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

GEOLOGY OF THE BIG THICKET
GEOLOGY OF BIG THICKET/INTRODUCTION 173
The geology of the Big Thicket is a subset of that of the Pineywoods as a whole (see page 76).
However, due to its position near the coast, the strata underlying the area are almost all quite
young—at most, no more than a few million years old (late Tertiary and Pleistocene). As
noted by Parent (1993),
As glaciers advanced and retreated during recent ice ages, sea level rose and fell along the low-lying
southeast Texas coast. During warm periods with high sea levels, the land flooded and rivers
deposited vast deltas and alluvial plains of mud, sand, and silt on the seabed. When the ice returned
and sea level fell, erosion cut into the newly deposited sediments. Over millennia, the weight of
increasing sediments caused the land to subside, slanting the layers downward into the Gulf of
Mexico. These layers are exposed to the surface as broad, irregular bands paralleling the gulf, with
the oldest layers to the north and the youngest lining the coast.
The rising and falling sea levels and the meandering rivers typical of relatively flat areas combined
to create an area of surprising hydrologic and soil complexity, despite the lack of topographic
relief. Such phenomena as shifting and abandoned stream channels, sandy levees, oxbow
lakes, and old river terraces contribute to the complexity (Watson 1975).
In a broad sense, the Big Thicket is a shallow basin sloping very gently from north to
south (about five feet per mile (0.94 m per km)—Watson 1975). In general, it occurs on
areas of low relief, with elevations ranging from approximately 365 feet (110 m) in the north
to only slightly above sea level (about 5 m) at the southern extreme (Deshotels 1978). In the
north, the topography is of low-lying hills moderately incised by numerous small streams
(McLeod 1971) and has a more developed ridge system and better drainage, while in the
southern part, the topography is lower and flatter and in general poorly drained. “The most
conspicuous physiographic features of the region are the broad flat floodplains along the
streams and rivers. They are generally well-defined by breaks or bluffs along the edges, and
meander scars, abandoned channels, and backswamps are common” (Marks & Harcombe
1981).
Also geologically important is the diversity of different strata/parent materials. These
result in a variety of soil and microhabitat conditions ranging from well-drained ridges to
bogs and from highly acidic to basic soils—these different conditions have profound effects
on the vegetation.
PRESETTLEMENT AND EARLY SETTLEMENT CONDITIONS AND HISTORY IN THE
BIG THICKET
PRESETTLEMENT AND EARLY SETTLEMENT VEGETATION OF THE BIG THICKET—Even though prior to
European settlement there were extensive areas of forest and “thicket,” the vegetation of the
Big Thicket was never a homogeneous impenetrable area. Instead, based on early explorer
and settler accounts and various studies of the vegetation (e.g., Schafale & Harcombe
1983—from original land surveys from the 1800s), we know it to have been an area of rich
vegetational diversity ranging from dry pine-covered uplands to majestic beech-magnolia
forests, bald-cypress swamps, impenetrable titi thickets, bogs, canebrakes, and even prairies.
Geraldine Watson (pers. comm.), based on nearly 80 years of research, personal observation,
and interviews with elderly members of families who settled the area, also notes that
the Big Thicket was neither homogeneous nor impenetrable. It was an area of varied vegetation
criss-crossed by trails made by Native Americans. These trails, which followed natural
features of the landscape (e.g., ridges, hummocks in the bottomlands), were later used by
the Spanish, French traders, explorers, and settlers. Eventually, in some cases, the trails were
widened and paved.