98 Gregory — Igiieotis Origin of the " Glacial Deposits''' 



west and northwest of the garnet deposits is not known. It is 

 probable that the garnet-bearing drift deposits are of greater age 

 than the glacial deposits of the San Francisco Mountains, for the. 

 former are covered with a stratum of hard white sandstone, and 

 are at almost as great an elevation as any of the surrounding 

 region. The presence of such quantities of crystalline and 

 ancient rocks in tbe drift cannot be explained by very recent 

 action, as these rocks do not outcrop near the locality."* 



The belief in the probable glacial origin of the gravels asso- 

 ciated with the garnets of the Navajo Reservation was strength- 

 ened by the discovery of similar materials near the San Juan 

 River at an elevation of 4800 feet. The deposits at this local- 

 ity have been described by Woodruff. 



"In the southeastern part of the [San Juan Oil] field there is a 

 small area covered by debris which is believed to be of glacial 

 origin. This material consists chiefly of fragments of sandstone, 

 but includes also a considerable amount of shale and lesser 

 amounts of interbedded limestone, conglomerate, and igneous 

 rocks. The conglomerate is of two types — (1) well-rounded 

 pebbles, in general similar to the Dakota conglomerate which is 

 exposed to the northeast of the field, and (2) apparently meta- 

 morphosed conglomerate. Neither type resembles any of the 

 other rocks exposed in this field. The igneous rocks comprise 

 schist and gneiss. The deposit is a heterogeneous mass which 

 shows no evidence of bedding, though some of the constituent 

 blocks show traces of their original stratification. Fragments 

 vary in size. One large block of conglomerate was found to be 

 more than 100 feet in length. The mass rests in an old channel 

 carved in the Moencopie and Dolores formations. The general 

 trend of the channel is north and south, and it terminates at the 

 south abruptly against a wall of shale and sandstone. Small gar- 

 nets were found in the anthills on the top of the debris. These 

 garnets are of interest in a study of the origin of the deposit, 

 because similar ones were found scattered over the surface and 

 in fragments of schist in the southeastern part of the field on the 

 divide between Gypsum and Chin Lee creeks, where it is crossed 

 by the wagon road. Sterrett suggested the glacial origin of 

 garnet beds immediately south of the San Juan field, where rocks 

 similar to the igneous rocks in this field are scattered over the 

 surface."! 



Granting that the deposits mentioned are glacial, the inter- 

 est of the problem presented by the interpretations of Sterrett 

 and of Woodruff is two-fold : 



1. If the glaciation be assigned to Pleistocene times, these 

 discoveries are remarkable in several respects. In the first 

 place, a new record for the lower limit of the ice cap south of 



* Sterrett, D. B., Min. Ees. U. S., for 190S, Pt. II, p. 825. 

 f Woodruff, E. F. : Geology of the San Juan Oil Field, Utah, U. S. Geol. 

 Survey, Bull. 471, pp. 85-86, 1912. 



