The Piedmont and Blue Ridge are dynamic landscapes that have
undergone substantial change since the orogenies that ended in late
Paleozoic or, as some believe, early Mesozoic time. The southern
Blue Ridge region south of Roanoke, Va., lies on the crest of a
topographic uplift that corresponds to the eastern continental
drainage divide. To the north, this uplift and divide cross the
Appalachian Valley and form the crest of the Appalachian Plateaus
as far north as central Pennsylvania. The northern Blue Ridge
Mountains as well as parts of the Piedmont are on the eastern part
of the uplift area. The southeastern margin of the uplift
corresponds to a line within the Piedmont physiographic province
that extends northeastward from the Tallapoosa River at the Fall
Zone and crosses the Rappahannock River at the Fall Zone. The
differential elevation on either side of this line is sharp in some
places, as, for example, northeast of Atlanta, Ga. In other places,
the difference in elevation is difficult to detect, and, in effect,
the line becomes a broad monoclinal slope. The region as a whole
can be divided into at least six broad subregions that have
somewhat different histories in late geologic time. The Piedmont
Lowlands subprovince, southeast of the uplifted area, is dominated
by a monotonous topography of low rounded ridges and ravines
largely underlain by saprolite on crystalline rocks. Isolated
ranges of hills of greater relief are scattered across the region;
those investigated are directly related to the presence of
erosionally resistant rocks. Stream patterns as well as broad
topographic forms indicate that although the southern part of the
Piedmont Lowlands was probably once covered by younger sediments,
this area has been exposed to erosion for a long time. In North
Carolina, the inner part of the Piedmont Lowlands has strongly
trellised stream patterns, which suggest that subaerial erosion was
active for an even longer time period, perhaps since the latest
orogeny. North of the Cape Fear River, the outer part of the
Piedmont Lowlands was covered by either fluvial or marine sediments
or both, probably during Miocene time. Tectonic activity has
affected the Piedmont Lowlands in late geologic time. The Fall Zone
that forms the southeast border is, at least in places, controlled
by faults active in Tertiary time. Late faults have also been found
in the Pine Mountain area of Georgia. Minor differences in relief
affecting large regions within the Piedmont Lowlands may be related
to different rates of uplift in addition to rock resistance, either
past or present. The Piedmont northeast of the Potomac River
(Northeastern Highlands) rises to more than 300 m in altitude. The
major streams have convex profiles that steepen as they near the
Coastal Plain. Unusually narrow valleys and broad upland surfaces
indicate an increased rate of erosion and show that the relief is
now or recently has been increasing because of uplift or tilting.
West of the southern end of the Piedmont Lowlands is an area herein
called the Southwestern Highlands that in some respects is similar.
The area is crossed by two large streams that have convex profiles.
The highest mountain ranges in the area rise to altitudes greater
than 600 m. Northwest of the Piedmont Lowlands, the topography and
relief are higher, and in some places, the rise is gradual, forming
a Foothill zone between the Piedmont Lowlands and the high Blue
Ridge. This zone is morphologically more complex than the Piedmont
Lowlands. North of the Roanoke River, the foothills are commonly
chains of isolated hills and ridges generally underlain by
resistant rocks. The hills increase in height near the Blue Ridge,
an indication that they owe their height to tectonism of late
geologic age. South of the Yadkin River, the hills are believed to
be residual, the remnants of a larger highland that has been only
partially reduced to the lower relief of the general Piedmont
surface.
General
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