Outstanding Natural Sites

in the Champlain Valley, NY

     AuSable Chasm is one of the most unique scenic spots in the entire Champlain Valley.  The AuSable River has carved a spectacular gorge here to expose a 455-ft thickness of the Keeseville Member of the Potsdam Sandstone.

     The chasm is located 1.3 miles north of Keeseville, NY, where U.S. Route 9 crosses the AuSable River.  The chasm is a privately owned attraction and fees are charged for access to the walking paths and the raft trip through the lower end of the gorge. 

Ausable Chasm photograph by L.B. Gillett, c.  1999

     This sandstone was deposited in shallow marine waters when this region was on the coast of North America.   The rock is composed of almost 100% rounded quartz grains with silica cement.   Quartz is a very durable mineral and is concentrated by natural sedimentological processes.  The mineral is easily recognizeable in hand specimen because of its glassy appearance.  The ripple marks in the adjacent photograph were created by waves or currents in the Potsdam sea.

photograph by L.B. Gillett, c. 1999

   Heavy rainfall in 1998 caused several landslides in the chasm, exposing glacial and proglacial lake sediments high on the chasm walls.  A thick section of interbedded diamicton (a poorly sorted till-like deposit), gravel and sand overlie the Potsdam Sandstone.  The contact between the sandstone and diamicton is an unconformity representing more than 500 million years of missing rock record.