STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

sandy phyllite, the Hortonville slate, outcrops over large parts of the
Castleton area. The rock is a sericite phyllite containing a few percent
graphite and up to 50 percent quartz in some places. The lower Hortonyule
has more graphite and the upper Hortonville more quartz. Where
the Hortonville is in contact with the Whipple marble, however, thin
gray quartzite bands are found in the slate. Local greenish-gray phyllite
is found in the Hortonville adjacent to the Whipple marble in two or
three places. These rocks have a Taconic aspect, and it is possible that
they are remnants of the overthrust sheet.
Bedding is rarely seen in the Hortonville; most of the rock is split by
undulating cleavage planes. In the less siliceous exposures the Hortonyule
consists of chips and cleavage fragments surrounded by black dust
or mud. Pods and veins of coarse-grained quartz bearing pyrite have
invaded the phyllite more or less parallel to the cleavage.
Thickness: The Hortonville in this area must be at least 1000 feet
thick. The Taconic overthnist has removed all but a few score feet of
the Hortonville at West Rutland, and throughout the area the younger
Hortonville appears to be missing because of erosion or concealment
beneath the Taconic allochthone.
Age: Keith (1932, p. 269) correlated the Hortonville with the Snake
Hill formation of eastern New York, which itself is probably "an eastern,
more sandy facies of the Canajoharie black shale" (Kay, 1937, p. 272).
Cady (1945, p. 558) correlated the Hortonville with the lower Canajoharie
and suggested the former presence of 4000 to 5000 feet of Canajoharie
clastics in Vermont. He further proposed a correlation between the
lower Hortonville and the lower Canajoharie Cumberland Head formation
as revised by Kay (1937, p. 274-275). Up to 150 feet of interbedded
blue limestone and black shale is found in the Cumberland Head, the
description of which resembles that of the Whipple-Hortonville sequence
in the Castleton area.
The Trenton unconformity: The interbedded Whipple marble and
Hortonville slate should be discussed as a unit. Throughout the Castleton
area there is probably an erosional break beneath the Whipple. The base
of the Whipple lies on the Bascom where the Bascom is present, on the
Boardman in the Clarendon Springs-Boardnian Hill area, on the Beldens
on the eastern side of the West Rutland Valley, and presumably on the
Middlebury at the base of the Taconic Range in West Rutland. One mile
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