In the Piedmont region of North Carolina, several mountains tower over the surrounding land, sitting eight hundred to nine hundred feet above the mean sea level (Bradley, 2015). These mountains, or “monadnocks”, seem to sprout from nowhere as they sit isolated from the nearby Blue Ridge Mountains. Tompkins (2013) explains, monadnocks are “remnants of a broad geological peneplain that eroded more than 100 million years ago,” creating the landscape in North Carolina we observe today. These monadnocks include Crowders Mountain and the Pinnacle. Generally, these mountains possess different bedrock than the surrounding land that has proven to be very resistant to erosion and movement over the course of 100 million years (Bradley, 2015). One of the specific rocks attributed to the resilience is “kyanite quartzite,” an enormously resistant rock unique to Crowders Mountain (Tompkins, 2013). However, North Carolina’s Piedmont region is not where the first monadnock was discovered.
Far north in Jaffrey, New Hampshire, sits Mount Monadnock soaring 2,000 feet above the surrounding landscape (Fowler-Billings, 1949). The mountain is underlaid with metamorphic and plutonic rocks from four different estimated geological formations: the Ordovician Ammonoosuc, Ordovician Partridge, Silurian Clough, and Devonian Littleton formations. Though all four of these periods contribute to the shape of the mountain, “Mt. Monadnock is held up by this resistant middle member of the Littleton formation …. composed primarily of a massive, rusty-weathering gneiss” (Fowler-Billings, 1949). Other rocks from the Littleton formation include “mica schist, sillimanite schist, and quartzite” with the remainder of rocks being “intrusives scattered over the area in large and small bodies” (Fowler-Billings, 1949). These rocks, similar to those in the Piedmont region, contribute to the resilience of Mt. Monadnock, maintaining its strength through millions of years of erosion, wind, and rain. Since Mt. Monadnock was the first isolated mountain formed from erosive events, geologists labeled it as precedent for similar peaks, establishing the geological term “monadnock”.
To explore the formation of the monadnocks in the Piedmont, geologists researched an interpretation of the process. Though this interpretation remains widely accepted, geologists understand that this could change as scientists continue conducting research on this history.
The process began more than half a billion years ago when a chain of volcanic islands was forming on the ancient continent conglomerate known as “Gondwana” (Bradley, 2015). On the island sat “generations of volcanoes,” erupting billions of tons of ash and sediment over the course of millions of years and causing a large part of Carolina, an amalgamation of these island chains, to undergo a new phase of volcanism (Bradley, 2015). This new volcanism phase of eroding debris occurred another millions of years after the previous period, and geologists assume the area underwent many cycles of buildup and destruction. Carolina sat on the coast of Gondwana, constantly spewing volcanic debris until it broke the ocean surface and erosion began working against it (Bradley 2015). Over time, the erosion destroyed the volcanoes on Carolina, leaving layers of sedimentary rocks on top, such as siltstones, sandstone, and conglomerate. Today, the schist and phyllite rocks evident under the park show geologists that this environment was once at the bottom of the ocean before sedimentary rocks were deposited on top, connecting back to the island chain of volcanoes (Bradley 2015).
Years later, Carolina slowly broke off of the Gondwana continent and began moving across the ancient ocean called the “Iapetus,” advancing towards the ancient landmass of North America (Bradley 2015). While rifting from Gondwana, Carolina recieved another enormous amount of sediment on top of the existing landscape, adding thousands of feet of more siltstones, sandstones, conglomerates, and limestones which can be seen in present stratigraphy data (Bradley 2015).
Once colliding with and attaching to ancient North America, the layers of rock folded into each other and metamorphosed together (Bradley 2015). According to scientific research, these metamorphose conglomerate layers exist in the modern-day Crowders Mountain State Park (Bradley 2015). Around 300 million years ago, the ancient African continent then collided with ancient North America, connecting to Carolina and creating the well-known supercontinent “Pangea”. Moving to 245 million years ago, Pangea began to split and formed a “system of rift valleys up and down the eastern edge of North America” (Bradley 2015). 200 million years ago, magma bubbling below the continental crust intruded the rocks, forming the rock type “diabase” which can be found in thin lines under Crowders Mountain. Pangea continued to split apart, and the current continents began forming individually, creating the Atlantic Ocean (Bradley 2015). Finally, after “millions of years of subsequent uplift and erosion,” the Piedmont region “slowly formed the landscape visible today within the Park and its surroundings” (Bradley 2015).
For the last 66 million years, the Piedmont has experienced major uplift and erosion, contributing to the lowlands sitting far below the monadnocks (Bradley 2015). Since the continental plates float on top of the earth’s mantle, instances of erosion cause the plates to float higher up due to the slow release of weight. Subsequently, the mountains seemingly rose higher up overtime as the lowlands continued to erode sediment away from the monadnocks. Sedimentary deposits formed from the erosion, transporting and redepositing broken, weaker rocks away from the mountains, gradually forming small sand-like particles (Bradley 2015). In the last 5 million years, erosion has created the current landscape, “forming monadnocks where rock[s that] were resistant to erosion” existed from continental development (Bradley 2015).
While the monadnocks feature igneous rocks, hydrothermally altered rocks, and sedimentary deposits, the majority of the rock types fall under the category of metamorphic (Bradley 2015). The metamorphic rocks include: schist and phyllite, quartzite, metaconglomerate, and metatuff. Schist and phyllite are composed of flakey minerals observable by the naked eye. Since the flakey materials are not very resilient to weathering, they sit under the mountains away from erosive elements (Bradley 2015). Today, the schist and phyllite that underlay the Park date back to over 300 million years ago from the rift between Carolina and Gondwana. However, the quartzite and metaconglomerate rocks that sit on top of the other ancient rocks are the resistant materials that maintain Crowders Mountain and the Pinnacle (Bradley 2015). Quartzite is mainly composed of the strong, resistant mineral quartz and is most responsible for the current landscape of the park. Metaconglomerate possesses similar strength to quartzite, deriving from a sedimentary rock conglomerate which is fragments of rock shaved down into rounded pieces from sand and silt (Bradley 2015). Finally, metatuff is metamorphic rock formed from volcanic igneous rock (Bradley 2015).
After the monadnocks in the Piedmont continued to maintain their shape, a new process called hydrothermal alteration began, creating the clay soils seen today. In this process, heated water rose up to the sea floor and created hydrothermal vents (Bradley 2015). The intense heat radiating from the earth’s core slowly turned rocks on the mountain into clay over millions of years of erosion grinding them up. These aluminum-rich clays then created the substance “kyanite quartzite” after metamorphism, a resistant mineral suitable for high-temperature ceramics. First, the Native American Catawba tribe utilized these rich clays to create their traditional method of pottery. Many years later, the kyanite began attracting mining companies due to its mechanical strengthening properties (Bradley 2015). Unfortunately, the mining practices grew increasingly destructive to the integrity of the monadnocks, leading conservation groups to protect the region as a state park in 1973 (Bradley 2015).
Today, the landscape we observe in the Piedmont exists due to the geological processes occurring over 100 million years ago. Due to their strong and resilient rock formations, the monadnocks survived enormous periods of erosion and weathering. Without these unique and ancient rock configurations, Crowders Mountain and the Pinnacle would not exist as they do now.
Works Cited
Bradley, P. (2015). The Geology of Crowders Mountain State Park, North Carolina – A Guide for the Non-Geologist. North Carolina Geological Survey, 1-38. https://carolinageologicalsociety.org/2010s_files/GeologyCrowdersMntStatePark.pdf
Fowler-Billings, K. (1949). Geology of the Monadnock Region of New Hampshire. Geological Society of America Bulletin, 60(8), 1249-1280. https://moodle.davidson.edu/pluginfile.php/377162/mod_resource/content/0/MonadnockPaper.pdf
Tompkins, R. (2013). Prairie-Relict Communities of a Piedmont Monadnock. Castanea, 78(3), 185-197. Retrieved December 7, 2020, from http://www.jstor.org/stable/24619733