STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

directly above the Columbian marble. Wherever the Bascom-Boardman
contact is seen, continuous deposition across the boundary appears to
have taken place. The upper streaked marbles of the Boardman are
identical in appearance to the lower marbles of the Bascom.
The highest beds in the Bascom are darker blue and more argillaceous,
although sandy and dolomitic beds are not absent. The Trenton Whipple
marble is similar to the underlying upper Bascom in the northern part
of this area, and the only certain way of distinguishing the two, in the
absence of fossils, is to observe the thick black phyllites commonly
found in the lowest Whipple.
Thickness and age: The maximum thickness of this formation is impossible
to estimate with any accuracy. Located as it is on the overturned
limb of a great anticline whose core is composed of competent
quartzites and dolomites, the incompetent Bascom has undergone extreme
flowage. It may be repeated at least once by crumpling, but its
greatest thickness can hardly be less than about 500 feet.
Cady (1945, p. 545) suggested that the lower part of the Bascom is
in the Lecanospira zone and the upper part belongs to the Eurystoniites
kelloggi zone and is equivalent to most of Cushing's "Cassin" formation.
The Bascom is approximately equivalent to "Beekmantown Division
D" and is correlated with the uppermost Roubidoux, Jefferson City,
Cotter, Powell, and Smithville of the standard Canadian sequence in
the Ozark Mountains (Plate I).
ORDOVICIAN—CHAZY GROUP
Strata that belong to the Crown Point and Valcour divisions of the
standard Chazy section crop out in one small part of the Castleton
quadrangle. Beds equivalent to the Day Point formation, of earliest
Chazy age, are found in Vermont only in the north-central Champlain
Valley (Cady, 1945, p. 553). The valuable West Rutland marbles are
almost the only Chazy beds in the Vermont Valley that were spared by
Trenton erosion. Three formations are recognized: Burchards limestone,
Beldens formation, and Middlebury limestone.
Burchards Limestone
Name and distribution: The Burchards limestone (Kay and Cady,
1947) is typically exposed between Cornwall Village and The Ledges,
Addison County, Vermont. Formerly (Cady, 1945, p. 548) the same
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