STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

Silurian time. Under the present hypothesis the Taconic rocks reached
their present position by overthrusting, and therefore the Taconic overthrust
is pre-Upper Silurian. Southwest of Rondout, New York the
earliest Silurian Shawangunk formation unconformably overlies folded
shales (Schuchert and Longwell, 1932). It is reasonable to assume that
the Taconic disturbance ended in the Hudson Valley before earliest
Silurian time, and it is not unreasonable to affix a pre-Silurian age to
the Taconic overthrust. That the Taconic faults "are post-Richmond in
age is evidenced by their cutting Queenston shales south of the St.
Lawrence River in Quebec" (Kay, 1937, p. 287). Thus the folding and
faulting of the rocks of western Vermont and eastern New York took
place probably in post-Richmond and pre-Silurian time, i.e., near the
end of the Ordovician period.
The folds and thrust of the Champlain-Vermont Valley were formed
probably just prior to the development of the Taconic overthrust. Continued
stress folded and sliced the transported Taconic sheet and further
deformed the underlying carbonate sequence. The shear cleavage has
been referred to the latter part of the Taconic disturbance, but it is not
impossible that this cleavage was produced during the Devonian Acadian
disturbance that strongly affected central New England. To be sure, an
Appalachian age cannot be excluded from consideration. It is by no
means clear when the Green Mountain and eastern Vermont region was
folded. At the present time we cannot determine whether most of eastern
Vermont owes its deformation to the Taconic disturbance, the Acadian
disturbance, or a combination of both. If the Taconic rocks were deposited
in or east of the Connecticut Valley, large-scale orogenic forces
at the end of the Ordovician must have been at work throughout Vermont
and possibly New Hampshire.
METAMORPHISM
The term slate as used here refers to a well-foliated argillaceous rock
found only in the western part of the Castleton quadrangle. Its component
minerals, chlorite and sericite particularly, are rigorously oriented
parallel to the flow cleavage. Quartz particles also have been elongated
by recrystallization in the foliation plane. Transverse sections show a
rather perfect mass extinction. Although Harker (1939) called similar
rocks "sericite-phyllites," the name slate is retained here because of its
commercial uses.
East of the slate belt rocks of similar composition but higher meta-
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