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

SOIL-RELATED GEOLOGY, PINEYWOODS, POST OAK SAVANNAH/INTRODUCTION 53
Because of the high clay content, in some parts of the Pineywoods small prairies develop on
the Fleming (e.g., Windham Prairie in Polk County—Brown et al. 2002b). Further, in areas
of lower rainfall to the west of the Pineywoods, the Fleming supports extensive grassland vegetation
(Sellards et al. 1932)—e.g., it underlies much of the Fayette Prairie (Smeins &
Diamond 1983).
OAKVILLE FORMATION—The Miocene Oakville, a continentally deposited layer of limey sandstone,
is found in limited areas of East Texas, particularly in the central Post Oak Savannah
(e.g., northeast of the town of Navasota in Grimes County, near Kountze Bayou in Burleson
County, and Monument Hill and Kreische Brewery State Historic Sites in Fayette County). It
also lies beneath a small portion of the Fayette Prairie (Smeins & Diamond 1983). Because
the sandstone tends to be more resistant to erosion than most adjacent sediments, the
Oakville often occurs as low ridges or hills (e.g., Monument Hill). It outcrops in a northeast
to southwest band about eight miles (12.8 km) wide across East Texas (Sellards et al. 1932)
and for much of its length is covered with soil. However, where hardened sandstone reaches
the surface, a rich and unusual flora has developed. There, on soil-less or nearly soil-less areas,
many plants are found that are more typical of the Edwards Plateau hundreds of miles to the
west (e.g., Diospyros texana—Texas persimmon) (Reed et al. 2002). Further, one of the two
known locations of the East Texas endemic Navasota false foxglove (Agalinis navasotensis) is
from such an Oakville site in Grimes County (Canne-Hilliker & Dubrule 1993).
CATAHOULA FORMATION—This Oligocene formation is made up of “thick, light-gray, fine- to
medium-grained quartz sand and interbedded light-olive-gray mud. The sand is locally
indurated and tuffaceous” (Jackson & Garner 1982). The formation was in general laid down
under continental conditions and, in addition to other materials, contains significant amounts
of volcanic ash and tuff, at least some of which probably came from southwest Texas (Sellards
et al. 1932). In places, the resulting sandstone and siltstone can be quite consolidated (Bridges
& Orzell 1989b). The surface outcrop of this formation in East Texas varies from about four
to six miles (6.4 to 9.6 km) in width (Sellards et al. 1932). Soils of the Catahoula Formation
are mainly Alfisols with a loamy surface layer over a dense plastic clay subsoil. In some areas
where the Catahoula sandstone outcrops, “sandstone barrens” characterized by shallow soils
FIG.25/CROSS SECTION OF THE UPLAND ISLAND WILDERNESS AREA SHOWING THE CATAHOULA FORMATION (OLIGOCENE IN AGE) AND THE TOPO-
GRAPHICAL TRANSITION FROM A MAJOR RIVER (THE NECHES), THROUGH FLOODPLAIN, TO SLOPE AND DRY HILLTOP WITH LONGLEAF PINES (THE “UPLAND
ISLAND”) (FROM REALMS OF BEAUTY:THE WILDERNESS AREAS OF EAST TEXAS BY EDWARD C.FRITZ, PHOTOGRAPHS BY JESS ALFORD.COPYRIGHT © 1986.
BY PERMISSION OF THE AUTHOR AND THE UNIVERSITY OF TEXAS PRESS).