The geologic story of the rock types and their geologic history was determined through many years of research by many investigators in the Piedmont of North Carolina and the immediate area surrounding Prairie Ridge. This story combines many geologic investigations conducted within the Raleigh area and from other locations throughout the Southeast. For a scientific review of the geologic interpretation of the various crystalline rock terranes in the Raleigh area, see Hibbard et al. (2002), Blake et al. (2001), and Stoddard and Blake (1994). Descriptions of the rocks in the immediate vicinity of Prairie Ridge can be found as part of the detailed geologic map of the Raleigh area by Blake (2008).
The Rocks of Prairie Ridge and a Brief Review of their History
The various rock types and the soils that are derived from the rock form the foundation of Prairie Ridge. But this foundation is rarely seen. In the Raleigh area bedrock exposure is primarily restricted to creeks and streams, were the erosion power of water has stripped away feet of soil. The rocks visible in the creek at Prairie Ridge can be generally separated into three rock types: 1) muscovite schist, 2) graphite-bearing schist, and 3) granitic gneiss (Figure 1).
|Figure 1: Map of Prairie Ridge Ecostation with major geologic features indicated.|
Schist and gneiss are metamorphic rock types that formed from the metamorphism (changed through heat and pressure) of pre-existing rock types. Typically schist is the result of the metamorphism of mudstones and siltstones. Gneiss can be formed from the metamorphism of a wide variety of rock types – including sedimentary and igneous rock types. Muscovite (a light-colored mica mineral) and graphite (a very soft mineral composed of carbon) are minerals that are prominent in the schists at Prairie Ridge. The term “granitic” refers to the original rock having been granite (a plutonic igneous rock).
To a geologist these rock types help tell the story of how a major portion of North Carolina formed. The rock that underlie Prairie Ridge and the surrounding areas of Wake County record a long and sometimes violent geologic past of over a half billion years of Earth history. Collectively the rocks of the Raleigh area record the formation of an ancient chain of volcanic islands (called Carolinia by geologists) that were active for millions of years with countless volcanic eruptions that formed piles of volcanic ash and lava flows. During this same period of volcanic activity, erosion wore down the volcanic areas and deposited layers of sedimentary rocks in the form of mudstone, siltstone, and sandstone. At Prairie Ridge, the schists are examples of the metamorphosed mudstones and siltstones that were deposited in this ancient volcanic area. Primitive life, such as mats of algae, grew within and on the mud and silt layers. At Prairie Ridge, the graphite-bearing schists are examples of the metamorphosed mudstones and siltstones that had abundant organic material (likely from algae) deposited in this ancient volcanic area. Also at the same time, deep within the Earth below the volcanoes, magma (molten rock) -that did not make it to the surface to erupt as ash or lava – cooled very slowly and formed plutonic igneous rocks (like granite or other granitic rocks). Over time, multiple generations of volcanoes and magma cut into older volcanic and their related sedimentary deposits. At Prairie Ridge, the granitic gneiss is an example of magma that intruded the mudstones and siltstones after they were buried deeply. The magma cooled slowly and formed granite (a plutonic igneous rock).
Eventually, after many millions of years of volcanic activity, the volcanoes went extinct, and the layers of volcanic ash, lava, and associated sedimentary rocks were buried deep. Very generally, through plate tectonics and many millions of years, Carolinia – the chain of volcanic islands - inched across an ancient ocean and likely collided with another chain of islands that caused the first of several phases of folding and metamorphism. Later, magma from a new period of volcanism started to erupt through the older rocks of the volcanic island arc. This period of volcanism eventually ended, Carolinia continued to move slowly toward ancient North America and eventually after millions of years collided and “welded” itself to the North American continent. This collision caused another period of metamorphism and folding.
Millions of years later the ancient ocean that separated ancient North America from the ancient African continent closed and the continents collided forming the Supercontinent Pangea. This collision again caused metamorphism, folding, and faulting and is last of the metamorphic events to affect the rocks at Prairie Ridge. In the Raleigh area this collision formed a giant geologic structure called an antiform - a type of fold. (An antiform is similar to an anticline but geologists are unsure of the ages of the rocks so it is called an antiform.) Geologic units are repeated from one side of the anticline/antiform to the other. Geologists call this fold the Raleigh antiform (Figure 2).
|Figure 2: Block diagram sketch of the Raleigh antiform in a portion of the Raleigh West Geologic map.|
Important marker units (rock layers) in the Raleigh antiform are schist layers that are composed almost entirely of graphite – called the graphite schist. The graphite-bearing layers exposed in the creek at Prairie Ridge are part of the layers that are related to the graphite schist layers exposed on the opposite limb of the Raleigh antiform (Figure 2). Geologists believe that the various layers of graphite-bearing and graphite schist were originally deposited as organic-rich mudstone and siltstone, as horizontal layers (like typical sedimentary layers), and through folding are now exposed as “thin strips” that are standing on their sides on both sides of the Raleigh antiform (Lumpkin et al., 1994).
Essentially, the layers of graphite-bearing schist in the creek at Prairie Ridge are on their side (nearly vertical in places) because of the folding and they are part of the Raleigh antiform.
Blake, D.E., Clark, T.W., and Heller, M.J., 2001, A temporal view of terranes and structures in the eastern North Carolina Piedmont, in Hoffman, C.W., ed. Field Trip Guidebook for the 50th Annual Meeting of the Southeastern Section, Geological Society of America, Raleigh, North Carolina, p. 149-180.
Blake, D.E., 2008, Geologic map of the Raleigh West 7.5-minute quadrangle, Wake County, North Carolina: North Carolina Geological Survey, Geologic Map Series – 15, scale 1:24,000.
Hibbard, J., Stoddard, E.F., Secor, D., Jr., and Dennis, A., 2002, The Carolina Zone: Overview of Neoproterozoic to early Paleozoic peri-Gondwanan terranes along the eastern flank of the southern Appalachians: Earth Science Reviews, v. 57, n. 3/4, p. 299-339.
Lumpkin, B.L., Stoddard, E.F., Blake, D.E., 1994, The Raleigh graphite schist, in Stoddard, E.F. and Blake, D.E., eds., Geology and Field Trip Guide, Western Flank of the Raleigh Metamorphic Belt, North Carolina, Raleigh, North Carolina Geological Survey, Carolina Geological Society Guidebook for 1994, p. 19-24.
Stoddard, E.F. and Blake, D.E., eds., 1994, Geology and Field Trip Guide, Western Flank of the Raleigh metamorphic belt, North Carolina, Raleigh, North Carolina Geological Survey, Carolina Geological Society Guidebook for 1994, 110 p.