There has been an increasing use of geomorphologic ("physiographic") criteria in the mapping of the Quaternary formations of the northwestern coastal plain of the Gulf of Mexico. Begun by Deussen, the employment of these criteria has been increased and improved by Barton, Doering, Fisk, Howe, R. J. Russell, and the writer. Meanwhile the method has been employed along the North Atlantic coast by Cooke, MacClintock, and others. The last few years has seen a rapid advance in the method through recent studies of deltas using precise topographic data, available for the first time, and by extensive use of soil groupings in the tracing of formation outcrops. Lithologic criteria now fall in second place.

The Rio Grande delta is relatively small and contour maps with one-foot contours and modern soil maps are now available for a strip 30 miles wide across the Texas side of the delta along the Rio Grande. No other Gulf Coast delta is now so thoroughly known. Correlations have been carried from the Rio Grande to the Mississippi delta and to the terraces of the Red and Mississippi. Formations recognized are: Recent, Lake Charles (not present on Rio Grande), Ingleside (two latter replace Beaumont), Lissie, Willis, and the Pliocene Goliad. The Willis is the probable equivalent of the Uvalde and the Reynosa term was brought into use because of calcareous soil-hard-pan deposits (caliche) in the older formations, erroneously grouped into a single formation containing "limestone." The Trowbridg and present U.S.G.S. mappings are entirely replaced.

The coastal plain delta is analyzed and its component parts described.

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Downwarping of the thicker coastal areas is balanced by upwarp of the interior parts of the delta plains. Oscillations of sea-level by glacial control caused entrenchment of streams between periods of high sea-level deposition. Continued warping on axes parallel with the Gulf shore lines caused each older plain to slope more steeply gulfward than the next younger one. In the younger plains, slope criteria are secondary to continuity of plains and similar relationship to shore lines traced by continuity.

Shore lines of the Cooke Atlantic coast series are recognizable at 12, 25, 45, and 75 feet above sea in spite of Gulf Coast warping, probably because the warp axes are parallel with the coast line. Higher shore lines may be present. Entrenchment is known to have followed the abandonment of the 12- and 75-foot shorelines. The formations are continued up the Rio Grande valley as terraces. Stream terraces continuous with the intermediate shore lines have not been found.

The subject is presented in outline. Detailed presentation is reserved for publication by the Geological Society of America under a grant from which a part of the work has been done.

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