The Hudson Valley Fold-Thrust Belt, route 23, near Catskill, NY, contains an excellent outcrop showing the cross-section of Upper Silurian through lower Middle Devonian strata. These strata have been folded and faulted, but the bedding of the original formations is well preserved and can be easily traced. Observations were recorded and collected one afternoon during a field trip down to Catskill to collect data for a structural geology report. The roadcuts are accessible directly from the highway and it is legal to park on the shoulder for short periods of time.

 Lithology

    These sedimentary rocks contain classic Lower Devonian North American faunas and examples of shallow-marine carbonate facies. The Austin Glen Formation consists of interbedded greywacke and shale, which contains well-developed pencil cleavage. The top of the Austin Glen Formation has been eroded away to leave an unconformable surface. The Roundout Formation lies above the unconformity. This Upper Silurian rock formation consists of dolostone rich in clay. The Helderberg Group, which consists of the Manilus Formation, the Coeymans Formation, the Kalkberg Formation, and the New Scotland Formation, grades upward from the Roundout Formation. These formations includes a range of units indicating many successive transgressions of a shallow sea. The Manilus Formation consists of calcarenite, a crystalline limestone with massive bedding and medium grains, and fossils of brachiopods. The Coeymans Formation consists of thickly bedded calcarenite containing chert and crinoids. The Becraft, also a part of the Helderberg group is present in Catskill. It is often compared to and grouped together with the Coeymans group. The Kalkberg Formation include of a limestone that is lighter in color, contains less fossils than the Coeymans Formation but more chert, and has thinner beds with more defined layers. The New Scotland Formation follows the Kalkberg Formation in a grading upward sequence. The New Scotland Formation consists of fine- to medium-grained limestone that contains bits of clay. It contains many fossils of a variety of sea bottom dwellers. The Helderberg Group then grade into the Tristates Group consisting of the Schoharie and Carlisle Formations and the Esopus Formation seen in Catskill. The Schoharie and Carlisle Formation ranges from siltstone to fine-grained sandstone. It contains brachiopods, cephalopods, and corals. The Esopus Formation is a dark, gray shale containing chert nodules. It contains some of the same fossils as the Schoharie and Carlisle Formation as well.

 Structures

    There are 10 major folds in the Catskill outcrops. These folds generally trend north-south and are gently plunging south- west. Some faults have cut through dipping layers and disturbed the bedding. At the first road stop, on the north side of route 23, west of the road bridge, the slaty cleavage in the Esopus shale radiates outward from the center of the fold hinge located there. There are also tiny crinkle folds at the top of the formation. In the Schoharie and Carlisle Formation the slickenslides seen on the rocks indicate slip surfaces between the two formations. Across the street in the same formations, there is a horizontal fault that runs towards the west, just on the very eastern edge of the outcrop. In the third and forth outcrop stops, both sides of the street do not mirror the other, displays evidence to suggest that a fault cut through the bedding exhibiting drag folding. Both the Kalkberg, Coeymans, and Manilus Formations contain slip surfaces at the contacts indicated by slickenslides. The cleavage is just about perpendicular to the layering in some areas on the outcrop, especially where the folding of these layers is more intense within the layers. In general the outcrops are characterized by faults in which beds in the hanging walls have been folded into anticlines. The same bedding down at the river's edge is completely vertical. The last outcrop along the road, which consists of the Becraft, New Scotland, Kalkberg, Coeymans, Manilus, and Roundout Formations, contains a steepening of the dips of bedding.

 Interpretations

    The sedimentary deposits of these formations occurred in the Upper Silurian and continued through the lower Middle Devonian periods. During Silurian time, a warm, shallow sea covered the area until the beginning of the Early Devonian. The unconformity between these two time periods indicate that the sea retreated exposing the Helderberg Group and eroding it a but. The Tristates Group indicate a return of the sea. At the end of the Early Devonian, erosion removed almost all of the Tristates Group from the area. The beginning of the Devonian period brought the island-arc collision with Proto-North America, closing the remaining part of the Iapetus Ocean. This collision was the result of the faulting and folding seen in Catskill today. The stress applied to the area at this time came from one direction. This evidence is indicated in the trending of the layers as seen in the great circles drawn (please refer to great circle packet). Most of the strike and dips are in the same directions and trend more steeply dipping on the western limb of the anticline present.

 References

Iaschen, Y. W. et al. 1991. "Geology of New York: A Simplified Account." New York State Museum and Geologic Survey, The State Education Department, The University of the State of New York, Albany, NY 12230.

 Marshak, Stephen, and Engelder, Terry. "Exposures of the Hudson Valley Fold-Thrust Belt, west of Catskill, New York" Geological Society of America Centennial Field Guide- Northeastern Section, 1987. (pg 123-128).