CHAPTER VII. 

 LOWER BEACHES WITH SOUTHWARD OUTFLOW. 



Extensive portious of the lower beaches that were formed while Lake 

 Agassiz outflowed to the south have been exactly mapped, with determina- 

 tion of their heights by leveling. These are described in the following 

 pages, the successive beaches being treated in their order from higher to 

 lower. Four well-defined levels of the glacial lake are exhibited by the 

 shore-lines of its southern part, which have been named, from localities of 

 their typical development in Minnesota, the Norcross, Tintah, Campbell, and 

 McCauleyville beaches. In advancing northward each of these beaches, 

 similarly with the uppermost or Herman series of beaches described in the 

 last chapter, is found to liecome subdivided into two or more separate and 

 distinct beaches or shore-lines. 



The attempts here made to correlate these multiple northern shores of 

 Lake Agassiz with the fewer southern shore-lines rest on the determination 

 of many altitudes along the course of these former planes of the ancient 

 lake levels. The several shores, both at the south and north, were separated 

 from each other partly by the progressing erosion of the outlet, and partly 

 by the gradual decrease of the attraction of the lake by gravitation toward 

 the waning ice-sheet, but more by the intermittent uplifting of this part, of 

 the earth's crust, due evidently to its relief from the pressure of the depart- 

 ing ice. Considerable irregularities in this uplifting would be expected, by 

 which the gradients of northward ascent of the beaches would be made 

 variable, being comparatively steep or changing abruptly in elevation in 

 some places, and elsewhere being of small amount or even showing a 

 reversal, that is, a descent toward the north. In this sm-vey, however, I 

 have not discovered any remarkable divergences or exceptions from an 

 approximate parallelism of the beaches. The northward ascent of the 

 highest beach in the Herman series, ranging from 6 to 18 inches per mile. 



