Ausable Chasm is a sandstone gorge and tourist attraction located near the hamlet of Keeseville, New York, United States,[1] due west of Port Kent.

The sandstone of the gorge, formed from ancient tidal flats,[4] preserves ripple marks, ichnofossils,[5] and rare mid-Cambrian Scyphomedusae jellyfish fossils.

[7] By the end of the last glacial period, the melting ice sheets formed the vast Champlain Sea, which inundated the current location of the chasm.

[2] Once the incipient Ausable Chasm had fully cut through the unconsolidated glacial till and reached the Potsdam Sandstone basement, the river fell over a buried scarp and formed the ancestral Rainbow Falls.

Other than the natural weakness of the vertical joints, the horizonal beds of Potsdam Sandstone are resistant to erosion, contributing to the gorge's depth and narrowness.

Ausable Chasm today is a box canyon, with the Rainbow Falls having carved a two miles (3.2 km) long gorge from its ancestral location to the present-day visitor's center.

[5] Modern development of the gorge has effectively been halted due to the construction of a dam upstream and the diversion of water from Rainbow Falls.

In addition to five miles of trails which extend around and inside the chasm, activities offered include rock climbing, whitewater rafting and a via ferrata.