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

38 INTRODUCTION/GENERAL GEOLOGY OF EAST TEXAS
exposed to the west crop out again at the surface on the Sabine Uplift (e.g., Carrizo
Formation). In general, however, there is a striking progression of younger sediments
towards the coast.
Some of the Tertiary sediments are rich in organic materials from the abundant plant life
in the ancient swamps. These are particularly important economically because their hydrocarbon
compounds (modified by heat and pressure) are the source of significant amounts of
modern day oil, gas, and low-grade coal (lignite). Oil and gas are widely associated with East
Texas, sometimes to the detriment of the environment because of the destructive techniques
used to extract them. Lignite, however, is less well known in East Texas. Probably best
described as somewhere between hard peat and soft coal, lignite represents the carbonized
remains of plants that once flourished in the densely vegetated swamps near the coast.
Examples include Eocene age (about 50 million years old) lignite mined near Winfield west
of Mt. Pleasant and near Longview west of Marshall. Such lignite is used as fuel for power
plants that generate electricity (Spearing 1991).
While predominantly sand, the Tertiary layers of East Texas show significant variation
(e.g., local areas of clay, shale, silt, marl, limestone, gravel, iron-rich layers, glauconite, lignite,
oil and gas-bearing strata, or some sandy layers particularly hard and thus more resistant
to erosion, etc.), which greatly affects modern day landscapes and economies. For example,
according to Spearing (1991), “Rolling countryside speaks of alternating sandy and shaly
sequences of rocks, where the sands form ridges and soft shales erode to form valleys.” The
color of the layers also varies considerably, with some strikingly red due to the presence
of oxidized iron. In fact, in limited areas (e.g., Cass and Morris counties), iron ore was
mined from the Weches Formation beginning in the mid-nineteenth century (Brown et al.
1969) and was used during the Civil War in the manufacture of guns and other metal
objects (Maxwell 1970).
1
2
2A
3
4
5
6
7
PRECAMBRIAN OF THE LLANO REGION
OLDER PALEOZOICS OF THE LLANO REGION
MARATHON BASIN
MISSISSIPPIAN AND ORDOVICIAN OF THE LLANO BASIN
PENNSYLVANIAN OF THE PALO PINTO SECTION
PERMIAN OF THE OSAGE PLAIN
TRIASSIC AND JURASSIC OF THE CAP ROCK ESCARPMENT
LOWER CRETACEOUS OF THE EDWARDS PLATEAU,
LAMPASAS CUT PLAIN, AND COMANCHE PLATEAU
FIG.18/GEOLOGIC AGE OF SURFACE MATERIALS 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 PUB-
LISHER.ALL RIGHTS RESERVED).
8
9
10
11
12
13
14
UPPER CRETACEOUS OF THE BLACKLAND BELT
OLDER TERTIARY OF THE GULF COASTAL PLAIN
LATER TERTIARY OF THE GULF COASTAL PLAIN
PLEISTOCENE OF THE GULF COASTAL PLAIN
QUATERNARY OF THE PECOS VALLEY
LATE CENOZOIC ALLUVIUM OF THE HIGH PLAINS
TRANS-PECOS BASIN AND RANGES