Because of its narrowness and low divide (altitude 244 m), the Isthmus of Tehuantepec has caused much speculation and discussion regarding possible seaways between the Atlantic and Pacific oceans. Böse indicated in 1905 that no Cenozoic seaways crossed the Isthmus, nevertheless several subsequent authors have insisted on such a connection.
A re-examination of the Isthmus region clearly indicates that no Cenozoic seaways have crossed it, although Miocene seas from the north penetrated more than half way to the Pacific. No post-Cretaceous marine sediments are present on the Pacific side of the continental divide, nor are there any marine terraces on the extensive Tehuantepec plain, contrary to earlier authors. The Tehuantepec plain is separated from the adjacent mountains in many places by faults. The adjacent continental shelf extends seaward about 100 km, apparently as an expansion of the plain.
Paleocene and middle Eocene marine beds occur in the Tuxtla Gutierrez area. Upon preliminary examination, the small foraminiferan faunas show some resemblance to California faunas of similar age. The molluscan faunas show no significant similarity to described North Pacific coastal faunas. The resemblances noted by Maldonado-Koerdell and Gardner and Bowles are not considered indicative of North Pacific affinities.
Thick gravels, apparently Pleistocene, represent several episodes of deposition on the continental divide and northward from it, and with probable lacustrine deposits they suggest greater local relief or heavier rainfall in this locality during the Pleistocene.
Several sequences of apparent nonmarine rocks of presumed Tertiary age are present along the highway in the mountains between Tehuantepec and Oaxaca. In one of the younger sequences, near the village of Nejapa, a Merychippus palate and bones of a camelid, indicative of Miocene age, were found.