Geology of the New York City Region

26. Route 23A Road Cuts Near Catskill, New York

Anyone who has taken the energy to drive to look at the geology of the Catskills, but didn't take the 15 minutes to examine the road cuts along Route 23 in Catskill, New York, would have missed an the opportunity to examine the region's "Rosetta Stone." This locality is actually a series of outcrops along Route 23 (not 23A), and are easily to examine on the way to North Lake State Park. The outcrops include a nearly complete cross-section of the Hudson Valley Fold-Thrust Belt formed during the Acadian Orogeny, and includes a remarkable exposure of the Taconic Unconformity.

Figure 60 shows the location of the outcrops of interest in relation to the New York Thruway Exit (#21) and access to Route 23A (to the Catskills). The localities are worth a stop to look, but be aware that most of the locations of the outcrops are posted "no stopping, standing, or parking" due to traffic safety concerns; however, some are not posted. The best time to examine the exposures is early in the morning on the weekend when traffic is light.

Outcrop map near Catskill, New York
Figure 60. Location of roadside outcrops near the town of Catskill, New York.

The Thruway Tollgate lead to Route 23B. Turn left (south) on 23B and proceed .8 miles and park well off to the right a short distance before the northbound ramp to Route 23 (where it is not posted). The exposure of the Taconic Unconformity is located on the off-ramp from Route 23 to Route 23B (on the northeast side of the ramp [Figure 61]). At this location the Roundout Formation (Late Silurian) is resting on the truncated surface of the Ordovician Austin Glen Formation, exposing a classic angular unconformity. The limestone of the Early Devonian Manlius Formation conformably overlies the Rondout Formation. All of these units have be rotated to a steeply dipping angle by deformation during the Acadian Orogeny. A slight folding of the Ordovician beds along the contact suggests that some thrusting-type movement has occurred along this surface. The occurrence of the calcareous beds of the Roundout on this surface is a significant contrast to the occurrence of the massive Silurian Shawangunk Formation that rests upon this unconformable surface further south (see Locality 28).

The Taconic Unconformity (an angular unconformity with possible faulting offset?)
Figure 61. The Taconic Unconformity, an angular unconformity between the vertical beds of the Ordovician Austin Glen Formation and the overlying, but steeply dipping, Late Silurian Rondout Formation.

The drive west on Route 23 is lined with outcropping of the Lower Devonian Helderberg Group and Middle Devonian Tristates Group in a series folds (anticlines and synclines) with numerous small thrust faults. The most well-exposed anticline is on the northbound side of Route 23 approximately a half a mile west of the Thruway overpass, and just before the Bridge over Catskill Creek. This anticline displays numerous thrust sheets that display westward movement of the overthust sheets, consistent with the deformation style of the Acadian Orogeny throughout the region.

It will be necessary to turn around and head east on Route 23 in order to turn southward on Route 9W, then westward on Route 23A to get to the Catskill Mountains region. This method, however, will allow you scout the outcrops for a better look on the way back.

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Last modified: 3/11/2019