STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

ock was a gray fragile limestone with white crystalline seams, a description
that fits the Middlebury in this area. Billings found the gastropod
Raphistoma (Pleurotomaria) staminea (Hall) and a single plate of
Paleocystites tenuiradiatus (Hall), a "never-failing guide to the Chazy."
The Middlebury limestone is, therefore, of upper Valcour age and
(Cady, 1945, p. 553) may be in part of Black River age.
ORDOVICIAN—TRENTON GROUP
With the exception of some possible Black River beds in the Orwell
limestone there appears to be a depositional hiatus in this area between
the uppermost Middlebury strata and the Trenton. The Trenton units
recognized (Cady, 1945, p. 555-561) in west-central Vermont continue
southward into the Castleton quadrangle, although the Glens Falls
limestone is not certainly identified. A blue marble that may be the
Shoreham member of the Glens Falls is called the Whipple marble in
the present area. It cannot be traced northward around the Taconic
Range into the Glens Falls outcrop in Sudbury, Vermont, although
certain patches of limestone southwest of Brandon, mapped by Cady as
inliers of Crown Point limestone in Hortonville slate, may be Whipple
marble.
Orwell Limestone
Name: This formation was described by W. M. Cady (1945, p. 556)
from exposures in the southeastern part of Orwell township, Vermont.
Distribution: A small exposure of Orwell located 1.5 miles N. 450
W. of the village of Ira was formerly quarried for lime. Another exposure
of Orwell lies in a stream channel about 1.25 miles N. 15° W. of
the village of Ira. These two exposures lie both at and east of the trace
of the Taconic overthrust. Limestone lithologically identical to the
Orwell crops out in the bed of a small stream southwest of the Middlebury
exposures at West Rutland. This exposure may be either Orwell
or a lens of Whipple marble in Hortonville slate.
Description: "Typically it is a massive, closely-knit, heavy ledged,
light-dove-gray-weathering, rather fine-textured black limestone cut
through by innumerable white calcite veins. It may stand out above
associated rock types in gleaming, almost white ledges" (Cady, bc.
cit.). Shallow channels parallel to bedding and containing comminuted
fossils are occasionally found in the Orwell.
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