44G J. E. WOLFF — GEOLOGY OF THE CRAZY MOUNTAINS. 



which become cliffs as the spurs rise toward the peaks ; but on approach- 

 ing the central peaks the valley suddenly narrows for a mile or more and 

 the stream falls from a higher level 400 or 500 feet by cascades and falls, 

 and beyond this the valley again widens somewhat with a more gentle 

 slope to the head. This " fall-line" is found on all the radial streams, 

 and is plainly due to the local hardening of the Cretaceous rocks pro- 

 duced by the central stock of diorite, as described later. 



The larger valleys have been occupied by glaciers, as shown by rock 

 scoring; and the markings are found 500 or even 1,000 feet above the 

 present stream levels. No lateral moraines were observed, but at the 

 leads of the streams there is considerable morainic material, and also 

 below the exits of the streams from the range, which here contains 

 large bowlders of the characteristic eruptive rocks occurring higher up. 

 The glaciation seems to have been entirely local. The broad benches, 

 stretching out for miles eastward, westward and southward, are covered 

 with water-worn pebbles from the range and may lie high above the 

 present stream beds, which have cut deep through them into the under- 

 lying sandstones. The change in elevation from these benches to the 

 spurs from the peaks is sudden, the difference of level between the base 

 and the summit of the range averaging perhaps 4,000 feet. 



That part of the mountains north of the head of Shields river is lower 

 and the summits have the form of ridges or flat-topped dome-shaped 

 masses. Both here and in the southern half, outlying peaked summits 

 or buttes form a topographical feature. 



( rENERAL STRUCTURE. 



The general geology is comparatively simple.* The range lies in a 

 region of nearly horizontal Cretaceous rocks, extending indefinitely east- 

 ward in the great plains and westward to the edge of the frontal range, 

 where sharp uplifts expose the older rocks. These Cretaceous rocks are 

 found throughout the range and either horizontal or, if disturbed, with 

 generally low dips. They consist of yellow or brown sandstones and 

 occasional conglomerates, interstratified with yellow, drab, red. or black 

 shales and impure calcareous beds. The conglomerates in one place 

 contain large pebbles of an older (Carboniferous?) limestone ; the shales, 

 plant remains and small beds of impure coal. Xo attempt is here made 

 to assign them to a definite horizon of the Cretaceous, but the base at 

 least seems to belong with the Laramie, which a few miles westward lias 

 over 8,000 feet of strata. f 



*J. E.Wolff: "Null's mi tli^ Petrography of tin- Crazy Mountains"; Neues Jahrb. Min., etc, 

 1885, i, p. 69, and 1890, i, p. 192. 

 f\V. II. Weed: Hull. Oc, |. Soe. Am., vol. •_'. 1890, |>. 360. 



