39
REGION 18: THE BLACKLAND PRAIRIE
THIS prairie, in Texas, occupies clay and silt soils containing much calcium carbonate disseminated throughout. That of the northern portion extends westward to cover a part of the lower as well as the Upper Cretaceous, the two being separated by the Woodbine Sand whose outcrop supports the Eastern cross-timbers.
Reference to the vegetation map (Plate I.) shows this region to begin at a point below San Antonio and extend northeastward as a gradually widening strip which above Waco broadens out greatly and is cleft by the Eastern Cross Timbers, growing, as above indicated, upon the outcrop of the Woodbine Sand. That portion lying west of the timbered strip is supported by the rolling hills of the Lower Cretaceous, that lying east, by the marly clays of the Upper Cretaceous.
The eastern or "black waxy" prairie extends into a rainfall region of 40 inches; the western into a region of 30 inches or less. Decreasing rainfall has a marked selective effect upon the species which comprise the plant population. Eastward, the dominants are of the tall bunch grass type; westward, these tend to give way to grasses of lower stature. Big and little bluestem, dropseed and needle-grass are representative dominants eastward; gramas, Triodia and buffalo grass increasingly so westward.
Included within both regions are occasional rough stony outcrops covered with cacti, Condalia and other thorny scrub. Local sand and gravel deposits support a growth of postoak. Mesquite is quite common in the whole region; less so in its northeastern portion than elsewhere. It covers some of the marly eastern portion, and also deep soiled, level areas in the western portion with a more or less dense stand of scrub timber, frequently admixed with prickly pear.
Being highly adapted to agriculture, the region is almost wholly under cultivation except in the extreme northwest where rainfall is insufficient. Such land as is untillable is devoted to pasture and usually greatly over-grazed. A few native hay meadows have been