1864..
HIND GLACIAL DRIFT,
129
§ 6. Parallelism of Escarpments in America.
In 1860* I described the remarkable paral-
lelism which exists between great escarpments
in America north of the 40th parallel of latitude.
1st. The Niagara escarpment.
2nd. The Riding, Duck, and Porcupine Hill
escarpment, west of Lake Winnipeg.
3rd. The escarpment of the Grand Coteau
de Missouri.
These are all roughly parallel to one another,
and are many hundred miles in length. The
lowest, the Niagara, varies from 600 feet to
1300 feet above the sea

the second, west of
Lake Winnipeg, from 1600 feet to 2000

the
third, the Grand Coteau de Missouri, from 2000
to 3000 feet and more above the ocean (see
fig. 3). They have all easterly, north-easterly,
or northerly aspects, in relatively different parts
of their lengths f , and appear to have a com-
mon origin. If it can be shown conclusively,
as Mr. Whitney believes, that the driftless
area in Wisconsin has never been overflowed,
these escarpments, as well as those of their
great outliers in the " far West," can only be
due to the same agent which excavated the
basins of the great American lakes.
The symmetrical escarpments of the Grand
Coteau de Missouri, the Riding Mountain and
its prolongations, and portions of the Niagara
escarpments, are probably the result, to a large
extent, of the action of glacial rivers under-
mining and washing away the soft strata of
the sedimentary rocks, and excavating in ad-
vance of the glacial mass itself; and they re-
present different and closely succeeding gla-
cial periods (the Missouri escarpment being
older than that of the Riding Mountain), with,
however, a distinct geological interval between
them. The close proximity of the isothermal
curves in these latitudes to the general direc-
tion of the escarpments of the Grand Coteau
and Riding Mountain is a very interesting
and important feature in connexion with the
cause which produced them.

See my ' Narrative of the Canadian Exploring
Expeditions of 1857 and 1858,' vol. ii. p. 266, for a
notice of these escarpments.
t The western exception at Cypres Hills has been
already noticed. Here the flanks of the Rocky Moun-
tains are approached.
VOL. XX. PART I.
T*H

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