268 HOWE & CROSS GLACIAL PHENOMENA, SAN JUAN MOUNTAINS 



into consideration, it seems more than likely that the gravels of the lower 

 terrace represent the deposits incident to this interglacial erosion. They 

 might, of course, be regarded as valley train deposited during an inter- 

 mediate period of glaciation, Avhose drift it has been impossible to differ- 

 entiate from that of the earlier and later periods. 



GRAVEL-COVERED TERRACED OF THE LOWER ANIMAS AND SAN JUAN 



RIVERS 



The existence of widespread deposits of terrace gravels in the region 

 south of the San Juan mountains has long been known ; the}?' are men- 

 tioned in the reports of nearly all the early explorers, but no explanation 

 of their presence has been offered. Recently Cross has made further 

 observations on them, but as the greater part of the Survey field work 

 has been confined to the mountains, it has been impossible to make as 

 complete a study as could be desired of their character and distribution. 

 All the tributaries of the San Juan river heading in the mountains ex- 

 hibit in their valleys several gravel-covered terrace levels, which extend 

 at least as far down the San Juan as the mouth of the Mancos and prob- 

 ably much farther. 



The terraces of the Animas valley are best known and may be taken 

 as typical of all those in the San Juan basin. The upper limit of the 

 lower and intermediate gravel terraces is shown in plate 31. These two 

 gravel plains expand greatly just below Durango, and their development 

 at 7 or 8 miles below the town is shown in plates 28 and 2.9. The lowest 

 is that of the valley train of the last glacial stage aJready mentioned ; its 

 surface is roughly 50 feet above the present flood-plain of the Animas. 

 The next higher terrace level is represented by the Florida mesa on tlie 

 east side of the iVnimas river, while on the west there is a corresponding- 

 slope which rises to the low divide which exists betwee nthe Animas and 

 La Plata drainages. Between these higher and lower terraces there are 

 several intermediate levels, some of which are especially well preserved 

 east of the Animas river between Durango and the moraines (plate 31). 

 Like the ones on the north side of the San Juan mountains, they are 

 gravel-covered, and resting on the gravels is a variably thick layer of fine 

 eolian soil. In plate 29 traces of a higher level or levels are seen in the 

 flat-topped hills or mesas in the far distance. 



These three main gravel horizons may be traced down the Animas to 

 the broad San Juan valley, and down the latter to the point, some 50 miles 

 southwest of Durango, where the upturned strata of the Mesayerde coal- 

 bearing formation cross the river at the Great Hogback, or , ''Creston, , ' as 

 it is called on the Hayden map. 



