diversity in the character of the oils. 

With but few exceptions there is approximate uniformity, as shown 

by standard physical tests. Considering the striking uniformity of 

the great body of shales, this general likeness in the character of the 

oils is to be expected. Thus far there is but one authenticated 

instance in which the pumping of a new well has influenced the 

production of an older well. In this case the two wells are less than 

200 feet apart. Curiously enough, these two wells found oil at depths 

differing almost 500 feet.

SHOOTING OF WELLS.

All but one of the wells which were pumped previous to the close 

of 1902 were shot. In 1903 this practice was less general. The 

amount of nitroglycerin used in these shots has varied from 10 to 

140 quarts. Dynamite charges have been as large as 500 pounds, 

TO per cent nitroglycerin. The effects of these shots have not been 

uniformly favorable. The beneficial effects in a few of the best wells 

have doubtless been responsible for most of the later attempts. 

More recently a number of wells have begun pumping Avithout being 

shot. The fact that the flow of some wells has decreased since the 

shooting will lead to greater caution, and it is to be hoped that it 

Avill lead to a more careful study of the nature and thicknesses of 

the strata passed through.

The beneficial or harmful effects of a shot must depend largely 

upon the texture of the stratum yielding the oil, for it seems to be 

true that some shales are compacted rather than shattered by the 

explosion. Suppbsably for this reason, shooting is not practiced in 

the Florence field, which, of all the older oil districts, the Boulder 

field most resembles. Owing to the difference in texture of the 

various beds yielding oil in the Boulder field, it is but reasonable to 

expect that the same shot which would prove beneficial to one well
PBNNBMAN.] SHOOTING OF WELLS. 85

would be ruinous to another. On this account, if on no other, the 

texture and composition of the oil strata should be carefully studied 

by methods far more discriminating than the superficial ones first 

used in this field. It is but just to the companies and contractors 

now remaining on the ground to say that there has been much 

improvement in the character of well records.

A second reason for injurious effects from shooting lies in uncertainty about the exact depth of the sand which it is intended to 

shatter. Measurements of depths by steel tape are now becoming 

more common. In' most of the earlier wells the depths of all formations are known only by cable measurement. It is not uncommon 

that the stated depths of important sands are thus liable to errors 

of 25 to 50 feet.

The possible injuries from a shot at the wrong place may be readily seen from the following consideration. If a porous rock is saturated with oil under pressure arid is pierced by the drill, the oil is 

pressed upward in the hole solely because it has no outlet in any other 

direction, being surrounded (as in this field), or at least overlain, by 

impervious rocks. If this well is shot in such a way as to rupture the 

impervious rocks that surround the oil sand, the oil may leave the 

sand by other openings besides the well and may thus be dissipated in 

other porous beds and the well may be ruined.

Such an injurious effect ma}7 be produced even by shooting at the 

proper depth if the charge employed be too heavy. In one instance 

a well was shot at 740 feet with 500 pounds of dynamite, 60 per cent 

nitroglycerin. The formation above the sand was a uniform dense 

shale. A good qualit}7 of sandstone was blown from the hole in 

large chunks, one of which weighed 14 pounds. The shale was 

ruptured to the surface. Open cracks an inch or more wide extended for some rods from the well. It may be presumed that cracks 

also reached a considerable depth below the sand Avhich was to be 

shattered. The well, of course, yielded no oil.

It can not be too carefully borne in mind that the one object in 

shooting is to shatter the rock which carries the oil and that only. 

With this object in view, it is plain that intelligent and discriminating shooting must depend upon the answers to the following questions : Is the oil stratum of such a texture that it will be shattered 

rather than compacted? What is the exact depth of its top (and bottom if drilled through) ? HOAV much of a shot will the overlying 

rocks bear without giving other outlets to the oil ? This last question 

is' one of great importance in this field. It is needless to say that such 

questions can be answered only by a carefully kept log and close study 

of samples, not only of oil sands but of all strata, in order to properly 

forecast their behavior under the influence of a shot.
86 GEOLOGY OF THE BOULDER DISTRICT. COLORADO. [BULL. 265.

 '

Wells ivhich -were shot previous to June, -1903.

Name.

McKenzie --.-...-..

Savanna No. 3 (second shot) .

OteroNo. 1 --------

Otero No. 2 _..__.__

Otero No. 2 (second 

shot).

Otero No. 3 ........

Otero No. 3 (fifth 

 shot).

Bradford No. 1 . . . . .

Bradford No. 2 . - ...

Bradford No. 2 (second shot) .

Boulder Illuminating. 

Crawford ...........

Boulder Valley

Hayden No. 1 ........

Do............

Central ............

Two-ten....--....-.

Hayden No. 2.......

Wisconsin No. 2. ... 

Boulder-Peoria

Independence ......

Boulder - Val m o n t

No. 2.

Boulder - Val m o n t 

No. 2 (second 

shot).

Disimike _..-----...

Charge of nitroglycerin.

Quarts.

140

40

100

50

 100 

60

10

60 

«10

60 

100

30

15 

100

100

80

80

100

120

40

40

40 

80

60

60 

60

60

Depth of shot, 

in feet.

2, 530-2, 587

2,115

1,579-1,600

- 1,579-1,600 

1,649-1,673

1,724-1,761

1,724-1,761 

1,600-1,900

1,807-1,837

1,650

1,720

2, 682-2, 734

2,021

2,005

Same depth.

2,600

2,111-2,141

2, 135

2, 670

2,035

1,100

1,650 

1,947

2. 064

Effect.

No change.

Abandoned after shot.

Intended to shoot at 2,100; abandoned after shot. 

Reduced production from 10 barrels

to 5 barrels.

Produced about 7 barrels after second shot.

Not pumped before; a good producer. 

Not pumped.

Little oil before; not pumped; a fair 

producer afterwards.

Shot four times: little oil.

Produced 3 or 4 barrels. 

Produced 4 or 5 barrels.

Went on drilling.

Very small producer. 

Not pumped before; a very small

producer. 

Fair well before; ruined.

Increased production from 6 barrels

to 15 barrels. 

Shot with 500 pounds dynamite, 70

per cent; no effect. 

Never pumped.

A little oil; soon abandoned.

Not pumped before; small producer

afterwards. 

Never pumped.

Do.

Small producer; drilled deeper and 

abandoned.

Never pumped.

Do.

Drilled deeper. 

A fair prodticer.

Not pumped before; a small producer afterwards.

For each shot.
FENNKMAX.] STRUCTURE OF THE OIL FIELD.

Wells which were shot previous to June, 1903 Continued.

87

Name.

Buffalo ............

Republic No. 1 ......

Republic No. 3 ......

North Bend ........

Charge of nitroglycerin.

Quarts. 

40

24

Depth of shot, 

in feet.

1,600

- 965

740

Effect.

No oil; did not aif ect gas production. -

Shot with 100 pounds dynamite; no

change; dry hole. 

Shot with 500 pounds dynamite, 60

per cent; hole ruined.

STRUCTURE OF THE OIL FIELD.

From all observations previous to the preliminary report made by 

the writer in January, 1903," there appeared to be no law governing 

the distribution of oil in the Pierre shales. Small bodies of oil have 

been found in various parts of Colorado and neighboring States, but 

their structural relations, so far as known, afforded only vague explanations of their presence, and left the prospector with the impression that oil might or might not be found at almost any place in the 

Pierre.

The want of any known evidence of anticlinal arches suggested the 

inference that such structures were unnecessary to form receptacles 

for oil and gas in so dense a rock as the Pierre shales. The idea of 

some connection with folds was, however, not given up, and renewed 

efforts were made to induce companies and drillers to preserve samples and careful records, with a view to determining the exact position 

of the strata. During the summer of 1903 a detailed structural 

study was made of an area of 150 square miles, of which the oil field 

occupies the center.

It is a noteworthy fact that, with the exception of two very small 

producers, all the wells which have thus far produced oil are in a very 

narrow north-south belt. It is n.ow known that this belt follows a 

monoclinal fold, the wells being near the top, and therefore in similar structural position to those of Ohio described by Orton as occurring on " arrested anticlines." The two very small producing wells, 

referred to above as located outside the narrow belt, are the Two-ten, 

one-half mile east, and the Boulder Illuminating, 1 mile east. It will 

be seen below (p. 92) that these wells occupy very significant positions with respect to another fold.

The monocline on which the wells are located is almost in line with 

an incipient echelon fold to the north-northwest, which affects the

"The Boulder, Colo., oil field: Bull. U. S. Geol. Survey No. 213, 1903, p. 322.
88 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BOLL. 265.

foothill belt after the characteristic manner "of such folds in this 

region. (See p. 43; also map, PL II.) For the present purpose it is 

best seen in the prominent offset in the outcropping edge of the 

Hygiene sandstone in sec. 6. The axis of this fold points east of 

south. In the outcrop of the same sandstone, but 5 miles farther 

northeast, a well-developed fold of similar character branches off, 

with an axis trending almost due south. Both these folds are 

described below. The monocline which determines the present line 

of producing Avells is the southward continuation of the western and 

smaller c one of the two flexures, but it is affected .both in degree of 

flexure and in the direction of its axis by the stronger fold to the east.

It may be confidently affirmed that whatever may be the nature of 

the occurrence of oils elsewhere in the Pierre y there is nothing thus 

far known in the field at Boulder to offer encouragement to prospectors, except in intimate relation with folds. The essential features of 

this particular flexure should, then, be determined with care. Such a 

study may afford a basis for inference as to the probable oil-bearing 

or barren character of other similar structures occurring throughout 

a distance of 50 miles along the front of the range.

The form and location of these folds are best made clear by tracing 

the outcrop of the Hygiene sandstone (see p. 31), which made possible the discovery of the folds. Near Boulder the stratum is thin 

and weak and is separated from the Niobrara on the west by not 

more than 1,000 feet of shales. Seven miles north of Boulder it 

is 250 feet thick, and the Pierre shales below it have thickened to 

nearly 3,000 feet. As the dip has also greatly decreased within the 

same space, the sandstone outcrop has been carried' nearly 2 miles 

to the.east.

Stated more definitely, the outcrop forms a prominent cuesta 

(a ridge of abrupt scarp slope and long, gentle dip slope) running 

from the southwest corner to the northeast corner of sec. 6, T. 1 N., 

E. 70 W. This northeastward strike in sec. 6 an exception to the 

prevailing trend, which is almost north is the surface expression 

of a mild tendency to echelon folding. It belongs to the same 

flexure as the monocline of the oil field, and its more prominent 

northwestward extension has been described on page 44. From''the 

northeast corner of sec. 6 the outcrop extends north by east through 

the west side of sec. 32, T. 2 N. For nearly a mile north of this, in 

the vicinity of Lefthand Creek, the ridge is lost, but it reappears in 

the west half'of sec. 20, running almost north. On the north side 

of this section the outcrop disappears under Table Mesa. If the 

mesa is crossed in a northeasterly direction, its northern side is 

reached in the middle of sec. 9. At this point the sharp crags of 

this sandstone are seen, forming a bold, bare ridge running far away 

to the northeast (N. 27° E.).
.J STRUCTURE OF THE OIL FIELD. 9

If the mantle rock were removed from the mesa it is believed that 

the outcropping sandstone ledge would be seen to describe a great 

loop (see map, PL II) instead of cutting straight across from the 

point where it disappears under the soutlrwest side to where it reappears on the northeast. The reasons for assuming such a structure 

will appear in a description of the outcrops.

As noted above, the strike for the last mile or two on the southwest 

side of Table Mesa is nearly north. Similarly on the northeast side, 

the ledge which approaches the plateau with a strike of S. 27° W. 

turns, just before its disappearance, to a direction differing only 8° 

from due south. The two outcrops, therefore, on the northeast and 

southwest sides would, if prolonged in straight lines, miss each other 

by about a mile.

There is a second outcropping ledge of sandstone on the south side 

of Table Mesa, a little more.than a mile east of the one mentioned 

above, and about one-half mile west of Haystack Mountain. The 

sandstone of this eastern outcrop also forms a IOAV ridge, and is to all 

appearances identical with that of the more westerly exposure. It 

shows the same succession of beds a strong ledge of sandstone and 

a second horizon, a few hundred feet higher, marked by great calcareous concretions. (See p. 32.) The strike here is N. 35° W., and 

the dip is 10° to the northeast.

This low ridge abuts, as do those of the other outcrops, against the 

steep mesa scarp. It is believed that this line of outcrop is directly 

continuous with that which approaches Table Mesa from the northeast; that it represents the eastern limb of an anticlinal fold whose 

crest has been eroded away; that if the soil cover could be removed 

and the edge of the Hygiene sandstone everywhere exposed it would 

be found again dipping west on the western limb of this anticline a 

quarter to a half mile west of the eastern exposure; that the loop 

would be seen as represented in fig. 9, the outcrop crossing Table 

Mesa with a southerly trend from the middle of sec. 9, passing in a 

southeasterly course past Haystack Mountain, looping perhaps a 

mile to the south, then turning back to the north for 3 miles or more, 

then turning sharply around once more to. the south along the course 

first described.

The structure here described is inferred mainly from dips, all of 

which within the area concerned agree with the features described. 

Although the Hygiene sandstone shows no good outcrop on the 

western limb of the anticline, westward dips as high as 18° are 

plainly seen within the sandy shales on the south side of Lefthand 

Creek near the line between sees. 28 and 29. At the north end of the 

canoe-shaped synclinal trough southward dips are found, according
90 GEOLOGY OP THE BOULDER DISTRICT,, COLORADO. [BULL. 2G5.

perfectly with the assumed structure. These may be seen along the 

high, level ditch skirting the northwest side of Table Mesa.*1

Such an anticlinal fold as the one just described must die out 

gradually at the end. (See fig. 10.) If the axis be traced southward, the dip of the western limb should become less and less 

steep. At a certain point the westward dip will become zero; there 

will be a structural bench, as it were, interrupting the general eastward dip. This bench will be horizontal, except for a southward

FIG. 10. Cross sections showing the folding of the Hygiene sandstone. The straight 

horizontal lines represent the surface of the ground. The solid curve indicates the 

present position of the stratum, and the dotted curve indicates the position of the 

stratum before .erosion. A, 10 miles north of the fortieth parallel, which is 1 mile 

south of Boulder; B, 9 miles north ; C, 8 miles north ; D, 6J miles north ; E, 5 miles 

north ; F, 4 miles north.

inclination. Still farther south the whole will again dip toward the 

east, but in line with the axis of the anticline the dip will be smaller 

than either east or west of that line.

In the north side of sec. 9, T. 1 N., northwest of the Beasley reservoir, the only dip is one to the south, as is made apparent on the map

" This structure was first described by the writer in a paper entitled Structure of the 

Boulder oil field : Bull. U. S. Geol. Survey No. 225, 1904, p. 383. Since that publication 

and since the above paragraphs were written Messrs. S. F. Rathvon and William Rathvon, 

oil operators, have had eight holes dug to ascertain the dip of the shales in sec. 33, T. *J 

N., R. 70 W., and sees. 4 and 9, T. 1 N., and to verify the structure as described above. 

The results have fully confirmed the statements made here.
PBNNBMAN.] STRUCTURE OF THE OIL FIELD. 91

by the outcropping of two arenaceous beds, the lines of outcrop 

having an east-west direction. This is on the axis of the larger 

fold. At this place the axes of the two folds (see p. 88) are in 

proximity, and the eastern and stronger fold uplifts the east side of 

the feebler western fold, making a horizontal bench out of what 

would otherwise be only a partial flattening of the general eastward 

dip. The result is seen on a line passing south through sees. 8, IT, 

and 20, along which the shales have no eastward dip. This belt is, 

however, narrow a small fraction of a mile and in short distances 

both east and west of it decided eastward dips appear.

Immediately east of this bench, perhaps on an average one-fourth 

of a mile, the producing wells are found. Their linear arrangement is all the more striking if the line be extended south to include 

the Buffalo gas well in the southeast corner of sec. 29. The position of the oil-producing strip is in exact accord with that of oilproducing belts of Ohio, where the conditions of occurrence have 

been made so well known by the very complete discussion of Orton." 

The possible finding of other pools remote from such a structure 

need not be discussed here. They may or may not be found in structures differing from the one to which they are here related. It can 

only be said that the example of the already developed portion of 

this field offers no warrant for prospecting where these structural 

features are absent.

It may safely be assumed that the one significant feature in the 

structure of the oil field is the decided arrest of the general eastward 

dip. The peculiar relation of the two folds whose effects at this 

place unite to produce the result is not in itself taken to be significant, except in so far as it explains the occurrence or location of the 

one valuable feature. Further, it must not be assumed that this or 

any similar flexure will necessarily be oil-bearing throughout its 

whole length. Even very small transverse folds affecting the axis 

are sufficient to break up the deposit into so-called " pools." The 

intermittence of good oil territory along the line thus far developed, 

from sees. 8 and 9 on the north to the Buffalo gas well on the south, 

is already known. It can not therefore be assumed that one or two 

dry holes indicate that a line marked by the above-described structure is entirely barren.

SUGGESTIONS AS TO FURTHER DEVELOPMENT.

If, then, the promise of future developments in this oil field be 

limited to a relationship with folds, it is highly important to exam-

" The Trenton limestone as a source of petroleum and inflammable gas: Eighth. Ann. 

Kept. U. S. Geol. Survey, pt. 2, 1889.
92 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

ine similar occurrences beyond the limits already developed. In the 

discussion, however, it must always be taken into account that the 

beds capable of containing deposits are small and of uncertain lateral extent. At the outset the further extension of the immediate 

group of productive wells demands attention. The axis of the fainter 

of the two folds, whose combined effects produce the monocline in and 

south of sec. 8, runs northward from the present developed field, but 

the very few exposures along this northern portion have "not shown 

dips less than 6° or 7°, and therefore no close approach to the structure described above as occurring farther south, in the vicinity of the 

successful wells.

The stronger fold to the east completely neutralizes the general 

eastward dip as far south as sec. 4, while farther north it has a 

westward-dipping limb. The axis of this fold runs north by a little 

west through sec. 4, T. IN., R. TO W., and sees. 33 and 28 of T. 2 N. 

The axis of this fold has not been tested by the drill. At its smithern extremity, however, where the strata begin to dip strongly southward (10° or 15°), is the Two-ten well, and one-half mile farther 

east is the Boulder Illuminating. Both of these wells are small producers (about 2 barrels a day from each well), but the finding of oil. 

even in small quantities, in this relation to the fold is significant. 

From a structural standpoint it may be expected that if larger quantities of oil are present they will be found nearer the axis of this fold 

rather than farther away.

Sheet No. XII of Hay den's Atlas of Colorado shows with considerable detail a series of large echelon folds, but those described above 

are, in general, too small to appear on that map.. The salient curve 

in the Dakota, however, shown on the Hayden map north of Lefthand Creek, must be correlated with the long loop in the Hygiene 

sandstone described above. If this latter formation were drawn on 

the Hayden map, the loop in question would be three-fourths of an 

inch long. A fair idea of the relative sizes of such folds may be had 

by comparing the effects of each on the same formation, as, for 

example, on the Dakota. 

The question of stores of oil in the larger folds naturally .suggests 

itself, in view of the known relation of oil to the smaller. It is to 

be remembered, however, that in the Appalachian region it is not the 

great anticlines which have conserved the hydrocarbons. Here,- as 

there, the greater deformations may prove to have made poor reservoirs, probably by the opening of cracks. Nor, indeed, has it yet 

been shown that the larger of the small folds in the Boulder field is 

an oil reservoir. If the future proves otherwise, there will be a 

strong presumption in favor of restricting the search for oil to folds 

of the smaller order. If the large folds shown on the Hayden map
B-KNNBMAN.] FUKTHEB DEVELOPMENT OF OIL FIELD. 93

are to prove oil-bearing, it will probably be in their remote ends, 

where the fold is dying out on the plains.

A condition of this nature has been put to the test in one instance. 

A fine fold of this kind, north and east of Lyons, flattens out in the 

vicinity of Hygiene (about 12 miles north and 5 miles east from 

Boulder). Here, in a line transverse to the axis, three wells were 

drilled to depths approximating 3,000 feet. They were not without 

showings of oil, but a better showing was obtained 4 miles farther 

down its axis to the southeast, where the fold has flattened greatly 

and was probably never considered a factor. Oil from a depth of 

about 600 feet has also accumulated in an abandoned Avater well, a 

mile north of Hygiene. This is also on or near the axis of the fold.

If the larger folds fail to yield oil, the one remaining hope (probably more promising from the start) lies in the smaller folds. For 

reasons given .above, these, in so far as they affect the Pierre shales, 

are betrayed only by the most obscure outcrops. Yet it is in this 

formation, at a distance of a few miles from the harder rocks of the 

foothills, that folds must be found if they are to serve as guides in 

prospecting for oil.

Herein lies the large significance of the similar though often very 

small flexures which affect the stronger and well-exposed strata of 

the foothills from Morrison to Niobrara, inclusive. In these, stronger 

rocks they may be very small, the looping back generally being less 

than, a quarter of a mile; but the same stress will be found to have 

produced larger results in the weaker Pierre. Therefore, if the 

Pierre shales are to be prospected in this region of echelon folding, 

much time will be saved by making a preliminary examination of 

the stronger strata. Where the Hygiene sandstone is exposed, it'is 

the natural index of structure and the best. Where it does not outcrop, the basal Niobrara limestone is the nearest criterion.

In these suggestions no account is taken of the possibility -of folds 

running strictly parallel to the foothill outcrops. This is of course 

possible, especially south of Boulder, where the echelon type is not 

prevalent. The lines of procedure suggested above are only those 

which have been derived from the known structure of the already 

developed field. They are not intended to exclude consideration of 

structures not yet exemplified.

The Maxwell gas well, less than 2 miles north of Boulder and a 

little more than half a mile from the Pierre-Niobrara contact, may 

seem an exception to all that has been said above regarding the location of oil and, by inference, of the closely associated gas. It might 

therefore be thought of as putting the generalization to a severe test. 

Near-by outcrops are too few and unsatisfactory to fix the structure 

with any definiteness, but it is a remarkable fact that this well lies
94 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BULL. 265.

south-southeast from an incipient echelon fold which aft'ects the boundaries of all outcrops from the Lykins to the Benton. (See p. 43.) 

The axis of this fold would seem to pass a little west of the Maxwell 

well.

SOURCES OF THE OIL.

The strata containing the original substances from which the oil 

of the Boulder field has been derived are not determined with exactness. Much of the Pierre is black with disseminated carbonaceous 

matter. The Niobrara below is bituminous, yielding1 a strong odor, 

especially from its more fossiliferous beds. The same is true of the 

Benton, whose shales are characteristically dark, and wliose bituminous odor is at least as well marked as that of the Niobrara. The 

Dakota bears oil in Wyoming, and an asphaltic substance oozes from 

its cracks at various places .from Wyoming to southwestern Colorado. 

Even the Morrison beds contain some oil, as seen near the Florence 

field.

The strata thus enumerated have a combined thickness of from 

5,000 to 6,000 feet below the horizon of the lowest oil reached in the 

Boulder field. The Benton and Niobrara are doubtless richer in 

organic matter than any higher strata and surely far richer than 

anything below. While these considerations are favorable to the 

beds below the Pierre as the ultimate sources of the oil, others favor 

a source not far from the horizon at which the oil is found. Neither 

hypothesis is established and neither can yet be abandoned.

The great density of the Pierre shales must make them a serious 

barrier to the passage of any form of matter. It is significant that 

the same dense shale which forms the so-called " cap rock," and is therefore considered impervious to oil or gas, also underlies the oil, and 

must therefore have been traversed by the hydrocarbons for thousands of feet if the ultimate source of these substances was in the 

Benton and Niobrara. It is also noteworthy that some of the purest 

white friable silica sand found in deep wells has contained no oil. 

Such sands must have been quickly discovered by the rising hydrocarbons working their way upward from great depths through the 

scanty pores of the shale. Both the general density of the shales and 

the absence of the oil from certain friable sands favor a hypothesis 

which locates the original source of the oil at horizons not very far 

from those at which it is now found. In this case it is safe to say 

that the tilting of the beds has given opportunity for the segregation 

of their fluids according to their several specific gravities. Theory 

would require that the oil should be found just about where the 

present wells, are located; that is, on the slope below the horizontal 

bench. The summit of the monoclinal slope, or edge of the structural
FE'NNE.MAN.] SOURCES OF THE OIL. 95

terrace, should be the approximate western limit of gas, or of oil, if 

there be little gas present. This is because fluids lighter than water, 

after rising through an inclined porous stratum to this line, could 

find no higher place to go.

A similar segregation might occur in beds whose dips are not interrupted by a horizontal bench. 'If oil, in mounting to higher levels 

through such porous beds, finds them gradually changed to dense 

shales, there must result an accumulation along the upper margin of 

the porous portion, regardless of folds and interrupted dips. It is 

not improbable that small accumulations of this kind exist in the 

Pierre shales and may in the future be found. The chance that such 

may exist is independent of the original source of the hydrocarbons. 

It matters little whether this was near the horizon of the oil-bearing 

strata or thousands of feet below.

PIG. 11. Hypothetical cracking in the formation of the oil-field monocline.

The suggestion of a deep source for the original constituents of 

the oil, based on the bituminous character of the Benton and Niobrara beds, is so strong that any plausible explanation of its rise 

through the Pierre shales deserves- consideration. While in general 

highly dense, it is not certain that the Pierre shales directly below 

the known oil deposits are unfractured. If affected by fracture 

they may not be so impervious as would be inferred from a study of 

samples. The folding along this belt may have had more significance than the making of suitable closed reservoirs in which to hold O

the oil. At the foot of such a monoclinal offset as has been described, 

the lower strata might well be expected to be stretched and those 

above compressed. (See fig. 11.) At the top of the monocline, 

the reverse is true; the upper beds should be stretched and those 

below compressed. If such folding, therefore, is to result in greater 

permeability of the lower beds along any belt, it must be along the
96 GEOLOGY OP THE BOULDER DISTRICT, COLORADO. [BULL. 265.

foot of the monoclinal slope. From all observations in this field, 

this tendency to fissuring in the deeper beds might well be the 

chief significance of the monocline. It has been pointed out above 

that the axis of the oil-producing belt so far as explored is a 

full quarter of a mile east of the north-south line along which the 

shales are horizontal. The dip of the shales through which the producing wells are drilled approaches 20°. Assuming the folds of the 

lower strata to be similar to those above, and remembering that no 

reservoirs of gas have been found, which might be expected to crowd 

the oil down the slope, this situation of the oil deposits does not f avor 

the analogy of the anticlinal reservoir. It seems to suggest that the 

foot of the monoclinal slope 'may be quite as important in relation to 

the accumulation of oil as is its top.

There are here, therefore, two possible ways in which folding may 

have affected these oil deposits: (1) It may have provided suitably 

constructed reservoirs; (2) it may have made the deep rocks sufficiently permeable to allow the ascent of hydrocarbons from the 

lower formations whose constitution makes them the most plausible 

.sources. If the deeper origin be assigned to the oil, then the second 

consideration is more urgent in this field than the first, though the 

two are quite consistent. The second consideration provides for 

communication (otherwise seemingly impossible) with the.most 

bituminous beds of the region. It also localizes the upward movement of the hydrocarbon's along lines of fracture, thus accounting 

for the absence of oil and gas in some of the most porous sands. It 

also harmonizes well with the situation of oils far down the monoclinal slope. Finally, such a structure in a mass of shales containing 

occasional sandy beds would result in accumulation at very diverse 

depths. Fractures leading up from below would here strike and 

there miss sands of limited lateral extent, just as the wells reaching 

down from the surface have done.

The stretching and possible fracture of the deeper beds at the foot 

of a monoclinal slope are, of course, not open to observation. The 

corresponding process in the upper beds at the top of the slope rias 

left the marks which were to be expected in this field. A considerable amount of fracturing is evidenced by calcite vein filling. In 

the SE. J sec. 6, T. 1 N., E. 70 W., where there is the incipient 

echelon fold that has been described (map, PI. II), the north side 

of the mesa shows broad exposures of the bare shales. The weathering and erosion of these has left the surface strewn with chunks of 

calcite which occupied the crevices of the shale. These veins are 

often several inches wide. Similar evidence is seen just west of the 

oil wells, where the horizontal position of the shales gives way to 

the eastward dip.
FENNEMAN.] OIL AND GAS. 97

It is not intended to suggest that the lower beds at the foot of the 

monoclinal slope could be affected by open fissures. The necessary 

depth to which permeability, resulting from deformation, must be 

extended in order to reach the Niobrara and Benton is even now well 

above half a mile, and it may l^we been one or two thousand feet more 

at the time the folding took place (assuming subsequent denudation 

to that amount). At such a depth it is not to be expected that crevices 

.in shale will open wide; indeed, the plausibility of the entire supposition here sketched depends upon the possibility of any fracture 

at all. However, inasmuch as the shale, when perfectly fresh, is a 

fairly strong rock, it seems at least probable that deformation 

within the zone concerned would be by fracture rather than by 

flowage. Still the connection between oil accumulation and permeability caused by folding can not yet be affirmed. Some questions

remain to be answered before this supposition can be made a working

hypothesis.

' PHYSICAL PROPERTIES.

In physical tests the products of the different wells show a noteworthy resemblance, despite the diverse depths from which they are 

derived. There are minor differences in color, but in general the 

oil is a light amber and has a paraffin base and an agreeable odor. 

Gravity tests range from 42° to 44.6° Beaume. In all flash tests 

reported the oil has flashed below 60° F., and the exact temperature 

of the point was therefore not recorded. The proportions of the 

more valuable products of refinement, notably that of kerosene, are 

very large. The current price of the crude oil, $1.10 per barrel, is 

the best indication of its quality from a commercial point of view. 

It is said by Oliphant to be the best oil west of Mississippi Eiver.tt

PRODUCTION OF OIL AND GAS.

As stated above (p. 77), there are nearly 70 wells in the productive 

area, though this number includes some very shallow holes. Twentyeight have pumped oil at some time. Twelve of the 28 were abandoned before October, 1904, and a few others were not at that time 

pumped with any regularity.

The oil production amounted to 11,000 barrels in 1902 and 39,000

barrels in 1903. While a very substantial gain is thus indicated

' when the output is stated by years, the monthly production at the

close of 1903 was smaller than at the beginning of the year. The

average daily production for 1904 did not exceed 70 barrels.

0 Mineral Resources Ui S. for 1902, U. S. Geol. Survey, 1904, p. 562, 

Bull. 265 05 M 7
98 GEOLOGY OF THE BOULDER DISTRICT, COLORADO. [BOLL. 265.

One gas well, the Buffalo, in the southeast corner of sec. 29, 

and therefore in the same north-south line with the oil wells, but 

farther south, has yielded considerable quantities of gas for several 

years. The gas is piped to Boulder and was at first used in connection with the manufactured gas. Since September, 1903, no gas has 

been manufactured, and the quantity of gas used from the well has 

been gradually increased. The rate of consumption has at times 

been nearly 3,000,000 feet per month. It is probable that the present 

rate of consumption could not with safety be greatly increased, but 

no diminution of the capacity of the well has yet been observed. 

Its quality is very satisfactory,
INDEX.

A.

Page.

Algonkian system, rocks of................. 21-22

Altona intrusive sheet, description of...... 38

Archean area, igneous rocks of............. 35-36

Archean system, rocks of................... 20-21

Atlantosaurus clays, probable occurrenceof. 27

B.

Bars in Fountain sea, migration of......... 60

occurrence of........................... 58-60

Bays and bars in Fountain sea, occurrence

of............................... 58-60

Bear Canyon, view across mouth of........ 30

Bedford limestone, comparison of, with red

brick ........................... 75

Benton formation, deposition of........... 63

intrusives in............................ 38-39

occurrence and character of........... 29-30

section of............................... 29

Benton sedimentation and uplift, discussion of.......................... 63

"Bobbers," oil wells located by means of.. 78 

Boulder arch, depression of................ 65

history of............................... 64-65

upliftof............................. 61,64-65

Boulder Creek, faults north of............. 54

Boulder district, area and location......... 9

economic geology of.................... 67-98

geologic history of...................... 54-66

geologic map of ........................ 10

stratigraphy of......................... 20-41

structure of............................. 41-54

topographic map of ..................... 9

Brackett, J.R., acknowledgments to....... 10

Brick industry, opportunities for........... 72-73

processes of manufacture............... 74

quality of brick ........................ 75

Bricks, light-colored ....................... 75-76

Buckley, E.K., cited ....................... 75

Building stone, occurrence of.............. 69-71

C.

Clays, occurrence of....'.................... 72-76

Coal, occurrence of......................... 76

Columnar jointing in Dakota'formation ... 29 

Cretaceous system, rocks of................ 28-34

"Crinkled" sandstone, occurrence and

character of.................... 25-26

Cross, Whitman, cited...................... 40

reference to ............................ 22

Crystalline rocks, area of, topography of.... 11

D.

Dakota formation, columnar jointing in... 29 

deposition of ........................... 62-63

intrusives in............................ 38-39

occurrence and character of......... 12,28-29

water in................................ 68-69

Dakota "hogback," position of............ 13

Dakota sedimentation, discussion of....... 62-63

Darton, N. H., reference to................. 20,22

Denver basin, monograph on, by Emmons, 

Eldridge, and Cross, reference 

to .......... 10,14,20,22,24,25,27,28,33,

34,35,40,41,53,59,62,65,66,72,76 

Dikes and intrusive sheets west of Boulder,

map showing.........................' 37

Dip faults, occurrence of, in foothills ...... 48

'' Doctor Bond sandstone,'' source of....... 26

use of, as building stone................ 71

Drainage, adj ustment of, to structure ...... 17

description of .......................... 16-19

examples of stream capture............ 17

Dun can, L., reference to.................... 75

E.'

Echelon folds, description of............... 43-46

Economic geology, discussion of ........... 67-98

Eldridge, George H., acknowledgments to . 11,20 

cited ........... 24,25,27,28,33,34,41,53,59,76

reference to ............. 22,24,25,27,53,65,72

Emmons, S. P., cited............... 14,35,62,65,66

Erosion during Quaternary time, history of. 66 

of shore during subsidence............. 56-57

F. 

Fan structure, discussion of................ 50-51

Faults and landslips, description of........ 46-54

Fenneman, N. M., cited............... 54,76,87,90

Flagstaff Mountain, cross section of east

slope of, figure showing ........ 49

Flagstaff intrusive sheet, description of.... 36-38

Flowing wells, water in.................... 67-68

Folds, echelon, description of.............. 43-46

Foothill monocline, description of......... 41-43

Foothill overturns, occurrence of.......... 50

Foothills, description of.................... 12-13

Foothills near Boulder, views showing .... 12,14

Fortieth Parallel Survey, reference to...... 10

Fountain formation, deposition of......... 57 58

occurrence and character of ........ 12,22-23

Fourrnile Canyon, Echelon fold south of ... 43-^4 

Fourmile Gulch, fault west of.............. 47
100 INDEX.

Page.

Fox Hills formation, occurrence and character of. ........................ 10,33

Fox Hills sedimentation, discussion of..... 64

FrontRange, denned....................... 11

G.

Gas, occurrence and development of....... 76,98

production of........................... 98

George, R. D., acknowledgments to........ 11

reference to .....................'....... 22,50.

Gilbert, G. K., cited........................ 60

Golden arch, history of..................... 65

Granite, occurrence of...................... 69

Green Mountain, fault near................ 46-47

structure of, figure showing............ 12

view westward from.................... 12

Grindstone, occurrence of.................. 71-72

H.

Hay, Robert, cited.......................... 35

Hayden and Aughey, cited................. 35

Hayden atlas, reference to ................. 78,92

Hayden Survey, reference to............... 10

Haystack Mountain, view of............... 14

Henderson, Junius, acknowledgments to.. 10

cited.................................... 50

History, geologic, of the district........... 54-66

Huggins Park, slide known as ............. 49-50

Hygiene sandstone, cross sections showing

folding of....................... 90

occurrence and character of........ 19,31-33

I.

Igneous rocks, description of............... 35-41

of Archean area........................ 35-36

Inoceramus, occurrence of, in Niobrara formation ......................... 30

Intrusions, differences in form of .......... 39

Intrusive sheets west of Boulder, map showing :............................ 37

Intrusives, occurrence of, in Dakota and

Ben ton formations ............. 38

in foothills......................... 36

in Niobrara and Pierre formations . 39

JJohnson,W. D., cited...........

Jurassic system, rocks of.......

...... 15,35,65,66

............ 26-28

K.

Kaolin, occurrence of. 76

L.

Lake basins, natural ....................... 19

Landslips and faults........................ 46-54

Laramie formation, occurrence and character of........................ 10, 33-34

Laramie uplift, discussion of............... 64

Lee, W. T., cited.....................I...... 14

Lefthand Creek, change in, by stream capture. ............................ 18

Lime, occurrence of........................ 72

Limestone beds, occurrence of, in Pierre

formation ...................... 31

Longitudinal valleys, occurrence and character of......................... 13

Page. 

Lykins formation, deposition of............ 61

occurrence and character of............ 24-26

Lykins sedimentation, discussion of........ 61

Lykins warping, discussion of.............. 61

Lyons sandstone, cross-bedding of......... 24

figure showing ..................... 58

deposition of........................... 58-60

occurrence and character of ........ 12,23-24

shore line during deposition of, figure

showing........................ 60

use of,.as building stone................ 69-70

M.

Marshall, plains faults at........... .-, ...... 63-54

Marvine, A. R., cited....................... 21

Mesas, description of....................... 13-16

former westward extension of strata... 16 

origin of................................ 14-16

slope and height of..................... 13

structure and covering of.............. 14

Mesas and foothills near Boulder, views

showing........................ 12,14

Monoclinal ridges, occurrence and characterof........................... 13

Monocline, the foothill..................... 41-43

Morrison formation, deposition of.......... 62

occurrence and character of............ 26-28

Morrison sedimentation, discussion of..... 62

N.

Niobrara formation, deposition of.......... 63

Inoceramus in........................... 30

intrusives in............................ 39

occurrence and character of........ 12,30-31

Ostrea congesta in...................... 31

Niobrara sedimentation, discussion of...... 63

Niobrara shales, outcrops of, plate showing. 30

0. i ~* 

Ohio, oil belt of, comparison with.......... 91

Oil, deposits of, effect of folding on ........ 95-97

drilling for, in Pierre formation........ 82

further development of................ 91-94

map showing location of wells......... 77

occurrence and development of........ 76-98

physical properties of .................. 97

production of........................... 97

reports received from drillers for....... 82-83

shooting of wells for.................... 84-87

sources of............................... 94-97

strata containing....................... 83-84

structure of the field ................... 87-91

surface indications of.................. 76-78

table showing location and depth of

wells ........................... 79-81

table showing wells shot for ........... 86-87

Oliphant, F.H.,cited....................... 97

Orton, Edward, reference to................ 91

Ostrea congesta, occurrence of, in Niobrara

formation ...................... 31

P.

Palmer, C. S., analysis by, of shale from

Austin brickyard............... 73

Petroleum. See Oil.

Physiography of the district................ 11-19
INDEX. 101

Pierre formation, deposition of............. 63-64

drilling for oil in....................... - 82

Hyglene sandstone member of......... 31-33

intrusives in............................ 39

limestone beds of....................... 31

occurrence and character of.... J... 10,31-33

Pierre sedimentation and folding, discus-

 sionof.......................... 63-64

Plains area, faults occurring in ............ 53-54

Plumely Canyon intrusive sheet, description of.......................... 38

Pole Canyon, slip fault in ............'...... 48-49

slip of Lykins formation in ............ 51-52

figure shpwing ..................... 52

use of name ............................ 13

Post-Laramie history, discussion of ........ 65-66

Pre-Pountain topography, description of.. 54-56

Quartzite, occurrence and character of.... 21-22

Quaternary deposits, occurrence and charr

acterof......................... 34-35

R.

Rathvon, S. F. and William, reference to.. 90 

Rathvon, William R., acknowledgments to. 10 

"Red Beds," use of term................. 22

Red rock, use of,' as building stone......... 69

Retreat of linear embankment, figure showing ............................. 60

S.

Sandstone of Lyons formation, use of, as

building stone.................. 69-70

Saurian sandstone, occurrence and character of........................... 27

Sec. 25, fault in............................. 47

Sees. 25 and 36, dchelon folds in............ 44-45

map showing....................... 44

Section, cross, near Boulder................ 12

Sedimentary rocks, occurrence and character of ........................... 21-35

Shale, analyses of .......................... 73,74

"Shells," use of term by well drillers...... 83

Shooting of wells for oil, discussion of ..... 84-87

Page. 

Shore erosion during subsidence, discussion

of............................... 56-57

Shore line during deposition of Lyons sandstone, figure showing........... 60

Sixmile Canyon, dip faults near ........... 48

Sixmile Gulch, echelon fold opposite....... 45

Slip fault, occurrence of, in Pole Canyon... 48-49 

South Boulder Canyon,Echelon fold near.. 43

quartzite of............................. 21

South Boulder peaks, fault between........ 46

figure showing structure ............... 12

Stoddard, W. B., analysis by, of shale from

Lee brickyard.................. 74

Stratigraphy of the district................. 20-41

"Streaks," use of term by well drillers..... 83

Stream capture, examples of............... 17

Structure, adjustment of drainage to....... 17

of the district........................... 41-54

Stuart well,.water in ....................... 67-68

Sunshine dike, description of.......'........ 36

Surface water, occurrence of............... 67

T. 

Table Mesa, Echelon fold of................ 45-46

Terrace origin of mesas, discussion of...... 15

Topography of area of crystalline rocks.... 11

pre-Fountain........................... 54-56

Triassic (?) system, rocks of................. 22-26

V.

Valleys, longitudinal, occurrence and character of......................... 13

Valmont dike,description of............... 40

Van Hise.C.R.,cited ....................... 21

W.

Water, occurrence of, in Dakota formation. 68-69 

in flowing wells..................... 67-68

Water resources, description of............ 67-69

Watts, Hugh F., acknowledgments to...... 10

map by, showing dikes and intrusive

sheets west of Boulder ......... 37

reference to .......................'..... 50,54

Wave erosion, effect of..................... 55,57

'' Wyoming'' group, formations included in. 22 

note on................................. 20

o