Geology and Physiography
The Hudson River valley runs more or less north to south down the eastern edge of New York State, cutting through a series of rock types including Triassic sandstones and redbeds in the south and much more ancient Precambrian gneiss in the north (and east). In the Hudson Highlands, the river enters a fjord cut during previous ice ages. To the west lie the extensive Appalachian highlands. In the Tappan Zee region, the west side of the river has high cliffs produced by an erosion-resistant diabase; these cliffs range from 400–800 feet in height.
The Hudson Valley is one physiographic section of the larger Ridge-and-Valley province, which in turn is part of the larger Appalachian physiographic division. The northern portions of the Hudson Valley fall within the Eastern Great Lakes and Hudson Lowlands Ecoregion.
During the last ice age, the valley was filled by a large glacier that pushed down as far as Long Island. Near the end of the last ice age, the Great Lakes drained south down the Hudson River, from a large glacial lake called Lake Iroquois. Lake Ontario is the remnant of that Lake. Large sand deposits remain from where Lake Iroquois drained into the Hudson; these are now part of the Rome Sand Plains.
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