A Little-Known Region in Northwestern Montana . 
195 
here, whatever they wer£, is this: The strata once continuous and level 
have been slightly flexed and broken into huge blocks from one to two 
miles in thickness, and five, ten or fifteen miles in each of the other di¬ 
mensions. These blocks have been heaved up to their present position. 
The mountains thus formed have two features in common. The side 
facing the plains has been most elevated. They thus present steep walls 
to the east, with long gentle slopes to the west. In other respects they 
differ. Some of them, for example, have not only steep walls on the 
east, but also on the south, with a gentle slope in the other two direc¬ 
tions. Others reverse this, having north and east facing walls, and 
slopes on the west and south. The structure here outlined is marked in 
the vicinity of the 51st parallel by an additional feature. The vertical 
displacement is reported by McConnell * as being over 15,000 feet, and 
accompanying this is a horizontal displacement or overthrust of from 
two to seven miles. This latter feature may occur south of the 49th par¬ 
allel, but I saw no evidence of it. 
Strata exposed. — The rocks exposed in the walls of the vallej^ of the 
Belly River give a practically continuous section of 5,000 or 6,000 feet of 
strata. They are all rather thin bedded and the whole series seem to be 
perfectly conformable. The beds consist, from below up, of yellowish 
gritty limestones, red sandy shales and sandstones, and green and black 
shales with more limestones at the top. The colors are quite distinct 
and give a broadly banded appearance to the high walls which form the 
sides of this valley, making it easy to trace the stratigraphic relations 
as we ascended. The dip was slight but towards the south or away from 
the plains, as it is along the range farther south. As we were climbing 
somewhat rapidly in the direction of the dip we were steadily getting 
higher in the series. No fossils were seen during the trip. The sand¬ 
stones often showed wind-drift structure and ripple-marks and the red 
shales were full of mud cracks. The black shales and limestones indi¬ 
cated deeper water. From base to summit there is an entire absence of 
crystalline rocks. 
The rock exposures on the Pacific side were not so numerous, but 
showed the same order of the same beds, the only difference being the 
addition of several intercalated beds of diorite, and the fact that the 
strata in the mountains west of the main range have been more strongly 
folded. The disturbance producing this folding occurred after the in- 
t 
trusion of the diorite. This is shown by the fact that the latter has 
shared in the folding and has been much fractured in the process. 
Igneous rocks .— 1. Location and character of outcrops. The outcrops 
of igneous rocks are on the headwaters of MacDonald’s Creek. The 
first one observed is about two miles from the summit of Ahern Pass 
(see 1 on map). The bed is about fifty feet thick, is smoothly inter- 
bedded with the stratified rocks of the region, and is exposed for about 
*'Annual Report Canadian Geological Survey, 1886, vol. 2, p. 33D. 
