STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

tion of the Cheshire. Sands from a western source were washed clean,
and muds were swept eastward. Thick widespread dolomite beds were
deposited on the sandstone to form the Dunham. More sandstone and
dolomite followed, constituting the Monkton and Winooski of the
Champlain basin. In central Vermont the Plymouth series is probably
partly equivalent to the Lower Cambrian of the Champlain basin, and
here mud and sand were more prominent than dolomite. At approximatelv
the same time further east the sand, mud, and limestone of the
Bomoseen, Mettawee, Eddy Hill, Schodack, and Zion Hill were laid
down. From the Lower Cambrian to the early Ordovician we have no
record of the Taconic section. Whether it was above the level of the sea
or submerged during the Middle and Upper Cambrian is not known.
That some subaerial erosion did occur before the Lower Ordovician,
however, is certain.
On the other hand, the most rapidly sinking part of the trough, the
eastern Vermont area, is assumed to have received thick sediments
throughout the remainder of the Cambrian. The Champlain basin was
the locus of sand and dolomite deposition during the rest of the Cambrian
except, possibly, during the Middle Cambrian. A muddy lagoon existed
in the St. Albans area almost continuously from later Lower Cambrian
through Lower Ordovician time. An uplift of the Adirondack area in
Upper Cambrian time furnished vast quantities of sand to the eastbordering
Champlain basin. Dolomites interfingered with sands away
from the western source of sands. These sediments are the Danby and
Potsdam formations, which were covered throughout Vermont during
latest Cambrian time by clean dolomites that in the present area are
called the Clarendon Springs dolomite.
Nowhere in this great trough is there a break that can be determined
without question as the Cambrian-Ordovician boundary. Continuous
sedimentation across this arbitrary boundary is probable. The Lower
and Middle Ordovician sediments of the Champlain basin were mostly
limestones. These include the Boardman, Bascom, Burchards, Beldens,
Middlebury, and Orwell. The Castleton area may have been above sea
level during the interval between the deposition of the Bascom and
Burchards and between the Middlebury and Orwell. The rocks deposited
in what is now central and eastern Vermont during the Lower and
Middle Ordovician were principally muds. During the same interval
the eastern part of the Magog basin received thick black shales, finegrained
grit, radiolarian chert, and sand, and all the rocks contain
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