250 ES nye CREA VEER 
interval preceding the glacial epoch as to make its general peneplain 
character recognizable only by the pronounced and uniform dis- 
cordance between surface and geologic structure. 
The southeastern border. of the plateau, where it overlooks the 
St. Lawrence lowlands and the estuary of the St. Lawrence River, 
is more rugged and uneven than the remainder, and is known as 
the Laurentide Mountains. The greater ruggedness of this border 
is due not entirely to residuals that survived the base-leveling of 
earlier cycles, but to some extent at least to the greater uplift of 
the peneplain along this line. An observer on the highlands looks 
across a plateau-like surface, but one in the lowlands looks up 
against the scarplike face of a range of hills of sufficient height 
to be known as mountains. It has been suggested that this 
escarpment may mark the line of a fault or a series of faults." 
Tributaries of the St. Lawrence cross the Laurentide Mountains — 
in deep, steep-sided canyons or gorges, which accentuate the rugged- 
ness of the local topography. The Saguenay, the Coldwater, the 
Hamilton, and the Ottawa rivers are typical of this class, and in 
places their canyons are more than 2,000 feet deep. These canyons 
may represent ancient river valleys deepened by subsequent 
glaciation, or they may be of the “graben” type and similar to 
many of the streams of the Adirondacks. 
The physiographic features of the Laurentian Plateau are 
reproduced in miniature in the Adirondack Mountains. ‘There also 
is a plateau, nearly uniform in elevation, but rising to somewhat 
greater heights along the northeast, east, and southeast borders.? 
The plateau is apparently an uplifted peneplain. The rougher 
margins may have been base-leveled at the same time as the 
remainder, but if so all traces of the old erosion surface have been 
effaced in subsequent changes. 
The drainage of the eastern Adirondacks is remarkably like that 
of the southeastern and eastern borders of the plateau proper. 
Many of the streams and lakes lie in steep-walled gorges or chasms. 
Escarpments produced by the compounding of several fault systems 
are also suggestive of the scarp along the border of the Laurentide 
Mountains. 
tE. M. Kindle and L. D. Burling, Geol. Survey Canada Mus., Bull. 18. 
2W. J. Miller, N.Y. State Mus., Bull. 164, p. 81. 
