The coastal prairie region of southeastern Texas is a deltaic coastal plain. A palmate series of irregularly branching, broad, low, sandy ridges in Harris, Fort Bend, and Brazoria counties represent the delta of an ancient Brazos River, and a similar series of ridges in Fort Bend, Chambers, and Liberty counties represent the delta of an ancient Trinity River. These old deltas controlled the development of the physiography of the coastal prairie region of southeastern Texas. Ridges that represent old natural levee systems are major topographic features of the coastal prairies. Most of the smaller streams of these prairies are interdistributaries that flow among the distributaries of the deltas. Those distributaries have aided in determining the positions of the larger streams. Galveston Bay is an interdelta bay; it lies between the ancient deltas of the Trinity and Brazos rivers.
A deltaic coastal plain has through major streams that are essentially without tributaries, as well as a branching dendritic net of smaller streams, most of which were originally interdistributary. A normal marine coastal plain, in contrast, has through streams that have as many tributaries as the local streams, or even more, and as large or larger drainage basins. The shoreline of a deltaic coastal plain may simulate closely a shoreline of depression, but an analysis of the erosion of low-lying coastal plains shows that funnel-shaped bays, such as Galveston Bay, that extend back into a youthful plain can not have been formed by the drowning of erosion valleys.
The drainage pattern of the area of the outcrop of the Lissie formation suggests the existence of a still older Pliocene-Pleistocene deltaic plain.