354 The American Geologist. June, 1905 
Between these diabase gorges the valley widens to about a mile, 
its western rock wall being an escarpment of almost horizontally 
bedded Cambrian sandstone and shales, easily eroded, while on the 
east it is inclosed by irregular slopes of the igneous Keweenawan 
rocks. Continuing south from the Lower Dalles, the valley, a half 
mile to one mile wide, is inclosed by escarpments of the horizontal 
Cambrian sandstone capped by dolomitic limestone, with overlying 
glacial drift. Returning and going up the river from St. Croix Falls, 
we find its valley there inclosed chiefly by eroded drift bluffs. 
Berkey and R. T. Chamberlin regard the very remark- 
able potholes or giants' kettles of the trap rocks at the Up- 
per Dalles as due to the ordinary river action while cutting 
down this rock barrier during Postglacial time. Instead, 
as shown in the second of my papers cited in the Bulletin 
of the Geological Society, these potholes seem to me due 
to torrents plunging through moulins of the ice-sheet, prob- 
ably at a time of stagnation of the glacial currents when 
the ice here was finally meltng away. 
Nearly coincident with the course of the St. Croix 
river through this Interstate Park was the junction of the 
diverse icefields, that on the east bringing the red drift 
from the region of lake Superior, while that on the west 
brought gray drift from the Red river valley and Manitoba. 
Though the parts of both icefields in the vicinity of the 
Dalles were moving easterly, their currents being turned 
toward the ice boundary and its concentrically curved 
moraine belts, we cannot doubt that some difference of 
slope of the ice surface was distinguishable on the east and 
west sides of this line of junction, when they both were 
"being fed by snowfall and by inflow of glacial currents. 
But when the final melting had reduced the ice here to a 
thickness of only a few hundred feet, or at last no more 
than a few scores of feet, with its former motion at a stand- 
still, or nearly so, we may readily see that the waters of the 
surface melting might run toward this line of confluence 
of the diverse Lake Superior and Keewatin icefields ; that 
waterfalls would pour through crevasses and the vertical 
tunnels called moulins ; and that such torrents could erode 
deep potholes, as here, with little general water-wearing 
of the contiguous rock surfaces. 
These giants' kettles, numbering about a hundred, the 
