CERTAIN VALLEY CONFIGURATIONS IN LOW 

 LATITUDES 



T. C. CHAMBERLIN and R. T. CHAMBERLIN 

 The University of Chicago 



To those who have studied the physiographic features of the tropics, 

 the phenomena described in this paper may be quite famihar, but 

 they do not appear to be well known to the average worker in the 

 higher latitudes, and this is our warrant for publishing what at best 

 are but passing notes. If the phenomena have been critically exam- 

 ined by students of tropical physiography it is to be hoped that they 

 may be stimulated by the imperfections of these notes to publish the 

 fuller truth for the benefit of geologists of the mid-latitudes. 



In September, 1906, while riding on the Ferrocarril Mexicano 

 from Esperanza on the high plateau down to Vera Cruz on the Gulf, 

 the attention of the writers was caught by the fact that there was 

 practically no talus at the base of the slopes, even when these slopes 

 were steep and rocky. The peculiar configuration of the valley 

 profiles also arrested attention. Further observations made it clear 

 that these features were prevalent. Preliminary to their discussion, 

 it may be remarked that the Mexican plateau has an elevation of 

 some 7,000 feet above the sea. At its eastern border the tableland 

 ends rather abruptly so that the railroad takes advantage of 

 several deep valleys heading back into the plateau and makes fre- 

 quent windings on their steep slopes to accomplish its descent to the 

 coastal plain. At first tributaries and then the main stream of the 

 Rio Blanco are followed to the city of Orizaba amid impressive 

 scenery. Between Orizaba and Cordoba the railway passes from the 

 valley of the Rio Blanca to the basin of the Rio Atoyac. Beyond 

 Cordoba, the railway line crosses a series of tributaries of the Rio 

 Atoyac through a region of steadily declining relief. The best 

 expressions of the distinctive configuration of the valleys lie in the 

 upper and steeper portions but in some degree they persist throughout. 



Above Orizaba the valley slopes are distinctly steep though not 

 strictly precipitous; and yet in favorable localities the stream possesses 



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