The quartz-rich conglomerate and sandstone and small amounts of shale of the Silurian-age Shawangunk Formation were deposited in largely terrestrial environments. Deep erosion of the Appalachian Mountains following the Ordovician-age Taconic Orogeny resulted in transport and deposition of large amounts of milky quartz in southeastern New York, seen as the white pebbles of the Shawangunk Formation.
The lowermost Shawangunk conglomerates sit on an ancient erosion surface (the "Taconic unconformity"). The erosion surface is estimated to represent approximately 20-30 million years. Below that are shales and sandstones deposited in deeper marine waters (a few hundred feet deep at most) during the Late Ordovician Period (Martinsburg Formation).
The Shawangunk Formation is thickest northeast of Port Jervis, where it is as much as 1400 feet (425 meters) thick. It thins to the northeast and disappears a little southwest of Kingston. In its thickest areas, it may have been deposited over as much as 15 million years or more—at the same time that a shallow sea covered central to western New York and much of the eastern U.S.
Cross-bedding, channel structures and other features in the conglomerates and sandstones indicate the rocks were chiefly deposited in "braided" rivers, which feature many small, interweaving channels. They are commonly seen draining mountainous areas. However, the finding of a trilobite fossil in 2004 indicates that there were times when sea level rose enough that the seaway of central to western New York flooded the Shawangunks area for a geologically short time. This is seen in some rock outcrops by layering that indicates the action of tides ("herring-bone cross laminations").
The very hard, resistant nature of the quartz-rich Shawangunk Formation rocks made them ideal for millstones, used for grinding grain. Mined from the slopes of the ridge and shipped out via the Delaware and Hudson canal during the 1800s, they were a major source of millstones in the U.S. Old quarry pits were worked by hand, and they are commonly seen in some areas. Lead was also formerly mined in the Shawangunks.
That same hard, resistant character of the rocks, along with the resulting high cliffs, also makes the Shawangunks ideal for rock climbers, who come to climb from around the world. It is said to be the most-visited rock climbing area in North America. Another famous mountain climbing site in the eastern U.S., called Seneca Rocks (in eastern West Virginia), is made of the same quartz-rich rocks deposited at the same time.
During the last glaciation of New York (which at its peak about 24,000 years ago covered all of New York except around Alleghany State Park in the southern tier) the Shawangunks were buried beneath thick glacial ice. Evidence of that glacial episode is found as scratches and gouges on the surface of the hard quartz-rich rocks ("glacial striations"), and as various sediment deposits including "glacial till", a mix of various sized-sediments from clay to sand to pebbles and boulders.
Thanks to Laura Conner (Minnewaska State Park, NYS Office of Parks, Recreation and Historic Preservation), Heidi Wagner (Sam’s Point Preserve, Nature Conservancy) for information and images, and to Craig Williams (historian, NY State Museum) for additional information and images.
Minnewaska State Park
Sam’s Point Preserve
Mohonk Mountain House
Shawangunk Ridge (Wikipedia)
Shawangunk Mountains Scenic Byway
http://pubs.usgs.gov/bul/1839l/report.pdf (a recent detailed geologic study of Silurian-age rocks of the Shawangunk Ridge; by Jack Epstein, 1993)