EASTERN FOOT-HILLS. 57 



sion, or bay, in the outline of the crystalline mass, around the lower end 

 of which the sedimentary ridges curve somewhat sharply, following 

 closely the Arohsean boundary in all its details. In this way, the narrow 

 spur forms an anticlinal axis, with sedimentary beds dipping away on 

 both sides. 



On the east side, the beds maintain the same dip as found to the north- 

 ward, averaging from 18° to 25°. Below the end of the Archaean body, the 

 sedimentary ridges extend, with an anticlinal structure, for some distance 

 farther southward, the Dakota sandstone still forming the outer member of 

 the upturned ridges. Near where the Dakota beds curve around to follow 

 up along the west side of the Archasan spur, they dip at 45°. The westerl}^- 

 dipping beds have a general trend of north 35° 40' west, with all the forma- 

 tions standing at a much higher angle than upon the east side, many beds 

 reaching nearly a vertical position, with the Dakota sandstones standing out 

 from the main ridge in a most prominent manner. About 2 miles north of 

 the crossing of the Big Thompson, the Mesozoic formations again curve round, 

 this time doubling upon themselves, and resting directly on the main 

 Archaean Range in a manner similar to the outer ridges next the plain. 

 The beds thus dipping eastward from the main range form, with the westward 

 side of the anticlinal fold, a broad synclinal valley within the bay or depres- 

 sion in the Archaean body, which at its broadest expanse measures 

 nearly 3 miles across, gradually narrowing toward the north, until 

 terminated by the coming together of the opposite sides of the fold in a 

 continuous ridge. The Colorado Cretaceous, influenced by the abrupt 

 bending of the underljdng formations, circles around the Dakota sandstone, 

 but, so far as observed, does not run up into the northern end of the synclinal, 

 the Dakota forming the upper member exposed. At least, on the east side 

 of the recess, Dakota beds occur standing nearly vertical, and on the west 

 side inclined at a much gentler angle, with a considerable intervening space, 

 so covered with accumulations of Quaternary material as to obscure all the 

 underlying formations, concealing, if present, the Cretaceous marls an,d 

 clays. 



Where the Big Thompson Creek cuts through the outer or anticlinal 

 ridge, it affords a very good exposure across the Red Beds, Jurassic, and 



