The Hudson River valley runs primarily 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; the cliffs range from in height.