STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

graptolites. These sediments are now the Schaghticoke, Deepkill, and
Normanskill formations. Whether such an assemblage of sediments
indicates that the Ordovician Taconic sequence was deposited in very
deep water is not yet clear. Between Deepkill and Normanskill times
the entire geosyncline may have been brought above sea level.
Toward the middle of Trenton time the Magog basin was uplifted and
probably folded. The resulting land mass, located in the vicinity of the
modern Connecticut Valley, has been called Vermontia. Following the
uplift of Vermontia, elevation and warping extended westward to the
Adirondack region. The Castleton area was exposed to subaerial erosion
for a comparatively brief time during the Middle Trenton. We find the
record of this in the Trenton unconformity in the Castleton quadrangle.
After the Champlain basin had again been submerged, the muds and
limestones of the Whipple-Hortonville succession were laid down. The
Hortonville shales are part of a great flood of dark muds and sands that
issued from Vermontia during the later Trenton. These elastics gradually
overlapped the Adirondack carbonates in a westerly direction.
During the Upper Ordovician Vermontia was extended, its western
edge being a subaerial delta continuously advancing westward. The
Taconic orogeny began in the eastern part of the former geosyncline
before the end of the Ordovician and moved westward into western
Vermont and eastern New York near the end of the Ordovician. During
the Taconic orogeny the rocks of the entire geosyncline, reaching an
aggregate thickness of perhaps 50,000 feet, were folded, faulted, and
moved westward. At the catastrophe the gigantic Taconic overthrust
carried strata that were deposited in the eastern Magog basin from the
vicinity of the present Connecticut Valley as far west as the Catskill
region. As uplift at the east continued, the Taconic Allochthone may
have become detached from its roots and slid downward and westward,
lubricated by the Hortonville slate. Eventual relaxation of the forces
was followed by large-scale gravity faulting peripheral to the Adirondacks.
The orogeny created a new land mass, Taconica. Eastward onlap of
elastic debris derived from the erosion of Taconica occurred throughout
the Silurian. At the end of that period and throughout much of the
ensuing Devonian period marine waters probably again covered Vermont,
but the greatest deposition of sediments occurred in New York
and New Hampshire. The mid-Devonian Acadian disturbance, accompanied
by extensive granitic intrusions in New Hampshire, deformed
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