STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

probably those on the eastern flank of the Taconic Range. in general it
is thought that successively younger beds crop out in a westerly direction
until the Bomoseen and Mettawee are encountered. Obviously, however,
all the rocks are intensely crumpled, and no simple sequence of older-toyounger
from east to west is conceivable.
Thin Bomoseen grit was found in Fennel Hollow along the contact of
the Mettawee slate and the Nassau formation. Elsewhere along this
vague contact the Bomoseen is either concealed or absent, neither of
which possibilities is in any manner unlikely or abnormal. The accordionlike
isoclinal folding of all the strata of the Taconic sequence can explain
adequately the obscurity of stratigraphic relations which, in regions of
less deformation, metamorphism, and forest cover, are relatively clear.
It is probable that the Nassau formation as mapped here includes
younger rocks that are not sufficiently well exposed to be recognizable
in the field. In addition to a large mass of probable Normanskill formation
differentiated on the map in the township of Middletown, there may
be other undifferentiated strata of Normanskill age in the Nassau.
Because of the lithologic similarity of the Mettawee slate and the Bomoseen
grit to some beds in the Nassau, some Mettawee and Bomoseen
probably have gone unrecognized in the Nassau terrane. It is not likely
that the Nassau as here mapped includes Schodack, Eddy Hill, or Zion
Hill strata.
Thickness: The Nassau formation in the Castleton quadrangle must
be at least 1000 to 2000 feet thick. In addition to this the main body of
Bird Mountain grit is probably 500 feet thick. To the south 200 or 300
feet of Bird Mountain grit may be present. The coarse elastics are probably
contemporaneous with some of the argillaceous rocks and may
represent deltas in a sea whose dominant deposits were muds and fine
sands.
Age: No fossils have been found in the Nassau or Bird Mountain in
this area. The Oldhamia-bearing Nassau beds of eastern New York lie
beneath rocks containing an Olenellus fauna. Ruedemann (1930, p. 83)
suggested that the Nassau may be very late Pre-Cambrian or transitional
from Pre-Cambrian to Cambrian.
Under the present interpretation the Bird Mountain grit is contemporaneous
with the Nassau formation. Although the Rensselaer grit or
graywacke of eastern New York is somewhat different petrographically,
enough similarity to the Bird Mountain exists to suggest contem-
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