Geologizing a Cuesta, Part 14: A Hudsonward View | Along Route 44/55, Shawangunk Ridge, New York, USA
Looking east-northeastward from the scenic pullout described in Part 5.
Here, spread out over the darkling brow of Shawangunk Mountain, is the bucolic valley of the Wallkill River. A bit of the town of New Paltz is visible at far left. The ridges in the distance lie just to the west of the Hudson River.
The fact that the main feature in this photo is a valley, and hence low ground, is a function of its bedrock. It’s soft and yielding Ordovician shale, much more prone to disintegration and removal than the harder, ridge-forming units on either side of it. Without meaning to, I have once again chanced upon the process of differential erosion that has been a leitmotif in a couple of separate photo-essays posted in other albums recently.
However, determining exactly which Ordovician shale it is can be a chore. The Lower Hudson Sheet of the Geologic Map of New York, published in 1970, identifies it as the Normanskill Formation from here to New Paltz, with the Austin Glen Formation lying a little farther east. And that is also what’s shown on the USGS Google Earth overlay.
The problem is that more recent sources imply without going into sufficient detail that it is the Martinsburg Formation that forms the Wallkill Valley floor on this side of the stream. The Martinsburg, stratigraphically higher in the Ordovician sequence than the Normanskill, is the unit exposed in the wonderful turbidite roadcut shown at this very spot in Part 5.
So I’m assuming that the Martinsburg is the more accurate identification. I cannot find the whole story on this reinterpretation, but it makes sense given what other, modern sources indicate about the geology closer to the Shawangunk Ridge itself. In any event, this locale sorely needs its own quadrangle or regional bedrock map, and preferably one produced in the current century.
As far as the high points in the distance go, it’s tempting to think they are part of the Hudson Highlands. But we’re too far north. That scenic and historic region, formed from glacially sculpted Mesoproterozoic crystalline rock, only extends up the big river to a little south of Newburgh.
So the distant ridges in this scene are made instead of another Ordovician formation, the Quassaic Quartzite. Or so it is shown on the 1970 map. But this too makes sense, because the Quassaic contains metaquartzite, which is very resistant stuff, as well as unaltered sandstone and conglomerate.
Of course, there’s always a chance that the stratigraphy over there by the Hudson has been changed as well. If I find it has been, I’ll do as I always do, and update this description accordingly.
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 14: A Hudsonward View | Along Route 44/55, Shawangunk Ridge, New York, USA
Looking east-northeastward from the scenic pullout described in Part 5.
Here, spread out over the darkling brow of Shawangunk Mountain, is the bucolic valley of the Wallkill River. A bit of the town of New Paltz is visible at far left. The ridges in the distance lie just to the west of the Hudson River.
The fact that the main feature in this photo is a valley, and hence low ground, is a function of its bedrock. It’s soft and yielding Ordovician shale, much more prone to disintegration and removal than the harder, ridge-forming units on either side of it. Without meaning to, I have once again chanced upon the process of differential erosion that has been a leitmotif in a couple of separate photo-essays posted in other albums recently.
However, determining exactly which Ordovician shale it is can be a chore. The Lower Hudson Sheet of the Geologic Map of New York, published in 1970, identifies it as the Normanskill Formation from here to New Paltz, with the Austin Glen Formation lying a little farther east. And that is also what’s shown on the USGS Google Earth overlay.
The problem is that more recent sources imply without going into sufficient detail that it is the Martinsburg Formation that forms the Wallkill Valley floor on this side of the stream. The Martinsburg, stratigraphically higher in the Ordovician sequence than the Normanskill, is the unit exposed in the wonderful turbidite roadcut shown at this very spot in Part 5.
So I’m assuming that the Martinsburg is the more accurate identification. I cannot find the whole story on this reinterpretation, but it makes sense given what other, modern sources indicate about the geology closer to the Shawangunk Ridge itself. In any event, this locale sorely needs its own quadrangle or regional bedrock map, and preferably one produced in the current century.
As far as the high points in the distance go, it’s tempting to think they are part of the Hudson Highlands. But we’re too far north. That scenic and historic region, formed from glacially sculpted Mesoproterozoic crystalline rock, only extends up the big river to a little south of Newburgh.
So the distant ridges in this scene are made instead of another Ordovician formation, the Quassaic Quartzite. Or so it is shown on the 1970 map. But this too makes sense, because the Quassaic contains metaquartzite, which is very resistant stuff, as well as unaltered sandstone and conglomerate.
Of course, there’s always a chance that the stratigraphy over there by the Hudson has been changed as well. If I find it has been, I’ll do as I always do, and update this description accordingly.
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