The second background process needed to understand geology is plate tectonics. The theory posits that the Earth’s surface is broken up into a system of plates and these shallow lithic rafts float on a molten mantle. Slowly, through convection processes, the plates move around, coalescing, subducting, scraping, and bumping into one another. This process gives rise to volcanoes, mountains, earthquakes, and even the continents and oceans themselves. Plate tectonics is the driver that creates and erodes rocks to form igneous, sedimentary, and metamorphic rock types. Plate tectonics is the primary factor creating the Blue Ridge, the physiographic provinces, and the landscape of Virginia more broadly. With a very basic geological background behind us, we can look at the specific events that created Oak Spring. Oak Spring’s features have large geologic controls – plate tectonics, volcanism, mountain building, and erosion.
Granite and Greenstone Oak Spring contains two primary rock types. Both are found over a broad regional area, but crop out at Oak Spring in unpredictable ways (J. Sawyer, personal communication). The older rock(s) is a metagranite or granitic gneiss. There are several specific rock types regionally described as a metamorphosed granite, or granitic gneiss. Without special analysis it’s not possible to know which of the lot occurs at Oak Spring (and there could be more than one). Nonetheless, all of the regional metagranites are similarly aged at 1.1 billion years. Despite their age, and the intense conditions following two periods of continental collision
causing recrystallization, when seen on the ground or in a stone wall at Oak Spring, the rocks still look like brightly colored granite. Sometimes the rock has bands showing its metamorphic history. Oak Spring’s second rock type is a 570 million year old metadiabase, commonly called greenstone. It’s the more common rock found in outcrops and stone walls at Oak Spring, but the less common rock regionally. Diabase is a volcanic rock compositionally similar to basalt. But because it cooled more slowly, the crystals and texture are coarser. At Oak Spring and the Loudoun Valley, diabase occurs as dikes, or veins intruded through the older granitic rocks. The diabase has been slightly metamorphosed resulting in metadiabase. A fine-scale geological map has not been produced for Fauquier County; however, an excellent, highly detailed map has been done for Loudoun County (Southworth et al. 2006), just 1.5 miles (2.4km) north of Oak Spring. Below is a clip from that map with Oak Spring geo-referenced to its true position slightly south to give an idea of the geological diversity and complexity of the local landscape. Dominant rock type descriptions are included. Considering what is found on the ground, it is expected that the geology at Oak Spring follows this pattern. However, Oak Spring seems to have broader diabase dikes than occur farther north.
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