70 C. L. DAKE 



Tolchico. — Gregory^ sums up briefly evidence presented by 

 several writers, pointing to unconformity at the base of the Moen- 

 kopi, which seems to be especially marked along the Little Colorado. 

 The writer has seen the area described by Gregory near Tolchico, 

 and while there is clear evidence both of pre-Moenkopi erosion 

 channels and basal Moenkopi conglomerate, neither the eroded 

 contact nor the conglomerate are as well developed here as near 

 Mule Twist Canyon, west of Henry Mountains, unless the condi- 

 tions described below are related to pre-Moenkopi erosion. 



At a sharp bend in the Little Colorado River, perhaps two or 

 three miles northwest of Tolchico, a deep sharp canyon is cut in 

 the Kaibab limestone, and a sharply cut tributary canyon enters 

 at the bend, from the southwest. Perhaps a quarter of a mile up 

 the tributary canyon from its mouth occur conditions of peculiar 

 interest. Within the canyon, which here cuts between seventy- 

 five and one hundred feet into the Kaibab, and resting against the 

 Kaibab walls with distinct and coarse basal conglomerate, are very 

 friable, intricately cross-bedded, rather coarse sandstones. In 

 color they are dark red, deep brown, and in places almost black, 

 in which respect they contrast strongly with the gray limestone 

 walls of the canyon between which they lie, and also with the cream- 

 colored, white, or gray drifts of dune sand which surround them, in 

 turn, and partly bury them. Their age was not determined, since 

 they appear to be absolutely unfossiliferous. At first the writer 

 inclined to the idea that they were a phase of the basal Moenkopi, 

 resting in an old pre-Permian channel, and while he still admits the 

 possibility of this interpretation, it is also considered possible and 

 perhaps probable that they are a wind-blown deposit of Tertiary 

 or early Quaternary age, consisting largely of materials derived 

 from the Moenkopi. That they rest in a channel cut at least one 

 hundred feet into the Kaibab Hmestone, that they contain a coarse 

 basal conglomerate of limestone bowlders, and that they are 

 distinctly older than the present dune sands, admit of no question 

 whatever. The general character and relations of these deposits 

 can be studied more clearly from the illustrations given (Figs. 7 

 and 8). 



' H. E. Gregory, op. cit., p. 21. 



