148 ROLLIN T. CHAMBERLIN 



♦ 



southward nearly halfway across the state of Montana.^ The 

 regular, linear, faulted chain here loses its distinctiveness and 

 identity, and gives way to an irregular group of scattered mountain 

 clusters in west central and southwestern Montana. Many of 

 these have resulted from igneous outbursts, and exhibit the results 

 of vertically acting forces fully as much as horizontal thrusting. 

 Progressing southward across Wyoming, the ranges of the Rocky 

 group tend to come together more closely in alignment and form a 

 more distinct, continuous chain. But the characteristic structure 

 of the system has changed. Gentle, open folding has replaced 

 thrust faulting as the key structure. This continues across 

 Colorado and into New Mexico. 



On many maps the western half of the state of Colorado gives 

 the impression of a tangle of mountains which manifest but little 

 order in arrangement. Such, however, is not the case. The Rocky 

 Mountains enter Colorado from the north as two distinct ranges — ■ 

 the Front Range on the east and the Park Range to the west. 

 Southward these two ranges gradually converge. Though the Park 

 Range dies out in the vicinity of Buena Vista, it is replaced, en 

 echelon, by the Sawatch Range. Farther south, in the neighbor- 

 hood of Salida, the Sawatch chain breaks down and is replaced in 

 turn by the Sangre de Cristo Range. In about the same latitude 

 the Front Range dies down and is replaced south of the Arkansas 

 River by the Wet Mountains. The convergence continues until 

 the two groups of ranges come together near La Veta Pass, from 

 which point southward into New Mexico the entire Rocky Moun- 

 tain system comprises but a single serrate ridge — the Culebra 

 Range, or southern Sangre de Cristo Mountains. The pattern 

 on the map is represented somewhat approximately by the letter 

 Y, though the two upper branches do not converge quite so rapidly 

 as in the printed letter, and the en echelon arrangement of the indi- 

 vidual ranges disturbs, to a certain extent, the smoothness of 

 alignment (Fig. i). 



This Y includes all of the Rocky Mountain system proper, 

 which is essentially a folded system in Colorado. West of the 



' Bailey Willis, " Stratigraphy and Structure, Lewis and Livingston Ranges, 

 Montana," Bull. Geol. Soc. of Amer., XIII (1902), 305-52; F. H. H. Calhoun, U.S. 

 Geol. Survey, Prof. Paper 50 (1906), pp. 9-10. 



