Geologizing a Cuesta, Part 4: A Joint-controlled Stream | Shawangunk Ridge, New York, USA
(Last updated on December 24, 2025)
Taken, I think, in the Rhododendron Bridge locale of the Mohonk Preserve. Here a number of hiking trails converge just south of Eagle Cliff.
If this stream has a name, it does not appear on the US National Map Viewer or the Mohonk Preserve trail map (see references, below) do not list it.
One of the best ways to disappoint a classful of college Earth-science students is to bring up the subject of joints—only to reveal that you're talking about fractures in rock where no appreciable displacement has occurred on either side. The sense of letdown is palpable.
Here in the Mohonk Preserve near New Paltz, the Silurian-period Shawangunk Formation (mostly white-quartz conglomerate and sandstone) contains a series of joints, including those oriented more or less vertically.
These may have formed when the beds now at the surface were relieved of the weight of overlying rock units when the latter were eroded away. In other words, the stone could have simply stretched when no longer under confining pressure. (This process of upward expansion causing sideways shrinking and cracking is known as the Poisson Effect.)
Alternatively, contraction caused by the beds cooling as they neared the surface may been an important factor. And other tectonic stresses and physical factors might have contributed to the fracturing as well.
Like most streams, this one seeks the least path of resistance and whenever possible uses these channels in the rocky surface. And in time it widens the fractures considerably.
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 4: A Joint-controlled Stream | Shawangunk Ridge, New York, USA
(Last updated on December 24, 2025)
Taken, I think, in the Rhododendron Bridge locale of the Mohonk Preserve. Here a number of hiking trails converge just south of Eagle Cliff.
If this stream has a name, it does not appear on the US National Map Viewer or the Mohonk Preserve trail map (see references, below) do not list it.
One of the best ways to disappoint a classful of college Earth-science students is to bring up the subject of joints—only to reveal that you're talking about fractures in rock where no appreciable displacement has occurred on either side. The sense of letdown is palpable.
Here in the Mohonk Preserve near New Paltz, the Silurian-period Shawangunk Formation (mostly white-quartz conglomerate and sandstone) contains a series of joints, including those oriented more or less vertically.
These may have formed when the beds now at the surface were relieved of the weight of overlying rock units when the latter were eroded away. In other words, the stone could have simply stretched when no longer under confining pressure. (This process of upward expansion causing sideways shrinking and cracking is known as the Poisson Effect.)
Alternatively, contraction caused by the beds cooling as they neared the surface may been an important factor. And other tectonic stresses and physical factors might have contributed to the fracturing as well.
Like most streams, this one seeks the least path of resistance and whenever possible uses these channels in the rocky surface. And in time it widens the fractures considerably.
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.