Sunday, July 7, 2013

Day 15: In Archaeology, Geology, Rocks!

Day 15- Geology and Rocks on Site at Monticello

My apologies, due to the rain and the hectic schedule of the last couple of weeks, my blog updates have fallen behind. But I can quickly update you on the lectures and experiences of the field school- it has been unbelievable!

The first thing you need to know about geology, which may seem simple, but often gets over looked- rocks vary regionally. The second step to understanding geology is being knowledgeable about the Wilson cycle. The Wilson cycle refers to theopening and closing of ocean basins caused by movement of the Earth's plates. The Wilson cycle begins with a rising plume of magma and the thinning of the overlying crust. As the crust continues to thin due to extensional tectonic forces, an ocean basin forms and sediments accumulate along its margins. Subsequently subduction is initiated on one of the ocean basin's margins and the ocean basin closes up. When the crust begins to thin again, another cycle begins. 

In the specific case of the field school, Monticello is located in the Blue Ridge Mountains of Virginia, which have eroded into the coastal plain. A long time ago, when ancient Africa and North American collided, Atlantic fills with lava flows forming black basalt build ups, forming the mountains, the mountains then erode, island chains slams in, thus causing a continual series of mountain forming events. This black basalt build-up was then squashed with heat and pressure and morphed into greenstone. The Atlantic Ocean then erodes these mountains to form modern-day Virginia. The lava continues to build, spills out, and forms tiny payers between basalt, forming beach dunes and various landscapes. Silt stones formed together with heat and pressure to produce quartz. This information is important due to the fact that greenstone and quartz are the most common rocks found at Site 8 on the dig at Monticello. Quartz is also resistant to weathering and the most common and resistant mineral on the planet. It is capable of surviving chemical weathering since it does not oxidize at all.

The teaching assistant who gave this lecture, Devin Floyd, explained to us that rocks in archaeology make up everything that came before the people and the buildings and the history. He also explained that we, as archaeologists need to question what is and is not a rock artifact, and we can answer this question with another question: Has the rock been altered (or worked) by humans? This is how archaeologists know if the rock fragment is an artifact which needs to be kept or simply thrown away.

At Monticello quartz sand has formed into quartz and mineral massive quartz cannot be tested or replicated. Seems and cracks form in the parent material, then groundwater passes through, and silica accumulates and fills the space with quartz crystals, the parent material decomposes, leaving the quartz vein to grow. Along with quartz deposits, Goethite, named after the German polymath Johann Wolfgang von Goethe, has been found. The mineral was used for devil’s dice (the mineral breaks in perfect cubes) and was used to make game pieces and toys at Monticello.

Back to knowing whether or not a piece of quartz has been worked or not, quartz has similar properties as glass, it creates sharp edges when broken and was primarily used for projectile points (weapons, knives, etc.) for prehistoric peoples. Local quartz found on Monticello have lots of heat fractures and small pieces were worked til thick and blunt before they break with concloidal fractures (cone of energy breakage), and the bulb in the rock can tell us if it was struck or worked in any manner to be cut down and used. Slate and soapstone are also very important artifacts which can be found at Monticello. Slate gives off a high-pitched ring if struck and we used in chimney chinking and soapstone is metamorphic and as its surface is refined, it turns black (steatite) and can then be used as gun flints which have been found on slave sites. This is a key piece of evidence which has recently been discovered by archaeologists and has led to the interpretation that the slaves on Monticello (and other sites where the steatite has been found) had access to guns- most likely for hunting. These are the important developments in archaeology which geology has led to and one of the many reasons it is an important aspect of the historical record. 

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