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

GENERAL GEOLOGY OF EAST TEXAS/INTRODUCTION 31
INTRODUCTION TO EAST TEXAS
OVERVIEW
East Texas is an area of approximately 62,600 square miles (162,200 square kilometers),
delimited on the east by the state border, on the north by the Red River, and extending
west and south to Dallas, Austin, and nearly to San Antonio and Houston. While a small
region compared to Texas as a whole (approximately 269,000 square miles = 697,000
square km), it is about the size of Georgia. It includes the Pineywoods, Post Oak Savannah,
Blackland Prairie, and the Red River Area (Fig. 2). The flora includes 3,402 species, slightly
more than two-thirds of the total for all of Texas (Hatch et al. 1990; Turner et al. 2003),
and 3,660 taxa (species, subspecies, and varieties) overall. This biological diversity is the
result of numerous factors, including the region’s geologic and climatic variation and its
location on the ecotone or transition zone between the eastern deciduous forests and the
central North American grasslands. Further, this biological crossroads is influenced by its
proximity to the southwestern deserts, the southeastern swamps, and the nearly tropical
areas of southern Texas. East Texas is thus a mixing ground for plants, with the result being
high species richness. For the past two centuries, humans have had, and are continuing to
have, a tremendous impact on the plants and animals of the region. Presettlement and early
settlement conditions were radically different from those found today, and environmental
change continues to accelerate. Given current trends, the present generation may be the
last with the opportunity to preserve even small remnants of the once extensive natural
ecosystems of the area.
GENERAL GEOLOGY OF EAST TEXAS
The geology of East Texas is a fascinating story which has unfolded over millions of years.
It is also a story that is key to understanding the plant life of the area, as well as the mineral
wealth (e.g., oil and gas) so important to this part of the state. The geological history
of the region is much more interesting and complex than the gently rolling to flat topography
of most of East Texas would suggest. It is a subtle tale that can be read in stream
beds, road cuts, or other areas where the soil is removed and small areas of the underlying
bedrock are exposed. In only a few places can rocks be seen at the surface (e.g.,
Daingerfield State Park in Morris County; Boykin Springs in Angelina County—Fig. 10).
In general, the easily erodible rocks and relatively heavy rainfall have combined to create
a thick soil layer obscuring the interesting geology beneath. During the Pennsylvanian
Period (325–286 million years ago [mya]) (Fig. 11), what is now East Texas was under an
ocean on the southern edge of a North American continent shaped very differently than it
is today. Later, as a result of plate tectonic movements, North America collided with Africa
and South America to become part of the supercontinent Pangaea. The outcome of this collision
was the uplift and formation of an extensive mountain system including the
Appalachians, Wichitas, and Ouachitas. The ancient Ouachita Mountains formed approximately
300 million years ago in a line roughly following the western edge of the current
Blackland Prairie, from near Sherman and Dallas, south along the Balcones Escarpment to
Austin and beyond (Fig. 12). This ancient Ouachita mountain belt also continued to the
northeast; the eroded Ouachitas seen today in southeastern Oklahoma and southwestern
Arkansas are remnants of this once much more extensive range. To the west of the ancient
Ouachita Mountains, crustal areas sagged and low basins formed. Shallow inland seas
invaded these low areas, and during the Pennsylvanian, and later the Permian (286–248
mya), western Texas served as a collection basin for the sediments that eroded from the
Ouachita Mountains east of the basin. These thick sediments harbor oil-bearing layers, so