STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

The "lower 10 to 40 feet of the Columbian deposit have slightly discontinuous
irregular dolomite veins" (Bain, 1934, p. 127) that "do not cross
bedding planes but vary in orientation and position within a bed."
Correlation and relations: This formation has a stratigraphic position
equivalent to that of the Shelburne marble plus the Cutting dolomite
and perhaps some of "Zone 1" of the Bascom formation (Cady, 1945,
p. 540-543). Cady has shown that the Cutting is unrecognizable south
of the boundary of the Brandon and Castleton quadrangles. The Cutting
equivalent, if any, in the Castleton quadrangle is marble that is indistinguishable
from the upper Shelburne as defined by Cady. Indeed,
the upper Shelburne together with the Cutting equivalent and some of
the Bascom forms an indivisible unit that is Bain's Columbian marble.
The name Cutting clearly cannot be used in the Castleton quadrangle.
Likewise the Shelburne is not a convenient unit because its top cannot
be recognized here. If the name Shelburne were to be used in the Castleton
area, its limits would have to be expanded considerably. Because
this is undesirable, the name Boardman formation is suggested to include
all strata between the uniform gray dolomite of the Clarendon Springs
and the lowest distinguishable beds of the Bascom, i.e., the first rock
other than calcite marble above the Columbian deposit, be it dolomitic,
sandy, or argillaceous.
Cady has cited evidence (1945, p. 541-542) for believing that the
variations in thickness of Beekmantown limestones and dolomites is a
result of secondary dolomitization of limestone. In the east limb of the
Middlebury synclinorium the Cutting appears to pass southward from
a dolomite into a "blue limestone that has a 'closely curdled' surface
formed by localized dolomitization." It may either pass south into the
upper Columbian blue marble or may pinch out stratigraphically. There
is no evidence of erosion in the Boardman-Bascom succession, and the
disappearance of the Cutting may be explained more reasonably by
lack of dolomitization to the south. Secondary dolomitization probably
increases northward and westward toward the Adirondack mass, which
probably was the locus of shallower seas during the Ordovician.
Thickness: The Sutherland Falls is regularly 90 to 100 feet thick,
although east of the Pittsford Valley quarries only about half of this
thickness is present because of tectonic thinning. The Intermediate
dolomite is "190 feet thick at places between Middlebury and Manchester"
(Bain, 1931, p. 509), and at one place in the Castleton quad-
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