PHYSIOGRAPHIC EXTENSION OF UNITED STATES 2409 
Straits of Belle Isle northward through Labrador, Baffin Island, 
Devon Island, and Ellesmere Island to Cape Sabine, a distance of 
approximately 2,000 miles. The ranges of this group are truly 
mountainous, with known elevations of 6,000 feet or more and with 
peaks estimated to approach 7,500 feet.‘ Allowing even a wide 
latitude of topographic variation, such mountains could scarcely 
be considered an integral part of the plateau. In the absence of 
topographic data the line separating plateau from mountains can 
be conjectured only, and that within wide limits of error. From 
the Straits of Belle Isle the southwestern boundary is the Gulf of 
St. Lawrence and the estuary of St. Lawrence River. Within these 
boundaries the Laurentian Plateau may be considered to be a 
topographic unit. Its chief features are those of a peneplained 
region of crystalline rocks which was uplifted, extensively dissected 
by ordinary erosion processes in preglacial time, and more recently 
heavily glaciated. 
The Laurentian Plateau has a gently undulating surface, 
diversified by such glacial features as moraines, lakes, swamps, 
muskegs, and outwash deposits. Over large areas a hill 150 feet 
high is a conspicuous feature of the topography, and though a few 
do actually approach 300 feet, they are of course residuals. The 
plateau is generally considered to be an uplifted peneplain, dissected 
to such a degree that it appears rough but not rugged. It should 
be borne in mind, however, as Adams? has pointed out, that the 
term peneplain is used merely as descriptive of the nearly level 
character of the country without any implication for a definite 
origin for it. Wilson’ has called attention to the fact that it is 
not a single peneplain, but probably a series of facets produced in 
widely separated periods. Very few of the qualities of the typical 
peneplain are present. There are no deep residual soils; in fact, 
there is no continuous cover of soil, except in the southern part, 
and no well-organized drainage courses. Instead there is a maze 
of youthful streams and lakes, and in places the altitude reaches 
2,000 feet. Unequal uplift and intense glaciation have so modified 
the topography that must have existed at the end of the erosion 
tF. D. Adams in Problems of American Geology (Yale University), p. 45. 
2F. D. Adams, op. cit., p. 49. 3A. W. G. Wilson, Jour. Geol., XI, 615-109. 
