STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

The composite thickness of the Valley sequence is approximately
10,000 feet in this quadrangle. These rocks were deposited in a northsouth
marine basin which has been called the Champlain trough. The
Champlain trough had a western boundary in the vicinity of the present
border of the Adirondack Mountains and an eastern boundary not far
east of the Green Mountain Front. The rocks of the Champlain trough
have been folded and faulted but lie today substantially where they were
originally laid down.
MENDON SERIES
Name: The oldest rocks in the autochthonous sequence of the Castleton
area belong to the provisional Mendon series (Whittle, 1894) and
are typically exposed in the township of Mendon in the Rutland quadrangle.
Until the true relations of these rocks to those below and above
are understood the name Mendon series should be retained.
Distribution: Rocks assigned to the Mendon series crop out in the
northeast corner of the Castleton quadrangle on and east of Cox Mountain.
A narrow band of Mendon extends along the eastern border of the
quadrangle from Pittsford Mills southward to Chippenhook.
Description: The Mendon series in the Castleton area has been separated
rather roughly into the following three rock types:
Type 3: This type includes black, gray, green, and brown quartzose
phyllite or phyllitic quartzite. Uncommon beds and lenses of fairly
clean quartzite range in thickness from Y4 inch to many feet. These
rocks show all gradations in composition from pure black phyllite to
clean quartzite. Type 3 is limited upward by the massive Cheshire
quartzite on Pine and Boardman hills and on the western flank of Cox
Mountain.
Type 2: This division contains gray and brown, coarsely crystalline,
impure dolomite in large lenses interstratified with chlorite schist and
fine- to coarse-grained gray grit and arkose. Mica flakes in the dolomite
give it a shimmering luster.
Type 1: This is a zone of gray- or white-weathering fine- to very
coarse-grained grit and arkose irregularly interbedded with fine-grained
green quartz-chlorite-sericite schist. The finer-textured massive grit
and arkose has a characteristic weathering surface resembling crepe
paper, and some of the medium-grained arkose is granitic in appearance.
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