STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

Cambrian period the Appalachian geosyncline in New England and
eastern Canada is supposed to have been divided into a western trough,
in which the sediments of the Castleton area were laid down, and an
eastern trough along the present Atlantic coast, the Acadian trough.
Some infer that a land barrier, probably trending northeast through
central and eastern New Hampshire, separated the two troughs. In any
case, the eastern, or Acadian, trough does not concern us here. In general,
throughout the Cambrian and Ordovician the eastern part of the western
trough was the locus of deposition of muds, sands, and coarse elastics.
Chiefly carbonate sediments were deposited in the western part of the
western trough. The autochthonous rocks of the Champlain Valley
sequence belong to the predominantly carbonate section of the trough,
which for convenience will be called the Champlain basin. The allochthonous
rocks of the Taconic sequence were laid down in the predominantly
clastic section of the trough, which will be called the Magog
basin. No implication that the Champlain and Magog basins are other
than the loci of deposition of two contrasting facies is intended; we have
no evidence of a barrier that separated the two basins.
The Taconic sequence probably was deposited in frequently shallow
seas, for we find coarse limestone conglomerates, edgewise conglomerates,
and abundant coarse graywackes and grits among the rocks of the
Taconic sequence. The small thickness of the Taconic sequence suggests
that it may have been deposited near the eastern shore of the Magog
basin. Here we have assumed that the Taconic sequence originated in
the present Connecticut Valley region. The Valley sequence also was
deposited under generally shallow water conditions. We do not know
the depth of the sea in the part of the trough that stretched between the
Champlain basin and the eastern part of the Magog basin, but this
central area, now central and eastern Vermont, was the most rapidly
subsiding part of the trough and received by far the thickest body of
sediments.
The coarse sediments of the Mendon series were the earliest deposits
in the trough. These were interbedded with and probably followed by
muds, sands, and dolomites in later Mendon time. At approximately the
same time the Connecticut Valley region was receiving the muds, sands,
and coarse elastics of the Nassau formation. The southern and central
parts of Vermont may have been folded after the Mendon series was
laid down.
The Cambrian period in the Champlain basin began with the deposi-
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