900 N. H. WINCH ELL ON THE KECESSION 



recession from Port Snelling, which is several miles above St. 

 Paul. There must have been a prior time when the Falls were at 

 St. Paul, and even below that point, inasmuch as the same conjunc- 

 tion of circumstances and the same formations extend several miles 

 below that city. It is not probable, however, that any data will 

 ever be discovered for computing that period of recession ; it must 

 have been during preglacial or interglacial time, and nearly all the 

 traces of that history have been obliterated by the ice of the last 

 cold epoch of the glacial period. That recession must have continued 

 past Port Snelling, along the old valley, and towards Shakopee, 

 when the Palls must have been reduced to nothing by the passage of 

 the river over other formations ; or they must have turned more 

 northwardly, caused, in that case, by the waters of the Mississippi 

 only. They may have continued their recession, and probably did, 

 through the intervening portion of Hennepin county, to the pre- 

 sent wide valley occupied by Bassett's Creek, and into the present 

 Mississippi valley above its mouth, to near the mouth of Shingle 

 Creek, where, owing to the " running out " of the Trenton and the 

 lack of artificial preventive measures, the preglacial Falls of St. 

 Anthony suffered a " going out," like that anticipated a few years 

 ago at their present site. 



Now it may not be plain how this bit of geological history, so 

 measured off, expresses the date of the last cold epoch of the glacial 

 period. It is by the following steps : — 



1. The reality of the glacial period, as announced by Prof. L. 

 Agassi z, with slight modifications, is taken as demonstrated by the 

 concurrent observations and opinions of the geologists of Europe and 

 America. 



2. The truth of the astronomical theory of the recurrence of cold 

 periods in the climate of the earth, as advocated by Dr. James Croll, 

 is also admitted. 



3. Hence the succession of glacial and interglacial epochs, as 

 demonstrated also by the study of the drift deposits themselves, of 

 varying lengths and intensity, must be admitted. 



4. The latest glacial drift, in the vicinity of the Palls of St. 

 Anthony, does not extend, in much force, to the south and east of 

 Minneapolis ; but in those directions a very different kind of drift 

 is found to prevail. 



5. Hence the choking up of the preglacial river-gorge, along the 

 west side of the present channel, must have been accomplished near 

 the acme of glacial cold, or at least when the effect of that cold 

 on the superficial materials was greatest. 



6. The river thus crowded out of its old valley made for itself a 

 new channel further east, and at the point where it reentered its 

 abandoned gorge it gave origin to the postglacial Falls of St. An- 

 thony by plunging over the limestone in which the old gorge had 

 been excavated. 



7. Hence the gorge from Port Snelling to the mouth of Bas- 

 sett's Creek is postglacial, above Bassett's Creek and below Port 

 Snelling preglacial or interglacial. 



