Shawangunk Ridge - Geology

Geology

The ridge is primarily underlain by Shawangunk Conglomerate, a hard, silica-cemented conglomerate of white quartz pebbles and sandstone that directly overlies the Martinsburg Shale, a thick turbidite sequence of dark gray shale and greywacke sandstone. The Martinsburg Shale was deposited in a deep ocean during the Ordovician (470 million years ago). The Shawangunk Conglomerate was deposited over the Martinsburg Shale in thick braided rivers during the Silurian (about 420 million years ago); both sequences of sedimentary rock were subsequently deformed and uplifted during the Permian (about 270 million years ago). As a result of this deformation, strata within the ridge are involved in a northward plunging series of asymmetric folds (e.g., anticlines and synclines) that dip gently towards the west. These same folds, involving strata that overlie the Shawangunk Conglomerate, are exposed north of Shawangunk Ridge in the Rosendale natural cement region, where they can be directly examined in abandoned cement mines. Strata along the eastern margin of Shawangunk Ridge are truncated by erosion, resulting in the prominent cliffs characteristic of Shawangunk Ridge. The Shawangunk Conglomerate is very hard and resistant to weathering; whereas the underlying shale erodes relatively easily. Thus, the quartz conglomerate forms cliffs and talus slopes, particularly along the eastern margin of the ridge.

The entire ridge was glaciated during the last (Wisconsin) glaciation, which scoured the ridges, left pockets of till, and dumped talus (blocks of rock) off the east side of the ridge. On top of the ridge, the soils are generally thin, highly acidic, low in nutrients, and droughty, but in depressions and other areas where water is trapped by the bedrock or till, there are interspersed lakes and wetland areas. Soils on top of shale are thicker, less acidic, and more fertile. Topography on the top of the northern Shawangunks is irregular due to a series of faults that form secondary plateaus and escarpments.