Geologizing a Cuesta, Part 5: Flysch on the Road to Molasse | Roadcut along Route 44/55, Shawangunk Ridge, New York, USA
This shot was taken half a century ago, in a scenic pullout that's still here. Even the roadcut has changed very little. See the geotag for the exact location.
To the rocks-are-boring crowd, this picture is criminally deficient in autumn colors, supernatural sunsets, ducklings, deer, and scenes of rusting Americana. But if you're a rockhound or a geologist, it's a thing of beauty. It shows a required stop in your continuing exploration of the evolution of the Eastern US.
Here's why. Anyone who has hiked in the Mohonk Preserve—for example, farther along on the Undercliff Trail here delineated by the adult couple and the two boys—has seen the Silurian-period Shawangunk Formation. This striking sequence of white conglomerate and sandstone beds forms the ridge's dramatic cuesta scarp and giant slump blocks. However, few visitors notice the less eye-catching Ordovician-period Martinsburg Formation that lies just beneath it.
They may be drab and dark and crumbly, but the Martinsburg's shale, siltstone, and graywacke strata are every bit as interesting. They owe their origin some 460 Ma ago to the convergence of Laurentia (ancestral North America) and a volcanic island arc. As the two terranes collided, the Taconic Mountains rose, and that in turn prompted the formation of a profound depression in the crust just to the west.
Geologists call this kind of downward buckling in front of a mountain range a "foreland basin." Into this basin flooded clay, silt, and dirty sand that formed turbidity currents, which swept with tremendous force downslope to the basin floor.
When these sediments finally came to rest, they did so in repeating patterns of layers known as flysch deposits. And while this is one of the best flysch exposures I've seen, I doubt that the folks shown here were aware that they were standing on ancient submarine avalanches.
Later in the Taconic mountain-building process, continued crustal compression folded and tilted these strata, as can be seen here. The Shawangunk Formation exposed farther uphill represents an even later, Silurian phase of the Taconic Orogeny. Its white quartz pebbles and sand, eroded off the mountains, created molasse units deposited in very shallow water or on land, once the forearc basin was filled. The Shawangunk beds were eventually tilted, too, but by a subsequent mountain-building event.
Sources Consulted for This Essay
- Epstein, Jack B. Stratigraphy of Silurian Rocks in Shawangunk Mountain, Southeastern New York, Including a Historical Review of Nomenclature. U.S. Geological Survey Bulletin 1839-L. Washington, DC: United States Government Printing Office, 1993.
- Feldman, Howard R., Jack B. Epstein, and John A. Smoliga. “The Shawangunk and Martinsburg Formations Revisited: Sedimentology, Stratigraphy, Mineralogy, Geochemistry, Structure and Paleontology.” In New York State Geological Association 81st Annual Meeting Field Trip Guidebook, Frederick W. Vollmer, ed. New Paltz, NY’ SUNY New Paltz, 2009.
- Mohonk Preserve. Undercliff-Overcliff Trail Map. Accessed October 6, 2022. www.mohonkpreserve.org/wp-content/uploads/2021/07/WT_Sugg...
- Schimmrich, Steven. Geology of the Hudson Valley: A Billion Years of History. Self-published by Steven Schimmrich, 2020.
- United States Geological Survey. National Map Viewer. Accessed December 24, 2025. apps.nationalmap.gov/viewer/.
To see the other photos and descriptions in this set, visit my my Geologizing a Cuesta album.
Geologizing a Cuesta, Part 5: Flysch on the Road to Molasse | Roadcut along Route 44/55, Shawangunk Ridge, New York, USA
This shot was taken half a century ago, in a scenic pullout that's still here. Even the roadcut has changed very little. See the geotag for the exact location.
To the rocks-are-boring crowd, this picture is criminally deficient in autumn colors, supernatural sunsets, ducklings, deer, and scenes of rusting Americana. But if you're a rockhound or a geologist, it's a thing of beauty. It shows a required stop in your continuing exploration of the evolution of the Eastern US.
Here's why. Anyone who has hiked in the Mohonk Preserve—for example, farther along on the Undercliff Trail here delineated by the adult couple and the two boys—has seen the Silurian-period Shawangunk Formation. This striking sequence of white conglomerate and sandstone beds forms the ridge's dramatic cuesta scarp and giant slump blocks. However, few visitors notice the less eye-catching Ordovician-period Martinsburg Formation that lies just beneath it.
They may be drab and dark and crumbly, but the Martinsburg's shale, siltstone, and graywacke strata are every bit as interesting. They owe their origin some 460 Ma ago to the convergence of Laurentia (ancestral North America) and a volcanic island arc. As the two terranes collided, the Taconic Mountains rose, and that in turn prompted the formation of a profound depression in the crust just to the west.
Geologists call this kind of downward buckling in front of a mountain range a "foreland basin." Into this basin flooded clay, silt, and dirty sand that formed turbidity currents, which swept with tremendous force downslope to the basin floor.
When these sediments finally came to rest, they did so in repeating patterns of layers known as flysch deposits. And while this is one of the best flysch exposures I've seen, I doubt that the folks shown here were aware that they were standing on ancient submarine avalanches.
Later in the Taconic mountain-building process, continued crustal compression folded and tilted these strata, as can be seen here. The Shawangunk Formation exposed farther uphill represents an even later, Silurian phase of the Taconic Orogeny. Its white quartz pebbles and sand, eroded off the mountains, created molasse units deposited in very shallow water or on land, once the forearc basin was filled. The Shawangunk beds were eventually tilted, too, but by a subsequent mountain-building event.
Sources Consulted for This Essay
- Epstein, Jack B. Stratigraphy of Silurian Rocks in Shawangunk Mountain, Southeastern New York, Including a Historical Review of Nomenclature. U.S. Geological Survey Bulletin 1839-L. Washington, DC: United States Government Printing Office, 1993.
- Feldman, Howard R., Jack B. Epstein, and John A. Smoliga. “The Shawangunk and Martinsburg Formations Revisited: Sedimentology, Stratigraphy, Mineralogy, Geochemistry, Structure and Paleontology.” In New York State Geological Association 81st Annual Meeting Field Trip Guidebook, Frederick W. Vollmer, ed. New Paltz, NY’ SUNY New Paltz, 2009.
- Mohonk Preserve. Undercliff-Overcliff Trail Map. Accessed October 6, 2022. www.mohonkpreserve.org/wp-content/uploads/2021/07/WT_Sugg...
- Schimmrich, Steven. Geology of the Hudson Valley: A Billion Years of History. Self-published by Steven Schimmrich, 2020.
- United States Geological Survey. National Map Viewer. Accessed December 24, 2025. apps.nationalmap.gov/viewer/.
To see the other photos and descriptions in this set, visit my my Geologizing a Cuesta album.