Ausable Chasm is a sandstone gorge about two miles long and 150 feet deep, near Keeseville in the northeastern Adirondacks, and it is the landmark the Ausable River is named for.
The rock is Potsdam Sandstone, a 520 foot thick Cambrian exposure around 500 million years old, formed from ancient tidal flats. Because it was tidal flat rather than deep ocean, it preserves things that rarely survive: ripple marks left by moving water, ichnofossils, which are the traces and burrows of animals rather than their bodies, and rare mid-Cambrian Scyphomedusae jellyfish fossils. Fossil jellyfish are exceptional, because a jellyfish is essentially water and has almost nothing durable to leave behind.
The gorge is very much younger than the rock. It began forming about 10,000 years ago at the end of the Pleistocene, cut by headward erosion working back upstream from Rainbow Falls, a 91 foot waterfall at the southern end. Before that, the Laurentide ice sheet had carved deep cliffs along the Champlain Valley and then buried them in glacial till.
The Ausable River runs through the chasm and empties into Lake Champlain about a mile away.
You can see it several ways: walkways and bridges along the rim and down inside, a raft or tube float through the lower gorge, and adventure routes with cables and ladders across the rock faces. It sits in the northeastern Adirondacks, due west of Port Kent, and is seasonal.
The sequence that cut it is worth following. When the ice sheets melted, the Champlain Sea flooded this ground. Post-glacial rebound lifted the land, the sea receded, and the Ausable River, carrying far more water then than now, began cutting down through the loose glacial till. Once it had cut through the till to the Potsdam Sandstone beneath, the river went over a buried scarp and the ancestral Rainbow Falls formed. As those falls eroded backwards upstream they exploited vertical joints in the sandstone, which is why the gorge follows such a sharp, angular course. The horizontal beds resist erosion, so the river cut down rather than sideways: hence the depth and the narrowness.
Over 10,000 years the river changed course in places and abandoned old channels. Two of them, Big Dry Chasm and Little Dry Chasm, are open to visitors alongside the main gorge. A dam upstream and the diversion of water have effectively stopped the chasm developing any further.
Other places on the map within a couple of hours.
