As indicated by this (left) extract of the geologic map of Texas (1992. Bureau of Economic Geology, The University of Texas at Austin), floristic diversity along the North/South axis of the Brazos/Navasota drainages is, to some extent, concordant with substrate variation that is due to a series of geologic formations that transect the area.
This is especially evident when we encounter outcrop areas and take a close look at floristic composition. This trip took us into into the Carrizo Sand formation. We came into immediate contact with Carrizo Sand bedrock on our first stop, and again when we collected in the foothills of Sugarloaf Mountain.
Distribution of this formation in the Gause area can be seen from this extract from the Geologic Atlas of Texas (1981 - Austin Sheet - Army Map Service/Texas Department of Water Resources) as compared to our trip map.
As compared to Miocene/Oligocene-age formations that we encountered at Old River Ranch near Clay, and the Moore's Hill site, just East of Navasota, the Eocene-age sandstone has produced deep sand accumulations. Many distinctive elements of the state flora, including a suite of endemics, have adapted to the Carrizo sands and these are known locally as the 'sandyland flora'.
An indicator of these 'sandyland' floras is an oak that produces distinctive, elliptic leaves with a bluish cast. The plants were in full flower during out visit:
Also, associated with the Carrizo sands are bog areas that appear to be maintained by constant water flow through these sandy deposits. Many of these occur along the upper Navasota drainage in Robertson and Leon counties, but one - Wall Bog - can be found along Beaver Creek in Milam County.
As has been the case all Spring, the flora of this area is clearly 'depressed' and this lack of growth/development was evident among bog-adapted plants associated with the pond and sand-adapted plants inhabiting the slope above the bog. We saw many interesting taxa that were not in flower and best exposure to the Carrizo Sand/Bog flora is probably - at this point in time - via the TAMU Herbarium Flynn Bog System WWW pages. We will be visiting this area soon.