. Bulletin of the Geological Society of America. Geology. FORMATIONS OF PACIFIC COASTAL PLAINS OF TEHUANTEPEC. 23 feet of water-worn pebbles below, which are overlaid by 3 feet of the red loam. These accumulations rest upon decayed gneiss. Near by, the river exposes the gravels and loams filling an old valley, where the thick- ness is 50 feet without reaching the underlying surface. The thickness of the deposits usually varies from 10 to 20 feet of loam above and from 2 to 8 feet of gravel below. On the higher undulations of the plain these accumulations are sometimes wanting and low bosses of
. Bulletin of the Geological Society of America. Geology. FORMATIONS OF PACIFIC COASTAL PLAINS OF TEHUANTEPEC. 23 feet of water-worn pebbles below, which are overlaid by 3 feet of the red loam. These accumulations rest upon decayed gneiss. Near by, the river exposes the gravels and loams filling an old valley, where the thick- ness is 50 feet without reaching the underlying surface. The thickness of the deposits usually varies from 10 to 20 feet of loam above and from 2 to 8 feet of gravel below. On the higher undulations of the plain these accumulations are sometimes wanting and low bosses of the underlying rocks come to the surface. The stratification, while often apparent, is in many places obscure, especially where the loams become case-hardened when they stand in vertical walls. In the vicinity of the rivers and of. Figure 3.—Section along Rio San Geronimo. B, Eroded surface with valleys excavated out of decayed basaltic rocks ; L, Marl with water-worn pebbles ; C, Gravels and loams resting unconformably upon the marls. the mountains, the greatest amount of the gravel was seen. This forma- tion of gravels and loams has been a great leveller of the plains by burying the old valleys and hollows; accordingly, the old courses of the streams have often been changed. Thus the Tehuantepec river leaves the broad, open plain, where its former course has been buried, and flows through a narrow channel between two high isolated hills rising out of the plains near the sea. The formation thus described as covering the Pacific coastal plain is identical with that forming the upper surface of the Gulf plains (the Co- lumbia), while the underlying marly limestones are regarded as the equivalent of the lower mechanical deposits occurring on the Gulf side of the isthmus (the Lafayette). Later Formations of the Atlantic Coastal Plain of Tehuantepec coatzacoalcos formation Character, extent, and relations.—The reason for a nomenclature so diffi- cult arises from the isthmus no
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