Page 6 of 19
4
buttes in the area of the Edwards Plateau and do not occur in the Coastal Plain, except in one small outcrop along the eastern foot of Mount Barker, too small to be shown on the map.
The yellow clays of the Walnut formation most commonly occupy a terrace or bench resting upon a hard stratum of the Glen Rose formation, with a nipple-like hill of the Edwards limestone occur ring above. This bench forms a conspicuous feature of the landscape of the Edwards Plateau.
Comanche Peak limestone. —This is a persistent bed of white, chalky limestone, presenting a shat tered, reticulated appearance on weath- Persistent ering. It is partly characterized by an chaikV^meV abundant fossil fauna containing a large number of Exogyra texa?ia, which is especially abundant in its basal portion. It is from 40 to 50 feet thick, thinning toward the Rio Grande. Although it is insignificant as regards thickness, and may be considered the base of the Edwards limestone, it is one of the most persistent paleon tologic horizons of the Texas Cretaceous sections. In the Austin quadrangle this formation occurs only within the area of the Edwards Plateau, usually as a chalky slope immediately below the cap rock of Edwards limestone and above the bench occupied by the Walnut formation. A typical exposure of this formation may be seen on the top of Mount Barker, northwest of Austin, and its section is shown on Columnar Section sheet 2.
Persistent bed of white, chalky limestone.
Edwards limestone. —The Comanche Peak lime stone is the base of a thick group of limestones consisting of the Comanche Peak and Edwards formations. The upper formation contains a large number of flint nodules with vast quantities of Rudistes and aberrant Cham idee, and was called by Shumard the Caprina limestone.
This formation is the most conspicuous and extensive in the Texan-Mexican region. It is composed mostly of limestone strata, but there are some marly layers. It shows slight variation in color, composition, texture, and mode of weath ering. In general the beds are whitish, although layers of buff, cream, yellow, or dull gray are frequent. These colors depend much upon weath erftig. In composition many of the beds are as nearly pure carbonate of lime as can be found in nature, but some have small carifonateoi admixtures of silica, epsomite, chloride of sodium, and perhaps other salts as yet unde termined. Occasional bands of soft brownish yellow stone are intercalated with the limestone. These bands are popularly called " magnesian," and are composed largely of exceedingly fine-grained siliceous material, like tripoli. As these beds often contain flints, the silex may be of organic origin. Clay is absent except as a minor consti tuent in the few marly layers. Iron is sparingly present as pyrites, and is revealed by the red color of the clay that weathers out of a few beds. Exceedingly fine siliceous particles occur, espe cially southward from Comanche County — but no sand grain, pebble, bowlder, lignite, or other undoubted piece of land-derived debris has ever been found.
Beds of pure carbonate of lime.
The limestones vary in degree of induration from hard, ringing, durable strata to soft, pul verulent chalk that crumbles in the fingers and resembles very much the prepared article of com merce. Some of the beds are coarsely crystalline, with calcitized fossils, and are susceptible of high polish. The beds also vary in texture. Some of them are porous and pervious, while others are close grained and impervious. Some are homo geneous throughout; others have hard and soft spots, the latter dissolving by the percolation of underground water and constituting what is popu larly termed " honeycombed " rocks. The harder spots in some cases seem to be in process of indu ration, suggesting a step in the formation of flints. The holes in the honeycombed layers often repre sent what were once spots containing soluble salts of iron and other accessory minerals.
The formation can usually be distinguished by the immense quantity of flint nodules embedded in and between the limestones and scattered over the surface everywhere. «tfes o" an= inese are ot many shapes; some are fusiform, like elongated roots; others are knotty, like warty potatoes; others are parts of extensive sheets or very flat lenses. They vary in size from
Great quantities of flints.
that of a hen's egg to a foot or more in diameter. They also vary greatly in color. On fresh frac ture some are almost jet black; others are light blue, gray, or opalescent; still others are delicate pink in color. There is some, but not conclusive, evidence that each particular kind of flint occupies a definite horizon.
In most cases the Edwards limestone may also readily be distinguished by the peculiar aberrant mollusks of the genera Monopleura, Meqtrienia, and JRadiolites — bivalve fossils which have cor nucopiate form, suggesting a resemblance in shape to the horns of cows, goats, and sheep.
The formation is stratified into a succession of massive beds accompanied by very few flaggy and marly layers. Some of the strata are harder than others and project beyond the softer layers in the profile of the hills as overhanging shelves; others are soft and erode very rapidly. In the Austin quadrangle the Edwards limestone is almost inseparable from the underlying Comanche Peak, since both are composed chiefly of carbonate of lime. The Comanche Peak strata are less con solidated, and, as they are somewhat argillaceous, possess a more marly texture than the Edwards limestone, which is usuall) r a firm, white, ringing limestone of great hardness and durability; so that the Edwards weathers into cliffs, while the Comanche Peak is wrought into lower slopes; but in most cases reliance must be placed on paleonto logic determinations to distinguish the two forma tions.
Neither is the Edwards limestone always sharply defined from the overlying Georgetown, except by paleontologic criteria. It is true that the Georgetown limestone is somewhat more arena ceous, but the differences are so slight that their detection requires the trained eye of the geologist.
The Edwards limestone, being more purely calcareous than any other of the Comanche series, probably corresponds to the deepest submergence of the Comanche epoch. de Ve'p e snu Cb- 0 t • i /~ii t> f mergence. It is true that m the Glen Rose forma tion occasional thin beds of chalk occur, and that some of these are composed almost entirely of foraminifera, but such chalks usually contain a considerable percentage of clay, recognized as an offshore deposit.
Evidence of deep submergence.
Topographically the Edwards limestone is one of the most important formations in Texas. In fact, it is the determining factor in the topography of the whole of the Edwards Plateau. Its hard ness being superior to that of the overlying and underlying beds, its consequent resistance to ero sion has preserved it as the capstone of the innumerable buttes and mesas of this portion of the State and of the extensive Edwards Plateau and Grand Prairie regions. Not only are most of the buttes and mesas capped by it, but these are accompanied by scarps overlooking the lower lying valley prairies which follow the stream. The walls of the canyons which many of the streams have cut are also composed largely of the Edwards limestone, especially the portions of those crossing the Bear Creek belt. The cliffs of the Colorado between Austin and Bee Creek form a conspicuous exposure of this limestone. To its hardness is also largely due the topography of the limestone mountains of Mexico.
It shows many types of weathering. Some of the strata make bold cliffs nearly 50 feet in height, the faces of which, although apparently of homogeneous texture, weather into small open caverns. This weathering sometimes brings out a thinly laminated structure associated with white efflorescence. The bottoms of caverns of this character are filled with a layer of white, pulver ulent earth. The residual products of other massive ledges weathering into caverns are ver milion-colored clays, in which are beautiful fossils composed entirely of crystallized calcite.
The hard limestones weather into vertical, square-cut bluffs, while the soft and more homo geneous beds of marly or chalky tex- ger|es of ture form slopes. Where these hard cvf efs n aand and soft beds occur in alternation there is a corresponding alternation of scarps and slopes in the topographic profile.
Series of alternate cliffs and slopes.
Where the Edwards formation makes extensive stretches of level country, such as that of the Bear Creek country between Manchaca and Oak Hill and that upon the summit of the plateau,
and where the surface stratum is of homogeneous texture, it weathers into innumerable little ridges, crests, and drainage lines, illustrating in miniature the processes of erosion and mountain carving. These minutely eroded limestone surfaces are tech nically known as " karrenfelder," and are formed by the solvent effect of rain water upon the sun heated limestone surfaces. The crevices in these level areas of Edwards limestone country are usually grass covered, with occasional patches of scrub oak. The surface is very rocky, the karren felder protruding in jagged points through the rich but scanty soils. Sometimes residual flints occur in such immense quantities over these sur faces that one is apt to mistake them for a water rolled gravel formation.
The base of the Edwards limestone is found capping many of the high buttes of the Edwards Plateau, as shown on the geologic map, while the upper and greater portion is exposed along the Bear Creek country, east of the Balcones scarp. This is well displayed in the banks on the south side of the Colorado in the western part of Austin, between McDonald's brickyard and the city dam. Owing to faulting, this section is somewhat com plicated and is not continuoush T exposed at any single locality. The details as made out at three localities at Austin are shown in the accompany ing sections and figures. It will be noticed that in the lower portion of the Bee Creek section, which is still above the base of the whole of the formation, arenaceous marls and limestones are rather numerous. These play an important part in the artesian conditions from the Colorado south westward.
The following sections represent the entire thickness of the Edwards limestone exposed on the downthrown side of the fault in the bluffs of the Colorado between Austin and the river level at the mouth of Bee Creek. (See Columnar Sec tion sheet 2.)
The base of the beds is concealed in the Coastal Plain, lying probably less than 100 feet below No. 1 of section C, but can be seen on the upthrown side of the fault, capping the remnants of the plateau.
Section of bluff on Barton Creek about 1 mile above Barton Spring, Travis County.
Section of Deep Eddy Bluff, south of the Colorado River, west of Austin.
* The Georgetown limestone is assumed to be 70 feet thick at Austin. Future study may modify this estimate. The uppermost layers, characterized by Kingena wacoensis, are here missing. t The beds in the Barton Creek section below 43 can not be correlated layer for layer with the Deep Eddy Bluff section; therefore numbers are not used in the description of the former section for beds below the one numbered 43 except the one lettered b. which is equivalent to 29 of the section below.
Section of bluff at mouth of Bee Creek.
Georgetown limestone: * Ft.
In.
5. Grayish limestone, irregular fracture, with Alectryonia carinata and Qryphcea wash- itaensis 1
0
4. Yellow or reddish calcareous shale 4
4
3. Alternating layers of hard and soft limestone
with Alectryonia carinata, Qryphcea wash- itaensis, Exogyra americana, etc 18 2. Hard, grayish limestone 33 1. Soft, chalky limestone, with a saline taste.... 13
0 0 0
Total thickness of Georgetown limestone... 69
4
Edwards limestone:
49. Nodular limestone full of requienias (first
req uienia bed) 3 48. Nodular limestone; nodules as large as one's
0
head 2
0
47. Hard, chalky limestone 3 46. Thinly laminated limestone (the so-called "lithographic flags") 8 45. White, sublaminated, chalky limestone. The lower part of Nos. 45 and 46 contain many
0 9
fossils — Exogyra texana, Pholadomya
knowltoni, etc 8
5
44. Nodular limestone, no requienias 1 43. f Nodular limestone with many requienias
0
(second requienia bed) 3 e. Laminated limestone . 1
9 0
d. A series of hard limestone ledges (eight in number), separated by the thinly laminated layers. There are some flints, about as
large as a man's fist, lladiolites and Chon-
drodonta munsoni 45
8
c. Flaggy layer with discoidal flints 2 (29.) b. Hard limestone, forming a shelf along this portion of Barton Creek and its bottom at the bridge below, eroded into deep pot holes. The lower 2 feet of this layer con-
4
tains very large blue flints, often 1 foot across. Some of them are oval, others
flattened out and very irregular in outline. The upper part of bed contains small flints.. 12 a. Limestone ledges, with some flattened flints. All of the flints in this section belong to the
8
blue variety 11
0
Base of a is Barton Creek bed.
Total thickness of strata in bluff 171
11
:3. Nodular limestone with requienias at top (the second requienia bed of the Barton Creek sec-
tion).
42. Limestone ledges 5 41. Limestone ledges containing requienias. The three layers above described form a slope to the top of the hill (or bluff) above the face proper of the bluff 1
0 0
40. Ledge of hard limestone, 10 inches above basal sheet flint. The upper part of the ledge con- tains rather small nodular flints 15
0
39. Limestone weathering out and giving rise to a good deal of red clay, apparently representing the zone of calcitized fossils found in the high
bluff above McGill's Ford 6
6
38. Massive, thick ledges of limestone; detail not
exposed 23
8
37. Soft, white, arenaceous limestone 2
2
36. Soft, arenaceous limestone 3
10
35. Ledge of limestone, rather soft, emitting odor of petroleum 2 34. Chalky limestone, forming little caves, composed of a good many small ledges; discoidal flints at
10
top 4 33. Hard limestone, emitting odor of petroleum under blows of hammer. Texture of limestone rather
0
mealy. Nodular flints; occasional discoidal
flints in top 2
1
32. Two thin ledges of limestone; layer of sheet flint
in top 1 31. Ledge of thick, massive limestone 5 30. Hard, yellowish limestone 2 29. Hard, thick, massive ledge of siliceous limestone, ringing under blows of hammer. At the base there is a layer, about 9 inches thick, of opales- cent, pinkish or brownish flint. Apparently the limestone is being converted into flint by
9 0 0
replacement, and the process has not yet been
completed 6 28. Soft, chalky limestone, dissolving and forming
0
small caves 3
0
27. Soft, chalky limestone with very large (may be 1 foot long), irregularly shaped blue flints at top. 2 26. White, chalky limestone, apparently siliceous; zone of flint near top. The flints blue, discoidal,
3
and tending to form sheet 6
8
25. Massive ledge of hard, bluish limestone 7 24. Very hard limestone 0
0 6
23. A layer of enormous blue flints, in some places
over 1 foot thick 1
0
22. Thick, massive ledge of limestone, rather soft, yellow in color, and slightly arenaceous 5 21. Ledge of hard, yellowish limestone with a zone of flints tending to form a sheet at base 1 20. Soft, white, slightly arenaceous limestone, com- posed of thin ledges; upper 2 feet, middle 4 feet, lower 1 foot 7
5 4 0
19. Soft, yellowish or whitish limestone with layer of flattish, bluish flints forming a sheet at top. This is really three ledges; upper ledge, with flints at top, 2 feet; middle containing concre- tions of calcite in lower part, 4 feet; lower ledge exposed at low water, 1 foot 7
0
Total, Deep Eddy section 121
5
Ft.
In.
Limestone slope, detail not exposed
11
0
23. Layer of enormous blue flints
1
0
22. Arenaceous limestone
5
5
21. Hard, yellowish limestone with sheet flint at base. 20. Yellowish, rather hard limestone, somewhat sili- ceous; thin band of chalky limestone at top;
1
4
calcite concretions near base
6
0
19. Sheet flint at top (sheet flint at top of lowest ledge of Deep Eddy Bluff); three ledges of limestone: upper, 1 foot; middle, 2 feet 6 inches; lower (containing calcite concretions), 3 feet 18. Sandy limestone, with two zones of nodular flints near middle; sheet flint at base; mass of requi-
6
6
enias just above the sheet flint 17. Soft, yellow, calcareous sandstone, a part of the
10
0
preceding ledge, about 16. Yellow, cherty limestone, about 15. Three or four ledges of rather soft, whitish or yel- lowish limestone; the upper ledge containing a great mass of requienias, the others fewer 14. Solid, white limestone, granular, not very hard; contains a great many requienias near top
3 0 8 6
0 6 1 11
13. Yellow, arenaceous limestone
4
0
12. Blotched, arenaceous limestone 11. Soft, yellow, arenaceous limestone 10. Hard, yellowish, granular limestone, with shell
3 2
8 0
fragments, gray on fresh exposure
1
6
9. Soft, yellow, arenaceous limestone or calcareous sandstone
2
4
8. Ledge of nonindurated, granular limestone, with indurated blotches, which are structureless and
flinty looking
1
0
7. Ledge of white, rather soft limestone, with many very irregularly shaped flints in a zone about the middle of the ledge. The flints are mostly
small, bluish in color, and do not show con- centric banding; about 6. Ledge of white, rather soft limestone; no flints; a few fragmentary fossils
3 2
0 6
5. A soft, arenaceous ledge. The lower 1 foot 10
inches is a subledge. In the upper part (near top) are concretionary bodies that in form resem- ble flints, but are not flints in texture. These
bodies are hard, apparently siliceous, and con- tain white blotches, some of which appear to be of foraminiferal origin 4. Hard limestone, whitish or bluish, without flint;
5
5
not fossiliferous
4
0
3. Arenaceous limestone, has a tendency to lamina- tion, but in the ledge the laminated character
is not always evident. The upper part of the ledge by solution becomes porous. The rock has a considerable absorbent power for water,
and has a dark (wet) appearance, due to con-
tained water
5
9
2. Thick ledge of white limestone, not very hard, oxidizing yellow from contained iron. Contains
a large number of irregularly shaped flint nodules. These may be as much as 1 foot long, but usually are rather small — 3 or 4 inches in length. They are bluish in color and have a
concentrically grained structure, resembling the graining of pine wood. Their long axes are not
