CHIMNEY ROCKS PARK

GEOLOGY

PENNSYLVANIA U.S.A

 

The Chimney Rocks Park is located along the Catfish Ridge within the Appalachian Mountain chain. The Allegheny Structural Front is visible looking over Hollidaysburg, the geology and deformation styles are relatively different on either side of the Allegheny Structural Front to the northwest. It is the dividing line between the Valley and Ridge Province and the Appalachian Plateaus Province.  

 

The deformation of the region was caused by the Alleghanian Orogeny during the Carboniferous/Permian period (325 to 260 million-years-ago), as the collision of the African and North American Plates was occurring.  

The Chimney Rocks Park lies within the Valley and Ridge province, which is a mix of Fold-Thrust belts that lie west of the eroding Ancestral Appalachian Mountains. The rock strata has been forced westward by tectonic collision, this has formed massive thrust faults. Most faults are flat-ramp thrusts that do not break the surface, thus remain invisible but are extremely abundant.    

CHIMNEY ROCK LIMESTONE COMPANY

The cliffs at the Chimney Rocks Park are only visible because of its status as a former Limestone quarry, the Chimney Rock Limestone Company once mined the high-grade 'calico' Limestone rocks at the base of the Keyser Formation (Top of the quarries highwall). 

The Limestone was burned for Lime in a Kiln, along with other layers being crushed for aggregate and road material. The last documented information on the quarry suggested that it was still active in 1962. 

 

The highwall of the abandoned quarry exposes the highest layer of the Tonoloway formation along with two members of the Keyser Formation: the Chimney Rocks Member and the Jersey Shore Member.

ROCK LAYERS

The above Keyser Formation is distinguished from the lower Tonoloway Formation by a change in nodular limestone as the Tonoloway Formation is far more laminated then the above Keyser Formation. A possible unconformity between the lower Tonoloway Limestone and the Keyser Formation marked out by fauna found and lithologic breaks is also present.    

TONOLOWAY FORMATION

The Tonoloway Formation was deposited in the Late Silurian. The limestone is mostly finely laminated and thinly-bedded but does on occasion contain thin beds of calcareous shale.

The formation was formed in a widespread ocean bed, to the east it was bounded by the Bloomsburg Delta, and towards the south, a low landmass called the Auburn Promontory. The ocean once covered an area from Maryland to New York but got shallow enough to form mud cracks which have been preserved in the Formation. 

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KEYSER FORMATION

The Keyser Formation ranges from 27 to 62 meters throughout Pennsylvania and averages out at 30 meters around the Hollidaysburg area. The Keyser Formation is Silurian and Early Devonian in age and was deposited in a shallow widespread ocean environment like the underlying Tonoloway Limestone.  

The Keyser Formation consists of 4 members:

.Chimney Rocks Member (Below Images)

The Chimney Rocks Member is the lowest member visible in the park and lies conformably on top of the Tonoloway Formation. This member is a mix of different types of limestone, the base layer of the member is called "calico rock" and is comprised of 9 meters of medium-grey massive fossiliferous lime mudstone, bioclastic wackestone/floatstone which also contains vugs of calcite, white, yellow and pink in colour. Different fossils are also visible within the member like, corals, crinoids, stromatoporoids, brachiopods, along with stromatolites that are mound-like and columnar in shape. The fossils are most abundant in the top 3 meters and stand out within the matrix.  

 

.Jersey Shore Member   

The Jersey Shore Member is above the Chimney Rocks Member and is very similar, the only difference is with the marine invertebrate fauna as its more diverse, fossils like horn and halysitid corals are visible and not in the below Chimney Rocks Member. Another small difference is the addition of slightly yellow shaly layers, the contrast between the two members ranges from gradual to very sharp within the park. 

 

.LaVale Member (Not Present in this Location)

The LaVale Member represents a sea-level drop and another rise, the member was most likely deposited in a nearshore environment influenced by the tide as shale is present towards the top. 

.Byers Island Member (Not Present in this Location)

This member possibly represents the deepening of ocean water to a wave basin rather than a nearshore environment. The member is 12 meters high and is comprised of nodular limestone that becomes even more sporadically nodular further up.

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Limestone CHIMNEYS

The chimneys are comprised of 2-3 meters of Tonoloway Formation at the base with 4-6 meters of "calico" limestone mounted ontop. (The Tonoloway Formation isn't visible in the below images).

The chimneys visible on Catfish Ridge were formed by upward fault-related displacement, the fault has a throw of 4.6 to 6 meters in a vertical direction. Because of fault-related movement, the more resistant "calico" rock is located on the northwest side of the fault while the less erosion-resistant nodular limestone and grainstone are located on the southwest side.

 

Another reason the chimneys are so distinct is because of several well developed N15°W-striking joints that have prompted dissolution in the limestone which has aided erosion even further and made the more prominent chimneys stand out compared to the rest of Catfish ridge.

 

REFEreNCES

Behr, R.-A., Hand, K. L., Pennsylvania Geological Survey & Pennsylvania Department of Conservation and Natural Resources, (2017, September). Bedrock Geology of the Mingoville quadrangle, centre county, Pennsylvania | Pennsylvania Geological Survey Fourth Series Harrisburg. Research Gate.

Kerrigan, R., & Northern Alleghenies Geological Society. (2017). Structural Geology of the Allegheny Front near Chimney Rocks Park, Hollidaysburg, PA. The University of Pittsburgh at Johnstown.

Christopher D. Laughrey, & C.H. Shultz ed. (1999). Part II, Stratigraphy and Sedimentary Tectonics, Chapter 6: Silurian and Transition to Devonian. Geology of Pennsylvania: Pennsylvania Bureau of Topographic and Geologic Survey. Special Publication 1, p 91-107. 

Ruffolo, R. M., Ciampaglio, C. N., & Section, G. S. A. N. (2011). From the Shield to the Sea. Geological Society of America. Page 127. 

Geologic Unit: Keyser. (n.d.). Geolex (USGS). 

Reese, S. O. (2017). Outstanding Geological Feature Of Pennsylvania Chimney Rocks, Blair County. Trail of Geology 16–038.1. 

Butts, C. Description of the Hollidaysburg and Huntingdon Quadrangles. Page 7. United States Geological Survey (USGS). 

Wood, G. H., Jr., J. Peter Trexler, & Thomas M. Kehn. (1969). Geology of the West-Central Part of the Southern Anthracite Field and Adjoining Areas, Pennsylvania. United States Geological Survey (USGS).  Geological Survey Professional Paper 602.

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PENNSYLVANIA, U.S.A
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