ABSTRACT

Archaeological sites along the San Jacinto River valley near Peggy Lake are associated with four major geomorphic elements, valley floor, valley wall, upland, and headward-eroding stream, that have developed during the late Quaternary. Peggy Lake is located 7 mi upstream from Galveston Bay, where the San Jacinto River valley consists of a valley floor (floodplain) at or below sea level bounded to the northeast and southwest by valley walls 25 ft high. The valley walls separate the valley floor from the uplands, which are drained by headward-eroding, intermittent streams. Archaeological sites near Peggy Lake consist of Rangia cuneata shell middens interstratified with Holocene colluvial and alluvial sediments that mantle a soil on the Pleistocene Beaumont Formation.

At least four Quaternary events are evident near Peggy Lake. The first event, Beaumont fluvial-deltaic deposition, probably occurred during the last Pleistocene interglacial stage (Sangamonian, or oxygen isotope stage 5e). Despite a regional dip to the southeast, the Beaumont near Peggy Lake has a gentle dip component to the northwest. If the northwesterly dip component is a primary depositional feature, Beaumont sediments in the Peggy Lake area were probably deposited by the ancestral Brazos River. The second event, an extended period of nondeposition and weathering, caused a 10-ft-thick soil to form on the Beaumont. This soil development and minor downcutting and entrenchment of the San Jacinto River occurred principally during the last Pleistocene glacial stage (Wisconsinan, oroxygen isotope stages 2 to 5d). The third event was migration of the San Jacinto River to its southwestern valley margin near Peggy Lake, where it cut a relatively steep valley wall and truncated the Beaumont soil. This migration occurred in the middle to late Holocene, after the San Jacinto River entered an aggradational phase. The fourth event, late Holocene migration of the San Jacinto River northeastward, slowed erosion of the valley wall and allowed the accumulation of colluvium and the preservation of evidence of human occupation.

Most of the known archaeological sites occur on the valley wall, which is the youngest of the four major geomorpphic elements in the Peggy Lake area. Artifacts from these sites are no more than 4000 yr old. Older valley-wall sites, if present, were eroded during valley expansion caused by lateral migration of the San Jacinto River; other sites predating valley-wall erosion may exist within intermittent stream, upland, and unreworked valley-floor sediments.