PHYVSIOGRAPHIC EXTENSION OF UNITED STATES 251 
The remainder of the Adirondack region, the western half or 
plateau portion, “‘is not absolutely flat, but is more or less diversified 
with low hills and intervening broad valleys. Occasional summits 
give views of moderate extent, but no elevations can properly be 
called mountains, and the‘general term plateau is most expressive.’”* 
In the Superior Highlands of northern Wisconsin, northwestern 
Michigan, and northern Minnesota there is another area topo- 
graphically similar to the Laurentian Plateau proper. 
Like the latter its sky line is distinctly even and gives little hint of the 
mountainous structure that prevails almost everywhere within it. Its principal 
topographic feature is an uplifted and dissected plain of erosion which has 
been glaciated and hence has many secondary features due to ice erosion. It 
lies neither in the region of pronounced glacial aggradation nor in that of 
intense glacial denudation; hence those forms that are of glacial origin are 
due in some cases to ice scour, in others to ice accumulation. A certain amount 
of glacial detritus occurs here and there; in other places the surface is swept 
practically clean by glacial erosion. The glacial material is irregularly disposed 
in characteristic fashion and blocks the drainage to such an extent that ponds 
and lakes occur in large numbers.” 
e 
The Laurentian Plateau with its outliers or spurs is an ancient 
peneplain which has undergone differential elevation, has been 
denuded, subsequently dissected around the margins, and exten- 
sively glaciated. Further than this little may be said regarding 
its physiographic history. It is fairly well established that a 
peneplain was developed in pre-Cambrian time over the main part 
of the plateau, including the Adirondacks and probably the Superior 
Highlands. There is some evidence of a second peneplain developed 
upon Paleozoic sediments. Owing to the almost complete removal 
of these sediments and their existence at present only in small 
down-faulted blocks, the fact of this peneplain is not fully estab- 
lished. There is unquestionable evidence of a third, probably 
Cretaceous, peneplain produced over the greater part of the 
plateau, including the Adirondacks and possibly the Superior 
Highlands. However, in the present state of our knowledge, 
sweeping generalizations regarding the number, dates, or extent 
of the several peneplains are not justifiable. 
tJ. F. Kemp, Pop. Sci. Mon., LX VIII, 199. 
2 Isaiah Bowman, Forest Physiography, pp. 572, 573- 
