304 Gkicial and Modified Drift 



not continuously, along the distance of seven miles from the 

 vicinity of Fort Snelling to the east part of the city, varying 

 from a few feet to about 30 feet in thickness. With its com- 

 monly more or less waterworn masses derived from the Tren- 

 ton limestone, which makes up nine-tenths or often nineteen- 

 twentieths of the deposit, are everywhere scanty drift pebbles 

 and less frequent boulders, brought from great distances at the 

 north, which show for this unusual deposit an intimate rela- 

 tionship with the glacial and modified drift. It is nowhere 

 overlain by till, nor by valley drift of the usual type. No de- 

 scription of this singular formation, illustrated in Plate IX, has 

 been previously published. 



Guided by helpful suggestions of Prof. C. W. Hall and 

 Dr. F. W. Sardeson, I am led to ascribe this very coarse valley 

 debris to erosion by the river at some time during the final 

 recession of the ice-sheet, when the ordinary' modified drift,, 

 continues with the wide floodplains of the Mississippi at Min- 

 neapolis, Fort Snelling, South St. Paul, Newport, and Lang- 

 don, had filled the valley just to the height of this limestone 

 terrace. It is needful, however, to go back to a much earlier 

 part of the Glacial period and thence bring forward a very- 

 important part of this explanation. 



During some long interglacial stage, probably the Bu- 

 chanan time of glacial recession next after the Kansas glacia- 

 tion, southern Minnesota had been uncovered from the ice and 

 the Mississippi here had sculptured its valley to nearly its 

 present form, allowing prolonged erosion by rivulets and by 

 weathering on this limestone tract, which reaches seven miles 

 along the valley from southwest to northeast. This part of 

 the valley, it should be noted, lies transverse to its general 

 course both above and below ; and it is also transverse to the 

 directions of the glacial currents during both the earlier and 

 later advances of the ice-sheet. Parts of the limestone sur- 

 face became very irregularly channeled and decayed during 

 this interglacial exposure of perhaps 15,000 years, as its dura- 

 tion is computed by Prof. N. H. Winchell from an interglacial 

 drift-filled gorge of the Mississippi in the west part of Minne- 

 apolis. Afterward the valley here and nearly all of southern 

 Minnesota w^ere again covered by the readvancing ice-sheet 

 during the Illinoian and lowan glaciation, and were next un- 

 covered, as I think, during the Wisconsin stage of the final' 



