. Bulletin of the Geological Society of America. Geology. FORMATIONS ON GULF SIDE TEHUANTEPEC ISTHMUS. 29. Figure C>. -Junction of the Columbia (C) and Lafayette Formations (L). As shown at the 17-kilometer post of the Tehuan- tepee railway. lumbia red loams are from 5 to 10 feet thick in this section, with the un- derlying fine quartz gravels (mostly subangular) of about equal thickness. It rests unconformably on the Lafayette red loams, in which there is a streak of clayey matter one to two feet thick. The hills immediately on the coast are covered with loose sand in the form of dunes. Be
. Bulletin of the Geological Society of America. Geology. FORMATIONS ON GULF SIDE TEHUANTEPEC ISTHMUS. 29. Figure C>. -Junction of the Columbia (C) and Lafayette Formations (L). As shown at the 17-kilometer post of the Tehuan- tepee railway. lumbia red loams are from 5 to 10 feet thick in this section, with the un- derlying fine quartz gravels (mostly subangular) of about equal thickness. It rests unconformably on the Lafayette red loams, in which there is a streak of clayey matter one to two feet thick. The hills immediately on the coast are covered with loose sand in the form of dunes. Be- tween the coast and the mountain zone this Columbian formation occupies all the lower depressions of the undulating plains, while occasional ridges of the Lafayette red loams and gravels rise through them. At Jaltipan, as noted be- fore, the hills are composed of arkose, which may be a local representation of the Lafayette. While the Columbia and upper loams prevail eastward of the Jaltepec, beyond that river, in the higher hills, the Lafayette is frequently observed with its surface eroded and succeeded by the Columbia gravels and loams. These accumulations are best exposed between the 146 and 152-kilometer posts. The Lafayette is typically a red sand or sandy loam, more or less distinctly laminated, although frequently no bedding- is seen. It sometimes includes layers of sandy clay, which may be 4 feet thick. In many places the base of the bed is represented by gravel, which, however, may appear in other portions of it. When no other structure is seen, a vertical cleavage is apt to prevail, which, in this as in other Lafayette and Columbia loams, is a physical distinction from the underlying Coatzacoalcos formation. These red loams are also liable to be case-hardened. The stratification is nearly horizontal, although varying two or three degrees. Near the 136-kilometer post the Lafayette formation is notably faulted with a local dip of 10 degrees, and near by there is a dip
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