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These difrences of diction which in detail are imper-
ceptibly merged are clearly evident when separated ara are
examined. The extreme northwestern part of the Llano quad-
rangle, for example, conpawd with other portions of the
plateau, is almost flat. In the northst corner of the Llano
quadrangle dissection has proceeded further, producing numer-
ous low rounded hills. In the northern part of Burnet quad-
rangle, along the Colorado, steep canyons are developed and the
country is in every way more rugged. This sharply dissected
portion does not continue southward. In the region southwest
of Burnet only a rolling remnant of the plateau remains, rep-
resented by Backbone Ridge and its northeast extension.
Edwards Plateau.-The territory underlain by Cretaceous
rocks, topographically considered, is all a dissected portion of
the Edwards Plateau, and is confined in the main to the east-
ern half of the Burnet quadrangle. The remnants of the
plateau level formed by the Edwards limestone extend as a
number of ridges in a northwest-southeast direction from the
town of Burnet. Within this Cretaceous territory nearly flat-
lying rocks produce a distinct topographic type-benched and
terraced slopes, conical hills contoured by hard and soft beds,
and wide stretches of smooth, grass-covered plains. (See PI.
XI.) The extreme southern portion is maturely dissected,
though here also the nearly horizontal attitude of the alternat-
ing softer and harder layers results in forms of extreme regu-
larity. The relief in this portion is considerable, being greater
than in any other part of the Cretaceous area.
DRAINAGE.
The whole of the Llano quadrangle and about two-thirds of
the Burnet quadrangle is drained by Colorado River and its
tributaries. The northeastern third of the Burnet quadrangle
drains into tributaries of Brazos River, the chief of which
are North Rocky Creek and the forks of San Gabriel River.
Colorado River.-Colorado River is a perennial stream
rising northwest of this region on the flank of the Llano
Estacado and flowing into the Gulf of Mexico. It enters the
Burnet quadrangle near the northwest corner and flows south-
ward and eastward in a series of winding curves, leaving the
quadrangle near the southeast corner. For the first 7 or 8
miles of its course in this area the river has cut deeply into
the Ellenburger limestone, forming precipitous bluffs along a
portion of this section. Passing through the canyon-like
stretch by a series of rapids and quiet reaches, the stream, after
crossing Cambrian strata for a few miles, enters the pre-
Cambrian basin, through which it meanders past generally
low banks. Where it again flows upon Ellenburger limestone
and then upon the Carboniferous rocks, however, in the south-
eastern part of the quadrangle, another canyon-like stretch is
developed, and at Marble Falls the river descends 25 to 30 feet
within a short distance. In this portion of its course, where
the river crosses areas underlain by shale, it has a relatively
broad flood plain.
Colorado River falls in its journey of about 70 miles across
the quadrangle from an elevation of approximately 1025 feet
to one of 650 feet. This gives a total fall of 375 feet and an
average grade of nearly 5.4 feet to the mile. Its volume of
flow varies through a wide range, and the stream is known and
feared for its sudden rises or floods. At low stages of water
the Colorado occupies only a portion of its bed, but during
periods of high water it becomes an impassable, raging torrent.
The more important tributaries of the Colorado from the
east and north are Deer, Beaver, Morgan, Sparerib, Hamilton,
and Cow creeks. From the west and south, besides Llano
River, the largest tributaries are Fall, Campground, Redrock,
Sandy, Pecan, Slickrock, Flatrock, and Doublehorn creeks.
Sandy Creek, which flows eastward and joins the river in the
Burnet quadrangle, drains the southeastern part of the Llano
quadrangle.
Llano River.-Llano River enters the Llano quadrangle
near Castell, flows eastward across the quadrangle, and joins
the Colorado at Kingsland. The stream is perennial, though,
like the Colorado, subject to wide fluctuations in volume. Its
course in this area lies entirely within the pre-Cambrian basin,
and it descends by a series of deeps and shallows, due to the
variable hardness of the rocks over which it flows, from an
elevation of 1200 feet near Castell to one of 800 feet at its
mouth. The river carries a heavy burden of sand and during
low water occupies only a part of its channel. Its flood plain
is limited to narrow strips along its course.
Its principal tributaries from the north are Little Llano
River and Elm, San Fernando, Johnson, Pecan, and Mitchell
creeks. On the south the larger streams are Hickory, Bull-
head, Sixmile, Flag, Oatman, and Honey creeks
Subsidiary drainage.-For convenience in pointing out the
salient features the smaller, streams of the region may be
grouped with spet to the geologic age of the rocks over
which they flow. The strams in the are of pre-Cambrian
rocke owe their character both to the nature of the mews and
to the maturity of the erosion which has taken place. The
pre-Cambrian area, a has been stated, is a brodly eroded,
shallow, and plainlike baing The strums within it are,
arcterized by broad, open vlbys and moderate gradient
and are generally overloded with waste. In most of these
streams little or no water is visible during u& of the year,
Specially in seasons of light rainfall, and the slowly moving
currents can be found only by digging through a variable
thickness of sand. The streams rise rapidly during torren-
tial downpours but soon fall agin and disappear beneath
the sand. During protract droughts even relatively l4e
strums such as San Fernando Creek, Johnson Creek, Sandy
Creek, and others of their type, become partly or wholly dry.
Many of the strums flowing in the ara underlain by
Paleozoic rocks have more sharply cut valleys than those just
described and are apt to flow for longer periods on account of
the more general prevalence of perennial springs in these areas.
Deer Creek, Beaver Creek, and Morgan Creek, for example,
have, along their lower or middle reaches, rather steep walled
canyons cut in more or 1 horizontal limestone and are fed
by springs at several pla along their courses. Unlike the
streams of the basin area, these streams are generally actively
cutting but are often limited in the dissection which they can
accomplish by a scanty supply of water. (See PI. VII.)
Most of the streams in the areas of Cretaceous rocks erode
their channels when water is supplied by abundant rains. The
valleys of many of these streams, especially in the southeastern
portion of the area, where the plateau is rather sharply dis-
sected, take the form of walled canyons, on account of the flat-
lying, alternating hard and soft limestone layers which here
make up the Cretaceous strata. Back from thee bluffs the
valley sides slope away more gently, being cut as with a lathe
into terraced subconical hills. It may be said in general of the
minor streams that their flow is more or 1 intermittent and
directly dependent on a fluctuating and rather meager rainfall.
Exceptions are Cold Creek, Little Llano River, Hurley Creek,
and a few other streams fed by strong springs.
In the northeastern portion of the area, a region removed
from the stimulating effects of the Colorado drainage, the relief
is more gentle and the streams are characterized by relatively
broader and more open valleys.
Springs. - Almost all the springs are situated in areas
bordered or underlain by Paleozoic rocks. They are especially
abundant along the outcrop of unconformities and are also
localized by such structural features as faults. In addition
they occur at other horizons in the Paleozoic sediments.
Of the springs associated with a definite horizon, the most
numerous perhaps are those at the basal contact of the Cam-
brian strata. These springs owe their origin to downward-
percolating water reaching and flowing along the relatively
impervious pre-Cambrian floor. Where the unconformity cuts
the surface, as at the foot of the scarp surrounding the pre-
Cambrian basin, this underground flow issues as springs, most
of which are of excellent quality.
Springs occur also, perhaps in as great numbers as those just
described, at or near the base of the Ellenburger limestone,
which is an especially favorable horizon for their formation.
The overlying Cambrian and Ordovician rocks, being wholly
calcareous, afford many opportunities for water channels to
develop. The water, working its way downward, reaches the
practically impervious beds of the Wilberns formation and
must perforce flow laterally. If opportunity is afforded, as
along scarps or in stream canyons, it issues as springs.
Fault planes, too, have played an important part in the
localization of springs. Cold Creek and Little Llano River
are instances of streams fed by springs which owe their posi-
tion to fault planes acting as barriers to underground flowage
and at the same time permitting water to rise along these planes
under more or less head. West of Burnet, at the fault contact *
of the Hickory sandstone with the Ellenburger limestone, the
same condition exists.
In the Ellenburger and also in the overlying Carboniferous
limestone springs occur at favorable localities, where water from
channels that have formed in the easily dissolved limestone
finds egress to the surface.
AbOufd~ givnngto the la rid a as y
vaeties of for3 Mi gd Po b brdi and alog
stream co the iroea numns deiduouee kl gai,
lm, pcan, and 'oQonwoo bing, the 4m6r imjoit The
cutting dof car, whi h on aIou &n o and t of
ridges in the a undidain by PAleSio jine, has b
carried on fbr Y wrs. PoSt 6a and l ark 4e dominant
woods in ar unidern by kiinite ti h s W1*
their oated types, live oak and ii Oa re well d-
tributed ovr much of the region.
The rolling grassverd plateau, strhing northwa fo
miles in an Sba County and northern and noithstei
Burnet County, aods grazing land for lag f s of shep
and he of cattle.
DESCRPIVE GEOLOGY.
PRMOtlS WORK.
Before the detailed account- of the .geology of the LAfno-
Burnet region is given, rarence should be made to the reut
obtained by earlier workers. Theodore B. Comstoc& gives a
brief and concise account of the results accomplished in the
region prior to his studies, and it is not advisable to repet
such an account here. It is sufficient to mention the names of
B. F. Shumard, J. W. Glenn, Frederick Roemer, Arthur
Schott, S. B. Buckley, A. R. Roesler, W. E. Hidden, Robert
T. Hill, Ralph S. TIrr and Charles D. Walcott, all of whom
contributed to the knowledge of the region. Inasmuch as
certain important conclusions announced by a number of the
earlier writers, especially by Comstock, are at wide variance
with those reached by the author of this folio, it is necessary
to state clearly the more important differences.
B. F. Shumardb made valuable contributions to the knowl-
edge of the region by measuring a number of sections and
making important collections of fossils. His work, with a few
exceptions, was excellent as far as it went. He gave the fol-
lowing section at Morman Mill:
Section at Morman Hl, Tex.
Feet.
200
5
1. Massive beds suberystalline caleareo-magnesian lime-
stone, variegated with brown and purple and forming
rough OMif 8-- -----------------------------------
2. Gray mioaceous sandstone made up of fine grains
cemented with argillo-caleareous matter -----------
Shumard considered the limestone of this section as part of
his Calciferous sand group and the sandstone as Potsdam. He
therefore did not recognize the fault at that place. (See PI. III.)
The limestone is mapped in this folio as the Ellenburger lime-
stone, of late Cambrian and early Ordovician age, and the sand-
stone is regarded as a lentil in Carboniferous shale. Again,
though Shumard seemed to recognize that his basal sandstone
were " based on granite," he appears to have regarded at lest a
part of the granite as intrusive into Paleozoic beds, for he
speaks of these beds as being "highly metamorphosed near the
granite." No evidence of post-Cambrian intrusion was noted
by the author of this folio and the lowest Paleozoic bed were
invariably found overlying intrusive rocks of the pre-Cambrian
complex.
Charles D. Waleott" states that the base of the rocks of
Potsdam age rests unonforably on a great formation that is
stratigraphically the equivalent of Powell's Grand Canyon
series (Unkar and Chuar groups). For thi Se of pre
Potsdam strata he proposed the local name Llano group. He
says: "Massive reddish-colored sandstone rest on alternating
beds of shale, sandy shale, sandstone, limestone, and schist
The strata exhibit but little evidence of metamorphism, being
indurated but little more than the beds of the overlying
Potsdam and Carboniferous." On the same page he ys:
"Across the valley of Honey Creek, 4 miles west of Pa&-
saddle Mountain,. the stta of the Llano group have in more
metamorphosed, plicated, and broken by intrusive dike
of granite.>
The Llano se has proved to be a completely metamor-
phod series of s marbles, and gel_ and can be
ifiAas Aln s in contradistinction to Afehean only on
the very broadest evidence, such as the preponderance of
metamorphic sedimets over igneous material. Resler's
description of thes rc as granitic, metamorphic, and iameus
was therefore concise and, possibly except for his r nee of
the rie to the Archean, correct.
Lack of spae prevents individual disrusion of all the etot
ments made by Comstock which are now nsid4odto X
misinterpeations and to be readily traced to a fe bread moo
conceptions, as noted in the su*seding p ph
1. He did not recognize in fill the nature of the uncon-
formity at the base of the, Cambrian and did not iuticitily
coidet the uneven of the for of d;poin and the con-
seq oint variations 'in to t re and -.mn of i6 beds
doipted on it. ;;:-
CLIMATE.
The uncertainty of rains renders more or less precarious the
maturing of crops, and, although the droughts cause serious
injury to cattle owners, the small farmer is the greatest sufferer
during such periods. An avera of about 20 inches of rain
falls during the year," the heaviest precipitation occurring in
the spring and fall. The region is rarely visited by snow, but
destructive hail storms are not uncommon. High temperatures
at midday, ameliorated in the spring and fall by cool nights,
are the rule for half the year, but during the winter cold rains
and winds from the north, commonly termed northernrs," add
severity to the sason, When gr is plentiful, however, so
doe well without shelter.
VOETATION.
On the ring plains of Llano County quite, persimmon,
everal varieties of thorny buhe, and Phi of various tpes
ItaPh An. WL. M, 1
b To pt4tr. i ma i
JO 7 Ant Sd Adj~ Af1 82, 1
,Ain*. J6. S, SO "i; FI
Am.
a SlmnaLry of the limatoical data for the United Stye, by etions,
U. S. Woatlt Bureau, 19",
or