STRATIGRAPHY AND STRUCTURE CASTLETON AREA VERMONT

much of the New England region. Piedmont alluvial plains probably
covered Vermont and extended westward into deltaic deposits in western
New York and nearby areas during the remainder of the Devonian.
Erosion followed, and during the Mississippian period many dikes and
plugs of alkaline magma were emplaced throughout the New England-
Quebec region.
The remainder of Paleozoic and most of Mesozoic time is recorded not
at all in western New England. Fluvial erosion must have continued
throughout this interval. Perhaps in the Jurassic or early Cretaceous
period a lowland was achieved at a level that is probably not preserved
even on the highest peaks today. From the Cretaceous until the glacial
epoch, periodic general uplifts of New England took place. Drainage of
the region was by the Connecticut River toward the Atlantic Ocean and
by streams in western Vermont that probably emptied into an arm of
the sea that at times occupied the Hudson-Champlain-St. Lawrence
Valleys. Erosion during the Cenozoic developed terraces along the
streams, and these terraces were extended gradually upstream toward
the mountain watersheds. By early Miocene the Champlain Valley may
have attained substantially its modern aspect.
Mile-thick continental glaciers covered New England, probably more
than once, in the Pleistocene epoch. Their weight so depressed the land
that after their retreat marine waters again invaded the Champlain
Valley. Step-like uplift of the northeastern part of North America during
the last 15,000 years drained the Champlain sea and converted the
Hudson and St. Lawrence again into rivers.
ECONOMIC GEOLOGY
The rocks of the Castleton quadrangle are among the most valuable
natural resources of Vermont. In addition to abrasive sand and road
metal, the deposits of roofing slate and particularly of marble have long
been exploited.
The southwestern part of the quadrangle lies in the colored slate belt
of New York and Vermont, which extends northward from Greenwich,
New York to Lake Bomoseen in Castleton, Vermont. Several slate
quarries are now active in Poultney, Castleton, and Fairhaven. The only
formation being quarried at present is the Cambrian Mettawee slate,
although in the past a few quarries were opened in the Normanskill
formation. The purple and green Mettawee slate is divided into several
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