STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

east of Lake Dunmore Foye (1917-1918, P. 85) noted that the Cheshire
quartzite "was found to rest unconformably on the Mendon dolomite
and to have an arkosic conglomerate at its base. The basal conglomerate
of the Cambrian [Cheshire] quartzite is so unlike the Ripton conglomer -
ate, exposed less than a mile to the east, that the two cannot be considered
of the same age. The Lana conglomerate has small pebbles of
potash feldspar and includes dolomitic particles from the rock on which
it rests. It is only a few feet in thickness and passes gradually into true
Cambrian quartzite." Cady agrees that the contact at the Falls of Lana
"is without doubt an angular unconformity" (1945, p. 528) but points
out that regional relations in central and northern Vermont show no
evidence of significant unconformity between Mendon equivalents and
the Cheshire equivalent, the Gilman quartzite. Keith (1914, p. 39) has
stated that the Cheshire "transgressed the entire lower series." At the
Massachusetts border Cheshire quartzite lies unconformably on Stamford
gneiss with no intervening clastics (Pumpelly et al., 1894, p. 98-102).
Possibly pre-Cheshire folding increased in intensity toward southern
Vermont. This might account for the apparently discordant relations
beneath the Cambrian quartzite in the southern half of the state. On the
other hand, rapid lateral facies changes between great lens-like sedimentary
bodies in Mendon time, as stated above, could equally well
explain the phenomena in the Castleton area. The adjacent parts of
the Brandon, Rochester, and Rutland quadrangles must be remapped
before a decision can be made between the alternative explanations.
Thickness: The thickness of the Mendon in the Castleton area is
impossible to measure accurately because of structural and stratigraphic
obscurity. The black phyllite of Type 3 may change in thickness from a
feather edge near Pittsford to over 2000 feet thick on Cox Mountain.
Type 2 may be as much as 500 feet thick at the north. Type 1 is probably
somewhat thicker than Type 3, although obviously Type 1 and perhaps
some of Type 2 would be equivalent in time to Type 3 under the facies
hypothesis.
Age: The Mendon series is either Algonkian (Whittle, 1894) or Lower
Cambrian. In the absence of fossils definite proof of conformity with
the Cheshire would suggest a Lower Cambrian age. On the other hand,
proof of widespread angular unconformity would suggest a Pre-Cambrian
age for the Mendon series.
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