PHYSIOGRAPHIC EXTENSION OF UNITED STATES 243 
uplifted and well dissected probably in a preglacial stage of the 
Pleistocene epoch. The driftless area of Wisconsin is generally 
assumed to represent a slightly modified form of the topography of 
that stage. Then followed the advances and retreats of four or 
more ice sheets, with the consequent modification of relief and 
drainage lines. Since the older drift was deposited, erosion has 
produced a complex system of valleys in places, and some of the 
larger streams have developed broad, flat bottoms. In the area 
covered by the later ice sheets, there has been but little modification 
of the glacial topography. The rivers have terraced the outwash 
deposits, but the majority of glacial features remain unchanged. 
In the Canadian division of this section the drainage lines are still 
badly disorganized. 
THE EASTERN LAKE SECTION OF THE CENTRAL LOWLAND PROVINCE 
This is a region of general plain aspect immediately adjacent 
to the Great Lakes. It is difficult to give an exact definition of it 
that is inclusive in a broad way of all its topographic features, and 
at the same time delimit it from neighboring regions of apparently 
similar features. Writers on the physiography of the United 
States generally agree that the region adjacent to the Great Lakes 
possesses distinct glacial features which are easily recognizable 
from whatever point of the compass it is approached; and although 
the adjoining areas to the north, south, and west were also glaciated, 
there is sufficient contrast between the morainic, marshy, lake- 
dotted surface that characterizes the Lake Region, and the roche 
moutoné surface of the Laurentian Plateau and the till sheets of the 
plains to the west and south, to justify making this region a separate 
section. 
Within the United States this section is bordered by the Adiron- 
dack Mountains, the Allegheny Plateau, the -till plains, the 
Wisconsin driftless area, and the Superior Highlands. This 
boundary is generally marked by topographic breaks. In places 
along the border of the Adirondacks the break consists of the 
contrasting topography of mountain and plain; along the edge of 
the Allegheny Plateau it is an erosion scarp; in still other places it 
is a difference in topography not always to be comprehended in a 
