ILLUSTRATED FLORA OF EAST TEXAS - Brit - Botanical Research ...

42 INTRODUCTION/GENERAL GEOLOGY OF EAST TEXAS
Texas. In fact, if one were to travel from the modern day Texas Gulf Coast west across much
of the state, it would be a trip crossing older and older sediments deposited during the past
145 million years. Further west, to the west of the Cross Timbers and Prairies, lies the vast
area known as the Rolling Plains, underlain by the famous and even older Permian Red
Beds. This region, at least in part, is sometimes referred to as the Red Plains because of the
obviously red color of iron oxides in the Permian strata. The western edge of much of the
southern part of East Texas is marked by the Balcones Escarpment, a striking feature of the
Texas landscape. According to Spearing (1991), “The Spanish explorer, Bernardo de
Miranda, in 1756 named the escarpment ‘Los Balcones’, meaning ‘balconies’, which
describes quite well the stair-step, balcony-like topography rising above the plains.” The
displacement occurred ca. 10 million years ago along the zone of weakness associated with
the ancient Ouachita mountains (Fig. 21). The escarpment is most obvious from Waco south
through Austin and San Antonio. As one moves from east to west across this escarpment,
higher and higher benches or “steps” are encountered until the older Lower Cretaceous
rocks of the Edwards Plateau are reached—about 2,000 feet above sea level (Figs. 22, 23).
In one isolated region to the southwest of East Texas is a rugged area, variously known as
the Burnet Country, Central Mineral Region, or Llano Basin, which includes granite and
other very old Precambrian and Paleozoic outcrops. Here, ancient material has been exposed
by the extensive erosion of overlying Cretaceous sediments. This is one of relatively few
places in the state where materials of igneous origin can be easily viewed (e.g., Enchanted
Rock) (Spearing 1991).
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GUADALUPE MTS.
DELAWARE AND APACHE MTS.
DAVIS MTS.
MARATHON BASIN
SANTIAGO,CHALK, AND CHRISTMAS MTS.
CHISOS MTS.
CHINATI MTS. AND SIERRA VIEJA
QUITMAN AND FINLAY MTS.
HUECO MTS.
FRANKLIN MTS.
HUECO BASIN
GLASS MTS.
FIG.23/PHYSIOGRAPHIC REGIONS OF TEXAS (FROM HISTORICAL ATLAS OF TEXAS,
BY A. RAY STEPHENS AND WILLIAM M. HOLMES.COPYRIGHT ©1989,
BY THE UNIVERSITY OF OKLAHOMA PRESS, NORMAN.REPRINT BY PERMISSION OF THE
PUBLISHER.ALL RIGHTS RESERVED.