EROSION FEATURES 315 



where the southernmost Keweenawan rocks of Minnesota disappear 

 beneath the Cambrian sandstones, descend from an altitude of 1,200 feet 

 above the sea to 700 feet. The fall of 500 feet is sufficient to afford con- 

 siderable erosive power, yet, owing to the newness of the district physio- 

 graphically, very little rock-cutting has been done beyond the removal 

 of the glacial drift along the channels of the streams. 



The Kettle river in its course from Kettle River station to its conflu- 

 ence with the Saint Croix flows less than 30 miles and falls 220 feet. 

 While it has cut into the Cambrian sandstone 50 feet or more at the 

 quarry town of Sandstone and carved out at this place some of the 

 largest potholes thus far known, a few miles below, where the Kewee- 

 nawan eruptives are reached, but little cutting has been done. At Big 

 rapids of the Kettle, where much erosion should be expected, only a few 

 feet of Keweetiawan rocks have been removed. Glacial striae coursing 

 south 5 degrees east, magnetic are seen only 3 feet above the water. 



The Snake river, in a distance of 10 miles from Chengwatana to the 

 Saint Croix, falls 130 feet, showing a still sharper gradient than the 

 Kettle ; yet at the former place it has cut into the lava beds upturned 

 against its course not to exceed 20 feet at any place, and for the rest of 

 its course has done but little beyond trenching the glacial drift deposits 

 and soft Cambrian sandstones. 



Other streams as well as these, gathered on the broad and compara- 

 tively level and wooded glacial plain to the north and flowing down 

 over a rapidly descending slope into the Saint Croix, show identical 

 physical conformations. Lava flows are only slightly eroded, and the 

 cascade stage is the principal feature. This is true of Tamarack and 

 Crooked creeks, which, within a direct line of some 20 miles, fall 350 

 feet and show a succession of cataracts for several miles. It is along the 

 channels cut by these streams that the rock exposures lie. 



EROSION OF THE DALLES OF THE SAINT CROIX 



But still farther south at one or two localities, notably at the Dalles 

 of the Saint Croix, a remarkable extent of erosion is seen. According 

 to Doctor Berkey, the stream 



" lies for nearly 5 miles almost without a break between parallel ridges of diabase 

 standing on an average 1 mile apart and reaching an elevation of from 100 to 300 

 feet above the adjacent sedimentary rocks. . . . The present channel is not 

 believed to represent the original location of the river, although smaller streams 

 may have occupied portions of it. . . . All evidences indicate that the present 

 Saint Croix River gorge is post-Glacial. . . . The chief factor in making the 

 present channel the most available and permanent line of drainage was the glacial 

 erosion accomplished at this locality. ... At this time the volume of water 

 discharged was abundantly sufficient to account for all the erosion phenomena 



