PHYSIOGRAPHIC STUDIES IN SAN JUAN DISTRICT 451 



alluvial fans over the neighboring plateaus. The streams which 

 crossed the plateaus were first rejuvenated in their lower courses, 

 and as rejuvenation worked upstream across the broad plateaus 

 the growth of the great alluvial fans ceased and their dissection 

 began. The high-level bowlder-gravels are, therefore, . a deposit 

 which marks the beginning of a new period of erosion in the moun- 

 tains and a temporary period of alluviation about the base of the 

 mountains. The bowlder-gravels are, therefore, not of exactly 

 the same age, but a little younger than the small peneplain gravels 

 of the mountain area. 



Below the summit elevations in the mountains and in the 

 neighboring plateaus there are other broad bowlder-capped mesa- 

 like forms which appear to represent the base to which the streams 

 worked when the peneplain was first deformed. The bowlder 

 capping on these mesas is at places as much as thirty feet in thick- 

 ness. The Florida and Fort Lewis mesas just south of the San 

 Juan Mountains are typical of this bowlder-mesa stage in the 

 dissection of the area. Another uplift associated with the more 

 or less continuous growth of the mountains deformed the graded 

 surfaces of the bowlder-mesa stage, again rejuvenated the 

 streams, and opened another cycle of erosion. The surfaces to 

 which the streams then worked are represented by broad, open 

 valleys of late maturity or early old age in the softer rocks, and by 

 canyons in the harder rocks. This cycle of erosion has been for 

 convenience referred to as the Oxford stage, for there is an excellent 

 development of the typical lowlands of this stage near the village 

 of Oxford, a few miles southeast of Durango. It immediately 

 preceded the first epoch of glaciation as recorded by the moraines 

 and outwash deposits found on the south slope of the range. In 

 the mountain-canyons the bowlder-mesa and Oxford stages are 

 both represented by rock benches which in some instances carry 

 stream alluvium. 



These studies have opened certain large problems in the rela- 

 tionship of the mountains to the plateaus, and suggested a close 

 correlation in the physiographic histories of the two provinces. 

 Are the high-level bowlder-gravels resting on a true peneplain ? 

 Of what age is this peneplain ? In the area examined during the 



