158 The American Geologist. September, 
1904 
earlier stages of the Ice age, was practically contemporaneous 
with the remarkable Wisconsin stage of glaciation. Although 
in the central part of the continent the prominent moraines of 
that stage are far back from the drift border, belonging thus 
to icefields much restricted from their former a^ea, it is seen 
that on each side, eastward and westward, the snow and ice 
were then more abundantly accumulated than ever before, over- 
riding the older drift deposits and encroaching en ground not 
previously glaciated. When the local causes of the compar- 
atively late maximum growth of the eastern and western parts 
of the vast continental ice-sheet shall be satisfactorily deter- 
mined, consistent with the restriction at the same stage in the 
interior country, it will be a long stride of progress toward 
fully solving a difficult and most interesting problem, the gen- 
eral causes of the Ice age. 
Because of the great extent, both in length and width, of 
the groups of high Cordilleran ranges, they presented very 
favorable gathering grounds for glaciers during the Ice age. 
In remarkable contrast with the eastern half of our country, 
where the Appalachian mountain belt appears to have had no 
local glaciers, nor even any notable effect in deflecting the 
boundary of the ice-sheet from its direct course through New 
Jersey and Pennsylvania, the glaciation in the Rocky moun- 
tain region and onward to the Pacific was greatly influenced 
by the mountain masses. 
In northwestern ^Montana, heavy glaciation on a width of 
50 to 60 miles, extended along the frontal ranges, and on 
the upper part of the basin of the Flathead river and lake, 
to a distance of more than a hundred miles south of the main 
border of the continental icefields. Again, on the high area 
of the Yellowstone National Park and its environs, confluent 
glaciers formed an ice-sheet measuring about 75 by 150 miles; 
and several tracts of mountain glaciers, from one third to 
two-thirds as large, are mapped farther south, in Wyoming, 
Utah, and Colorado. On the great mountain ranges and 
groups of southwestern Colorado, and on the grand Sierra 
Nevada of eastern California, abundant ice accumulation and 
effective glaciation reached to the 37th degree of latitude, 
about 50 miles farther south than the utmost projection of 
the Laurentide part of the ice-sheet, in southern Illinois. 
