STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

Distribution: Dunham dolomite in the northeastern part of the quadrangle
is exposed in the bed of the Furnace Brook at Pittsford Mills
and northward. Exposures along the Otter Creek north of Proctor are
uncommon. It is inferred that a broad band of Dunham crops out beneath
the Otter Creek meadows in this vicinity. Dunham is exposed
in a northeast-trending band southeast of Chippenhook near the southern
border of the Castleton area.
Description: The Dunham in the Castleton area is a gray- and buffweathering,
compact, siliceous, gray dolomite containing irregularly
distributed quartz grains. Mottled patches of green, blue, and rarely
pink are not uncommon. The massive lower two-thirds of the Dunham
is so irregularly jointed that true bedding planes are hard to distinguish.
The upper third, which is similar in appearance to the Winooski dolomite,
is lighter gray, cream-colored, and generally less sandy. Thin shaly
partings at intervals of 6 to 8 inches are reported (Cady, 1945, p. 528)
in this upper zone from west-central Vermont but are not common in
the Castleton area. The upper Dunham just below the Monkton quartzite
has a faint pink color and contains thin pink dolomitic quartzites.
Because the Monkton decreases in thickness and may be absent in
much of the Otter Creek Valley, the pink quartzites are utilized as the
Dunham-Winooski boundary.
Thickness: The Dunham dolomite is the most poorly exposed stratigraphic
unit in the Valley sequence. Although this formation must crop
out over a large area, exposures are few. North of Proctor both the Otter
Creek lowlands and the northwestern slope of Pine Hill are cloaked
with surficial debris that hides bedrock. East of Pittsford, in the absence
of recognizable Monkton quartzite, it is difficult to separate any
Winooski that may be present from the Dunham. At Chippenhook
exposures of Dunham are also poor, and the Trenton erosion appears to
have removed beds far down in the Cambrian. The best that can be
said is that there are probably between 1500 and 3000 feet of Dunham
in the Castleton area.
Age: The Dunham is clearly Lower Cambrian. During this study
Hyolithes beds were observed in the Dunham east of Chippenhook.
These were discovered by Dale and identified by Walcott (Dale, 1894,
p. 535) as: H. americanus Billings, H. similis ?? Walcott, H. impar?
Ford, H. com,nunis? Billings, and Saltarella pukhella Billings. Such
19