THE CAUSE OF THE GLACIAL PERIOD. 523 



each area so closely extending to nearly the same maximum 

 limits in the earlier and the later parts of this Glacial period. 

 It is better, until proofs are obtained in the central regions 

 of the drift areas on each continent, to regard their time of 

 glaciation as one and continuous, with much areal oscilla- 

 tion, such as is proverbial of weather, both during the gen- 

 eral stages of growth and departure. 



My explorations of the Minnesota drift deposits, so far 

 as they appear to require great recession and later readvance 

 of the snow and ice accumulation, may be cited in the final 

 reports of our state geological survey, the most notable of 

 these observations being as follows, supplementing Prof. N. 

 H. Winchell's observations of an interglacial forest bed in 

 Mower county and other localities of southern Minnesota.* 



1. The exceedingly interesting and elsewhere unequal ed 

 chains of lakes in Martin county, one of the central counties 

 of our most southern tier, I can explain only by regarding 

 them as proofs of a fully developed interglacial system of 

 drainage running there from north to south, which became 

 afterward ice-enveloped in the Iowan and Wisconsin stages 

 of our Glacial period.f 



2. Somewhat the same conclusion seems again enforced 

 by the section of the drift close southwest of New Ulm, in 

 Brown county, about 40 miles north of these chains of lakes. J 



3. In the northern part of Chisago county, on the east 

 side of Minnesota, Rushseba township, in which Rush City 

 is situated, about 50 miles north of St. Paul and Minne- 

 apolis, has a considerable tract, § some 5 or 6 miles long and 

 of nearly as great width, where reddish-modified drift, spread 

 by streams flowing down from the receding northeastern 



* See below p. 605. 



t "Geology of Minnesota," vol. i, 1884, pp. 479-485, with the map 

 facing page 472. 



t Ibid, vol. i, pp. 581-3, with section, and with map facing p. 562. 



§ "Geology of Minnesota," vol. ii, 1888, pp. 409-415, 417, 418, the 

 last giving the record of that well, with the map facing page 399. 



