Geulogy of the St. Croix Dalles. — Berkey . 35U 
the different materials of which the drift is composed, the 
character of rock from which it was derived, and the source 
of supply, there is but little variation from observations made 
by Minnesota and Wisconsin geologists. The following state- 
ments are intended to summarize these points: 
1st. The eastern drift was derived largely from the igneous and iron- 
bearing rocks of the Lake Superior basin. The most abundant rock 
species noticed are: melaphyr (occurring in rather large boulders); gran- 
ite; amygdaloidal diabase: gabbro: quartz porphyry; diabase porphyrite: 
together with the sedinientaries, quartzite, red and yellow sandstone 
and a considerable admixture of highly ferruginous pebbles and frag- 
ments. The clayey material is red in color. Sand and gravel are 
abundant. 
2d. The so-called western drift carries many of the same rock spe- 
cies as the eastern. In addition, however, limestone boulders and 
pebbles are abundant. The til! and clay are usually gray or bluish gray 
in color and contain considerable finely ground calcareous material. 
It is evident that two such characteristically different deposits must 
have been deiived from two very different sources. Either the charac- 
teristic constituents of the gray drift were exhausted in a certain direc- 
tion and were theiefore of necessity succeeded by a quality very different 
on account of that lack; or we must suppose that there are at least two 
different areas of supply furnishing the material by two independent ice 
streams. The latter view is the more reasonable. In accord with the 
known character of the rock formations in different directions from this 
area, the red drift is Vjelieved to have come from the north and north- 
east, while the gray and blue has come from the west and northwest.* 
As to the periods of deposition, evidence shows that there 
were two and perhaps three different periods of accumulation. 
Whether any of the drift represented here was accumulated 
earlier than the lowan stage of re-advance may be doubted. 
Correlation with subdivisions accepted in other districts is 
difficult since this is so far removed from typical localities. 
The time intervals were comparatively short, indeed the ice 
fronts were, during a part of their duration, in actual contact. 
The eastern lobe, however, persisted longer than its western 
companion-lobe, and accumulated the extensive deposits cred- 
ited to the Wisconsin stage of advance. 
The Moraine. The Kettle range of the Wisconsin geolo- 
gistsf includes in many parts of its extent several quite dis- 
*Ge()I. and Nat. Hist. Survey of Minn., Final Report, vol. ir, 1888, p. 
410. 
tGeology of Wisconsin, vol. i, 188.'J, pp. 27.") :?S1. 
