PHVSIOGRAPHIC EXTENSION OF UNITED STATES 253 
rocks. Its elevation is but a few hundred feet above sea-level, and 
there are only occasional departures either upward or downward 
from the general level. It lies beneath the limiting provinces and 
is cut off from two of them by faults—the St. Lawrence-Champlain 
fault separating it from the New England region," and the Lauren- 
tian Plateau fault marking its northern boundary.” 
Of the few eminences rising above this plain, the Monteregian 
Hills are the most prominent. These hills are eight in number. 
They extend along an east-west line about ten miles apart, and rise 
abruptly from 600 to 1,200 feet above the surrounding country. 
Structurally they are cores of igneous rocks (probably conduits 
emanating from a dike or laccolith) surrounded and mantled by 
sediments in such a manner as to indicate that at the time of their 
intrusion the overlying Paleozoic strata were many hundreds of 
feet thicker than at present. This great mantle of rock has been 
since removed by erosion, except where local down-faulting has 
preserved remnants of it. Recent work on the Laurentian Plateau 
has discovered several outliers of Paleozoic rock scattered over its 
surface which were preserved in this manner. 
A variable thickness of glacial material which was assorted by 
the Pleistocene or post-Pleistocene marine invasion is spread upon 
the Paleozoic bedrock. Upon this in turn lies a thin veneer of 
marine sediments. The earlier erosion topography is probably 
neither accentuated nor subdued in any marked degree by the drift 
and sediments. 
The Champlain-Hudson Valley resembles the St. Lawrence 
Valley in several respects, namely, it lies at approximately the same 
altitude above sea-level; it is a low plain lying between adjacent 
mountainous provinces; it is floored with Paleozoic rocks; it has 
a mantle of drift spread over an old erosion surface; and it was 
subjected to a postglacial marine invasion. A part of this valley 
is occupied at present by the waters of Lake Champlain and 
Hudson River. 
In the St. Lawrence Valley physiographic history begins with 
the emergence of the Paleozoic rocks with which it is floored. The 
«¢. A. Young, op. cit., p. 02. 
2 FE. M. Kindle and L. D. Burling, o?. cit., Fig. 5. 
