314 C. W. HALL — KEWEENAWAN AREA OP EASTERN MINNESOTA 



Page 



Source of the eruptives = 335 



Development of the fault line 337 



Summary 338 



Explanation of plates 341 



Topography and Physiography 

 the area defined 



In eastern Minnesota south of lake Superior the area known to be 

 underlaid by the Keweenawan series comprises about 900 square miles. 

 It borders the state of Wisconsin for at least 100 miles, and lies almost 

 wholly within the valley of the Saint Croix riv^er. 



The northern limit of the district is near latitude 46° 25', at the 

 boundary line between Minnesota and Wisconsin; the southernmost 

 point exposed is in the lower Dalles of the Saint Croix near Franconia 

 (latitude 45° 15'), although the lava flows of the southern exposures 

 stretch beneath the Cambrian formations southward beyond Stillwater. 

 Westward the Keweenawan stops abruptly along a fault line which no- 

 where passes beyond longitude 16 degrees west of Washington. The 

 total length of the area is nearly 70 miles, and greatest width not more 

 than 30, with an average of 12 to 14 miles. 



S URFA CE FEA T URES 



The surface is general!}^ level. In the northern part the watershed 

 separating the Lake Superior drainage basin from that of the Saint Croix 

 is a morainic tract, which slopes southward into a well timl)ered and 

 generally level belt of country. Its i)rominent features are due to glacia- 

 tion, followed by post-Glacial river erosion. In pre-Glacial time the 

 Cambrian sandstones undoubtedly covered this entire region, from which 

 they are now mostly eroded. The distribution of the sandstone debris 

 has affected in no small degree the agricultural possibilities of much of 

 the district by the formation of extended sand plains, which locally sup- 

 port a partially xerophytic vegetation, the Jack pine, oaks, and under- 

 brush being especiall}^ characteristic of the more sandy belts. 



RIVER EROSION 



The visible erosion features of this part of Minnesota and adjacent 

 parts of Wisconsin are chiefly Glacial and post-Glacial. Exceptional is 

 the immediate gorge of the Saint Croix river through a part of its course, 

 an exception soon to be considered. The tributary streams form their 

 sources in the north to the level of the Saint Croix river at Franconia, 



