nmTZE, GEOLOGY OF WA8ATCII MOUNTAINS, UTAH 99 



The variation of thickness of the Algonkian rocks might be accounted 

 for in still another way. If they were laid down upon an irregular sur- 

 face, filling up the valleys and thus reducing the relief, the surface upon 

 which the Cambrian beds were laid down later might be relatively smooth 

 and their uniform thickness would be accounted for, as well as the change- 

 able thickness of the lower formations, but the relation of the beds above 

 the unconformity to those below would be different. There would have to 

 be practically no truncation of the lower members ; the relation would be 

 more nearly that of a disconformity, and the separation of the two series 

 would be much more difficult on account of the lack of discordance. The 

 advancing Lower Cambrian sea would rework the surface materials and a 

 gradual transition would be established. Moreover, the close resemblance 

 of the large rounded quartzitic bowlders in the conglomerate to the under- 

 lying quartzites is strong evidence that they were derived from them by 

 stream erosion. This implies that the lower series was consolidated into 

 hard sandstones before the invasion of the Cambrian sea, an inference 

 which points to the much greater age of the lower quartzites. It seems, 

 therefore, only possible to account for the great variation in thickness of 

 the Algonkian rocks by erosion, before the deposition of the I^ower Cam- 

 brian sediments upon their bevelled edges. 



Relation of Algonkian to Archean 



The relation of the Archean rocks to the Algonkian is not sliown in the 

 Cottonwood region. The base of the Algonkian series is not exposed. 

 From its relation elsewhere, an unconformity doubtless exists and has 

 been represented in the figure above given. The Archean oldland may 

 have been devoid of relief but probably had some low monadnocks in 

 places. If the Algonkian rocks are continental in origin, as is generally 

 believed, the surface upon which they were deposited must have-been rela- 

 tively low and flat. The spreading fans and deltas of the Algonkian 

 rivers slowly extended themselves and covered the surface of the Archean 

 rocks. An unconformity, then, with overlap away from the source of sup- 

 ply, must separate these pre-Cambrian formations from the underlying 

 Archean. 



The highest member of the Algonkian in the Cottonwood region is a 

 rock of somewhat unique characteristics. It is exposed at the head of 

 South Fork and may be traced south into American Fork Canyon but 

 soon disappears by overlap of the Cambrian beds. It has not been found 

 in any other part of the range, but upon one of the islands of Great Salt 

 Lake a rock of similar occurrence is found. The peculiar nature of the 



