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

36 INTRODUCTION/GENERAL GEOLOGY OF EAST TEXAS
FIG. 16/FIGURE SHOWING AN EXAMPLE OF HOW TEXAS WAS COV-
ERED BY AN INLAND SEA AT VARIOUS TIMES DURING THE CRETACEOUS
(ADAPTED FROM ROADSIDE GEOLOGY OF TEXAS, DARWIN SPEARING,
1991, ©MOUNTAIN PRESS PUBLISHING COMPANY,MISSOULA,MT).
The result is a complex intergrading and
interbedding of Tertiary sediments, some
representing off-shore marine layers (with
fossil shells), some coastal mud flats, others
sandy or gravelly coastal beach or delta
deposits, and still others materials laid down
in the swampy or marshy areas between the
meanders of coastal plain rivers. Numerous
layers were deposited over the area, the type
of layer depending on water depth, distance
from shore, and other factors. Often an
alternation can be seen between material
dominated by calcareous clay deposited under
shallow water marine conditions and layers
dominated by sand deposited on land. The
following sequence, exposed in East Texas,
is an example of such an alternation of geologic
formations: Sparta (sand—S), Weches
(calcareous clay—CC), Queen City (S), Reklaw
(CC), Carrizo (S) (Sellards et al. 1932; Bridges
& Orzell 1989a). These sedimentary processes continue today as modern rivers (e.g., Colorado,
Brazos, Sabine) dump their clay, silt, sand, and gravel near, at, or off the Texas coast.
(Sellards et al. 1932; Spearing 1991). However, a significant amount of the sediment moving
down Texas rivers is today being trapped in man-made reservoirs. This results in the filling
of the reservoirs and the eventual end of their usefulness, as well as the erosion of Gulf
Coast beaches as lost sediments are not replaced.
Eventually, pressure, heat, and time consolidated the sediments, in some cases producing
rather hard sandstones. However, a variety of other materials also formed, including
glauconite (a green-colored hydrous iron potassium silicate clay), iron ore (e.g., in the
Weches formation), and lignite (a low grade coal). Later, changes in sea level and tectonic
movements caused these sedimentary rocks, which now underlie East Texas, to be thrust
above sea level. Erosion then removed material, in some places exposing older layers, and
shaped the surface into its modern form—other than sedimentation, erosion is the most
important geologic factor shaping the landscape of East Texas. In fact, all of East Texas is
an erosional landscape, with sediments slightly more resistant to erosion occupying higher
points in the landscape and more easily eroded layers occupying lower positions.
As one drives from west to east across East Texas, progressively younger wedges of sediment
are exposed at the surface, “arranged like a tipped stack of books” (Spearing 1991)
or a stack of cards, with the youngest layers found near the present-day Gulf Coast (Figs.
19, 20). These sedimentary layers are more or less parallel to the coast (see Figs. 17, 18),
giving stark visual evidence of their history—layer after layer laid down over tens of millions
of years, little by little increasing the size of the North American continent. In fact, probably
the best short-hand description of the geology of East Texas is given by Spearing (1991)
who said the story “is one of tremendous sedimentation and progressive construction of the
southern continental margin of North America....” The one significant exception to this
pattern is the area associated with the Sabine Uplift in the northeastern part of East Texas
in the vicinity of Harrison, Panola, Sabine, and Shelby counties (Collins et al. 1980) (Figs.
17, 21). This structural feature represents an area of regional uplift resulting in slightly
older sediments being exposed at the surface. As a result, some geologic layers which are