Quaternary terraces of the lower Colorado River, Texas
Eight paired terraces, excluding the present flood plain, form low relief surfaces along the lower Colorado River Valley from Austin to Matagorda Bay, a distance of approximately 180 miles. The terraces converge down-valley and near the coast some are completely overlapped, while others broaden out into corresponding coastal terraces. The river terraces (cited first) correlate with the coastal terrace-stratigraphic units as follows: Willis = Citronelle; Gay Hill = Duck Lake (Doering, 1960) = "lower" Willis (Aronow, 1968); Bastrop Park = Bentley; Asylum = Lissie = Montgomery; Capitol = Oberlin (Doering, 1956); Sixth Street = Eunice (Doering, 1956); First Street = Holloway Prairie = Deweyville (?); Riverview and Sand Beach flood plain = Holocene offshore bar and beach deposits. The two younger terraces each possess two distinct levels less than ten feet apart in the Austin area. The terraces reflect high sea level stages throughout the Pleistocene Epoch, from pre-Nebraskan to the present. An abrupt decrease in the slope-profile accompanied by a distinct change from mostly siliceous gravel in the pre-Asylum terraces to granite- derived fragments and limestone pebbles in the Asylum and younger terraces reflects uplift west of Austin with consequent down-cutting of the Colorado River into the Cretaceous limestone formations and the pre-Cambrian rocks of the Llano Uplift. Decreased extent and vertical interval between the Asylum and younger terraces indicates increased frequency of eustatic and possibly isostatic adjustments. The "duality" of the Riverview and Sand Beach terraces may reflect a "pumping" of mantle material under the continent from beneath the Gulf of Mexico under the added load of water as sea level rose following the last glacial stage. The informal name St. Elmo is suggested for a more local terrace that is well-developed three miles south of Austin at the settlement of St. Elmo. This was referred to as the Uvalde terrace by Weeks (1941), but it is considerably younger than the Uvalde gravels which cap the hilltops west of Austin. This terrace is underlain by limestone gravel derived entirely from local sources by a tributary of the Colorado River.