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4    Wisconsin Waters 
Main Algonquin stage (slightly modified from J. L. Hough Geology of the Great
Lakes, p. 294. 
University of Illinois Press, Urbana. © 1958 by the Board of Trustees
of the University of Illinois) 
by glaciated regions: "It preserves a large sample of what the rest
of Wisconsin, 
as well as northern and eastern United States, were like before the Glacial
Pe- 
riod. Within the belts covered by the gigantic continental ice sheets of
north- 
eastern North America and northwestern Europe there is no similar region
of 
substantial size which was left bare of glacial ice" (Martin 1932:82).
Black (1959) 
suggested that early ice of the Wisconsin glaciation may have extended farther
into the driftless area than is shown on most published maps. 
   The most sharply contrasting ecological conditions over wide areas of
the 
state are between the driftless area and the area once covered by the glacier
and glacial drift. Greene (1935:223-224) noted: 
Compared with the drift-covered area, the Driftless Area is rugged. Its central
portion is 
floored principally by the Potsdam sandstone formation. In the northeast
and extending 
in tongues down the valleys of the Black and Wisconsin rivers, the Archean
granitic 
rock appears, while in the west and southwest the sandstone, overlain by
a layer of 
limestone, is exposed only in the stream valleys .... The erosional system
in the Drift- 
less Area has been at work over a relatively long period of time and streams
have carved 
for themselves a mature, efficient drainage system. Swampy conditions are
localized in 
the lower relatively flat river valleys, and the only natural lakes are Pepin
and St. Croix, 
formed in the courses of the Mississippi and St. Croix rivers respectively
by delta dams. 
The gradients of the streams in the driftless area are regular, with no natural
swamps and no lake reservoirs. Such streams are subject to extreme fluctuation
following wet and dry periods, and communities situated in their floodplains
are subject to periodic flooding. 
   In the glaciated regions, streams are controlled by slopes which are not
gen- 
erally conducive to efficient runoff. In these regions, lakes, ponds, and
marshes 
are the usual features. The phrase "imperfectly drained" is commonly
used in 
)GRAPHIC LABORATORY, UNIVERSITY OF WI!