MARYLAND GEOLOGICAL SURVEY 151

Mountain and the eastern slope of the Blue Ridge. The Blue Ridge
of Maryland is a continuation of the South Mountains of Pennsylvania
and extends as a sharply defined range from the northern border of
the state to the Potomac river, which it reaches at Weverton. Its
crest forms the border between Frederick and Washington counties.
The Blue Ridge reaches its greatest elevation of about 2400 feet at
Quirauk, not f ar from the Pennsylvania border. The Blue Ridge in
Virginia is not the direct continuation of the mountains so named
in Maryland, but of a smaller range, the Elk Ridge Mountains, which
adjoin them upon the west and which are pierced by the Potomac
river at Harper's Ferry. Occupying the larger portion of this eastern
district and reaching to its western border is the Hagerstown Valley,
a portion of the Great Valley of the Appalachian Region hitherto
described. It has an altitude of about 500 feet at Hagerstown, which
increases somewhat to the northward near the Pennsylvania line, but
declines considerably in the vicinity of the Potomac river. The
Antietam river and its tributaries occupy the eastern section of the
valley and the Conococheague river and its tributaries the western,
leaving the central portion of the valley somewhat higher than the
sides.

The central division, which comprises the Appalachian Mountains
proper, is bounded by the North Mountain upon the east and Will's
Mountain, near Cumberland, upon the west. Professor H. D.
Rogers describes this district as follows in his report of the First Geo-
logical Survey of Pennsylvania: " It is a complex chain of long, nar-
row, very level mountain ridges, separated by long, narrow, parallel
valleys. These ridges sometimes end abruptly in swelling knobs, and
sometimes taper off in long, slender points. Their slopes are singularly
uniform, being in many cases unvaried by ravine or gully for many
miles; in other instances they are trenched at equal intervals with
great regularity. Their crests are, for the most part, sharp, and they
preserve an extraordinarily equable elevation, being only here and
there interrupted by notches or gaps, which sometimes descend to the
water level, so as to give passage to the rivers [Potomac].... The
ridges are variously arranged in groups with long, narrow crests, some