STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

angle a thickness of 230 feet was computed. The maximum thickness
of the Columbian is 600 feet, but much of it has been removed in some
places as a result of erosion during the Trenton epoch. The total thickness
of the Boardman is approximately 900 feet.
Age: Cady has correlated the Shelburne marble with "Beekmantown
Division B", which is probably of Gasconade age (Rodgers, 1937, p.
1376-1577) and therefore lowest Ordovician. Wheeler (1942, P. 523)
placed this zone in the Upper Cambrian. The Cutting is the eastern
facies of "Beekmantown Division C", and parts of it (Cady, 1945, p.
545) are in the Lecanospira zone. The Boardman is, therefore, equivalent
to the Gasconade and most of the Roubidoux (Plate I).
Bascom Formation
Name: Cady (1945, p. 540) named this formation from a locality described
by Wing as "Bascom's Ledge" in the eastern part of Shoreham
township, Vermont.
Distribution: The Bascom formation thins markedly from the northern
border of the quadrangle two miles west of the Otter Creek to a point
one mile northwest of Proctor, where it disappears. It is not found elsewhere
in the Castleton area.
Description: The Bascom is a blue-gray and white banded marble
containing dolomite beds from a few inches to a few feet thick and beds
of sandy phyllite, phyllite, and sandstone. The distinctive variety of
lithologic types serves to set this formation apart from all other marbles.
The dolomites weather white, cream-colored, gray, and buff. In the
Hinesburg synclinorium in the northern Champlain Valley four zones
have been discriminated (Cady, 1945, p. 543), but the complex crumpling
and increased metamorphism of the Bascom in the present area has made
the succession obscure. In mapping, the Bascom has been separated
from the Columbian marble at the first appearance of dolomitic, sandy,
or argillaceous rocks, for no widespread single key bed or zone could be
found on which to base the Bascom. Whether the northern sandy beds
of the lowest Bascom pass southward into the uppermost Columbian
through loss of sand is unknown, but Cady's "Zone 1" seems to be missing
in the Castleton area. West of the Pittsford Valley quarries the
buff-weathering dolomites mark the lowest Bascom. Yet to the south
at Florence sandy and argillaceous beds accompany thin dolomites
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