THE PHYSIOGRAPHY OF MEXICO 93 



THE TEHUANTEPECAN PROVINCE 



Definition and boundaries. — This province comprises an area 

 of very old, low-lying rocks occupying the Isthmus of Tehuantepec. 

 If we were to draw province boundaries on a basis of topography 

 alone we should be obliged to include most of this area in the Gulf 

 Coastal Plain province, for there is scarcely any difference in 

 topography. But considering also structure and history, we are 

 compelled to make this a separate province, with boundaries as 

 indicated on the accompanying sketch. 



It is bounded north and south respectively by the Gulf of 

 Mexico and the Pacific Ocean. The greater part of the western 

 boundary is the scarp of the plateau, which rises 8,000 feet above 

 the isthmus, 1 while to the east the heights of Chiapas rise to an 

 almost equal altitude. 2 Between these two highland areas lies the 

 isthmus, like a great block dropped out of an arch. 



Topography. — The isthmus is an area of low, rounded hills, with 

 two regional slopes, the longer toward the north and the shorter 

 toward the south, the two slopes meeting to form a divide at 

 Tarifa Chivela. Both slopes are very gentle, and the maximum 

 altitude at the divide is less than 800 feet. 3 



The drainage follows the regional slopes^ regardless of structure, 

 the Rio Coatzalcoalcos and its tributaries draining northward, 

 and the Rio Geronimo, southward. 



Physiographic history. — The oldest rocks exposed in this prov- 

 ince are the Archean crystallines, in the vicinity of Tehuantepec. 

 Farther up the slope at Chivela are metamorphosed sediments of 

 probable Paleozoic age, and still higher, unaltered Middle Creta- 

 ceous and Upper Miocene. Down the Atlantic slope, Pliocene 

 deposits grade into the Recent sediments of the coastal plain. 



It may be a permissible figure to speak of the isthmus as a block 

 dropped out of an arch, but as a matter of fact it will require us to 

 postulate more profound changes than the mere dropping of a fault 

 block to account for the peculiar structure of the isthmus. 



1 Hill, Trans. A.I.M.E., XXXII, 163. 



2 Bose, Guides des excursions, 10th Inter. Geol. Cong. Mex., igo6, No. 31. 



3 Bose, op. cit., profile sketch. 



