338 C. W. HALL — KEWEENAW AN AREA OF EASTERN MINNESOTA 



ward and eastward of the lava beds and associated conglomerates. The 

 displacement must have been a continuous process, extending through 

 several periods of geologic histor}'- ; that it was partly pre-Cambrian is 

 shown by horizontal sandstones lying on the edges of Keweenawan 

 lavas, as in the Saint Croix valley ; that it was post-Cambrian is shown 

 by the contacts ever3nvhere of crushed and shattered rocks, both Kewee- 

 nawan and Cambrian ; that it was intra-Cambrian is a strong probability 

 in the problem, since Cambrian time was long and only in part repre- 

 sented by the rock formations of this region ; that it was partly post- 

 Ordovician is suggested by the beds of Trenton limestone in Washington 

 county, Minnesota, h'ing 200 feet higher than the same beds at Minne- 

 apolis, both areas being liorizontal and onl}^ 12 miles apart. 



There is everywhere in the upper Mississi[)pi and Saint Croix regions 

 evidence that local crustal movements have been comparatively sliglit 

 since Eo-Paleozoic time — at least, such displacement phenomena as we 

 have described are not yet elsewhere shown. 



Summary 



The foregoing discussion may be summarized under the following 

 eight propositions : 



1. The district is quite level and unbroken, not only in its modern 

 features but in tliose pertaining to ever}'' geologic age since the Kewee- 

 nawan. River erosion is very recent, and its exhibitions of water work, 

 save in the gorge of the Saint Croix, are post-Ghicial. In tliat gorge are 

 evidences of work performed during a i)re-Glacial or inter-Glacial period 

 of baseleveling. 



2. The Keweenawan rests unconformably on the earlier rocks and 

 is in turn overlain b}'' the Cambrian in unconformable superposition. 

 Its rocks represent a long period of sediment-building interru[)ted by 

 seasons of volcanic activity. Judging from the character of the rocks 

 formed during this time, the energy and frequency of the volcanic out- 

 bursts waned to a state of perfect quiet during the period designated 

 Upper Keweenawan. , Then the Cambrian was ushered in. 



3. The eastern Minnesota KeweenaAvan area is a S3Miclinal trough, 

 each side of wliicli is a series of lava flows and conglomerates. The 

 succession along the east side, as seen in the gorge of the Saint Croix 

 river, dips westward at 10 degrees about the mouths of Kettle and Snake 

 rivers, where some 2,500 feet of rocks are in sight, and at 15 degrees 

 southwestward at Taylors falls, where on both sides of the Saint Croix 



