The coastal margin of Southeast Texas occupies a band along the Gulf Coast from the shoreline to about 15 miles inland. This geographic area is a mixture of coastal prarie, woodlands, wetlands, brackish water lakes, and river deltas. Freshwater streams discharging into the Gulf become brackish in the coastal margin due to saltwater from tidal flow. The brackish water lakes are generally remnants of former river channels. A high proportion of the archeological sites on the coastal margin are Rangia cuneata shell middens. Some oyster shell middens are found on the coastline in a true marine saltwater environment. The general Galveston Bay area, including San Jacinto, Trinity, and East Bays, is particularly well-known for Rangia shell middens.

During the Early PaleoIndian era, sea level was around 400 feet lower than today, moving the coastline 40 to 100 miles 'offshore' (map of Gulf coast during ice age).