NATURAL AREAS AND REGIONS 



575 



mountainous region which occupies 

 most of the State of Oaxaca and is 

 limited to the southeast by the Isthmus 

 of Tehuantepec. This isthmus is a 

 broad trough-shaped pass, with an 

 elevation of only about 800 ft. in the 

 highest part, extending across the 

 narrowed continent between the Atlan- 

 tic and Pacific coasts. Beyond this, to 

 the east again rises a broad generally 

 mountainous area reaching an altitude 

 of more than 8000 ft., which extends 

 southward as the elevated interior of 

 Guatemala, and thence continues more 

 brokenly into other parts of Central 

 America. 



The principal mountain range of 

 Mexico, the Sierra Madre Occidental, 

 as already mentioned, is the southern 

 extension of the Rocky Mountain sys- 

 tem. It is broken into sections by 

 passes, each marked by a river canyon 

 and along the Lerma Valley by a broad 

 plain. An offset range located parallel 

 to the coast in Guerrero is known as the 

 Sierra Madre del Sur, which extends 

 southeasterly through Oaxaca to unite 

 with the southern end of the Sierra 

 Madre Oriental. The two great eastern 

 and western mountain systems of Mex- 

 ico reach their greatest development 

 in the southern extension, between the 

 18th and 20th parallels of latitude. 

 There they mark a region of tremendous 

 volcanic activity and the highest peaks, 

 from 15,000 to over 18,000 ft. in altitude, 

 stand up boldly, towering high above 

 the surface of the tableland which, 

 about their base, has elevations of from 

 7000 to 8000 ft. 



The Peninsula of Lower California 

 has a single, definite mountain range of 

 varying elevation extending near the 

 eastern coast nearly its entire length, 

 to end on the shore of La Paz Bay. 

 This range has an abrupt slope fronting 

 the Gulf of California but descends 

 gently toward the Pacific. The range 

 begins at an elevation of about 4000 ft. 

 on the California border as a southern 

 extension of the San Jacinto Mountains 

 of southern California, and about 150 

 mi. farther south rises to an altitude of 



more than 10,000 ft. on La Providencia 

 Peak. Near La Paz the mountains are 

 interrupted by a low gap south of which, 

 near the western side of the peninsula, 

 rises the short Sierra de la Victoria, 

 which has an altitude of from 3000 to 

 6000 ft., with occasional peaks rising 

 1000 or 2000 ft. higher. 



In addition to the main mountains 

 mentioned above, the land surface of 

 Mexico is here and there diversified 

 by minor ranges. 



The long, narrow valley of Toluca in 

 the State of Mexico is the highest in the 

 Republic. It has an altitude of about 

 8200 ft. and is inclosed on three sides 

 by high mountains, the drainage being 

 to the north. The valley floor about 15 

 mi. broad consists of open sloping, 

 treeless plains with extensive centrally 

 located marshes forming the principal 

 sources of the Rio Lerma, which flows 

 north and westerly into the Pacific and 

 is the longest river wholly within Mexi- 

 can territory. The valley of Mexico, 

 separated from the valley of Toluca by 

 the Sierra de las Cruces, is much larger 

 and more oval in form, but of similar 

 character. This basin in which is 

 located the City of Mexico, at about 

 7400 ft. altitude, contains lakes Tezcoco, 

 Chalco, Xochimilco, and several of 

 smaller size, together with extensive 

 marsh areas. 



Coastal plains form a well defined 

 lowland belt, between the shore line 

 and the basal slopes of mountains, 

 along the entire Pacific littoral of Mex- 

 ico, both on the mainland and in places 

 on the western side of the peninsula 

 of Lower California. These are inter- 

 rupted in places where the mountain 

 spurs or ridges extend to the shore line. 

 The seaboard of the Gulf of Mexico 

 has a broader coastal plain which merges 

 into that which covers all the peninsula 

 of Yucatan, including the states of 

 Yucatan, Campeche, and the territory 

 of Qutntana Roo, together with the 

 neighboring state of Tabasco, the 

 general area consisting almost wholly 

 of nearly level, or gently rolling tropical 

 lowlands, with slight elevation. 



