Geology: Difference between revisions
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The rock that composed the walls of the quarry was limestone. I knew this going in. What this stop gave us the chance to see was how limestone formed and how its position can change over time. What inferences we can make about the past environment and the geological history of the area was apparent in the quarry. So what are they? Because the rock is limestone, one can infer that the area was once a shallow sea. This is because limestone forms from the calcium-based shells of the organisms that float in the shallow, sunlit waters not far off coasts. The rock at the quarry may also contain the fossils of reef organisms. Although none were visible in the rock walls, fossils of the sort would further support the thesis that the area formed in a marine environment. | The rock that composed the walls of the quarry was limestone. I knew this going in. What this stop gave us the chance to see was how limestone formed and how its position can change over time. What inferences we can make about the past environment and the geological history of the area was apparent in the quarry. So what are they? Because the rock is limestone, one can infer that the area was once a shallow sea. This is because limestone forms from the calcium-based shells of the organisms that float in the shallow, sunlit waters not far off coasts. The rock at the quarry may also contain the fossils of reef organisms. Although none were visible in the rock walls, fossils of the sort would further support the thesis that the area formed in a marine environment. | ||
The limestone formed in layers | The limestone formed in layers as dead marine organisms sunk to the sea floor and accumulated. This would mean that the layers of limestone should appear in a vertical orientation. The layers at the quarry were tilted, however, indicating that an event had to have taken place during the history of the area to move the limestone. A more extensive study of the area would likely offer an answer. | ||
===Stop Three: Road Cut on Long Gap Rd.=== | ===Stop Three: Road Cut on Long Gap Rd.=== | ||
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====Interpretations==== | ====Interpretations==== | ||
The rock encountered in this stop | The rock encountered in this stop was likely shale, based on the texture of the rock, its pattern of breaking, and the presence of layers. This outcrop likely formed in a marine environment like the neighboring limestone in the College Quarry. Unlike the limestone, however, shale is wholly a clastic sedimentary rock: It formed from detritus, ground-up loose pieces of rock. The shale formed in deeper water, where darkness curtailed the presence of shell-toting life forms and calcium-based corals. Shale formed from debris in the water that accumulated over time on the sea floor. Logic would dictate that shale should have vertical bands, and indeed some examples of shale outcrops are vertically oriented. The shale at this outcrop was turned nearly ninety degrees, however, indicating a geologic event in the past that put it in its current position. | ||
===Stop Four: Waggoner’s Gap=== | ===Stop Four: Waggoner’s Gap=== | ||
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====Interpretations==== | ====Interpretations==== | ||
The rock at this stop is well-worn quartz sandstone. This is apparent from the grain size of the rock and its neutral reaction to hydrochloric acid. Quartz is also a mineral most resistant to weathering, and makes a logical | The rock at this stop is well-worn quartz sandstone. This is apparent from the grain size of the rock and its neutral reaction to hydrochloric acid. Quartz is also a mineral most resistant to weathering, and makes a logical mineral type for the composition of rock at the top of a mountain. How does the quartz sandstone of Waggoner’s Gap fit in with the geologic picture of the Carlisle area illustrated by the prior stops for this lab? As I mentioned in the observation section, banding was apparent in the rock. This banding is not all vertical, but tilted in different directions. This is indicative of a phenomenon called cross-beading. Cross-bedding occurs in dunes, where layers of sand blow over each other. These layers can harden into rock. The cross-bedding in the rocks of Waggoner’s Gap indicate that the area may once have been ancient sand dunes, perhaps even shoreline. | ||
Latest revision as of 15:15, 3 October 2007
Geology 103
North Mountain Field Lab
For Geology 103's first field lab, the class traveled to three different sites in the Carlisle area. The purpose of the lab was to develop the classes skills of observation and then using those skills to piece together the geologic history of the Carlisle area. The four stops the class visited are listed below, along with the observations I took and interpretations I made from the observations and from discussion.
Stop One: Outside of Kaufman Building
Observations
Light gray, splotches of darker gray material Deep cracks in surface of rock, banding, black rocks sticking out from light gray rock. Black Rocks: Layered, glassy, not banded. Veins run along surface of gray rock; white, rusty color.
Interpretations
The rock I viewed outside of Kaufman Hall was likely massive limestone. I did not see any grains upon looking at the texture of the rock. This led me to conclude that the rock is composed of a fine arrangement of minerals. Although I did not get a chance to test the rock with any hydrochloric acid, I did note that the gray color of the rock and the banding apparent in the rock were consistent with examples of limestone I had seen prior. I also used a process of elimination to come to my conclusion, ruling out the possibility that the rock was any other type of chemical sedimentary rock. For example, I was able to immediately rule out the likelihood the rock was coal because the rock was not glassy and black. The veins that lined the surface of the rock also led me to believe it was limestone after I learned the veins are calcite precipitate.
The black rocks that protruded from the surface of the gray rock I learned were chert. This rock was less susceptible to weathering than the limestone parent rock. Its glassy texture and dark color, and also my conversation with the professor, informed me of the rock’s identity.
Stop Two: College Quarry
Observations
Dark gray rock with bands, flakes at places, white veins, rust colors in veins. Bands are tilted in downward and eastward direction.
Interpretations
The rock that composed the walls of the quarry was limestone. I knew this going in. What this stop gave us the chance to see was how limestone formed and how its position can change over time. What inferences we can make about the past environment and the geological history of the area was apparent in the quarry. So what are they? Because the rock is limestone, one can infer that the area was once a shallow sea. This is because limestone forms from the calcium-based shells of the organisms that float in the shallow, sunlit waters not far off coasts. The rock at the quarry may also contain the fossils of reef organisms. Although none were visible in the rock walls, fossils of the sort would further support the thesis that the area formed in a marine environment.
The limestone formed in layers as dead marine organisms sunk to the sea floor and accumulated. This would mean that the layers of limestone should appear in a vertical orientation. The layers at the quarry were tilted, however, indicating that an event had to have taken place during the history of the area to move the limestone. A more extensive study of the area would likely offer an answer.
Stop Three: Road Cut on Long Gap Rd.
Observations
Rock mostly dark brown, reddish color. The rock does not indicate any coarse grains. Rock breaks in plates; flakes. Some rock appears to be more resistant to breaking than the vast majority of the rock composing the outcrop. Rock is layered, though layers are perpendicular to ground.
Interpretations
The rock encountered in this stop was likely shale, based on the texture of the rock, its pattern of breaking, and the presence of layers. This outcrop likely formed in a marine environment like the neighboring limestone in the College Quarry. Unlike the limestone, however, shale is wholly a clastic sedimentary rock: It formed from detritus, ground-up loose pieces of rock. The shale formed in deeper water, where darkness curtailed the presence of shell-toting life forms and calcium-based corals. Shale formed from debris in the water that accumulated over time on the sea floor. Logic would dictate that shale should have vertical bands, and indeed some examples of shale outcrops are vertically oriented. The shale at this outcrop was turned nearly ninety degrees, however, indicating a geologic event in the past that put it in its current position.
Stop Four: Waggoner’s Gap
Observations
Pale orange to pink. White also prevalent. Texture: Medium-grain, crystalline. Appear to be quartz crystals. Banding is apparent.
Interpretations
The rock at this stop is well-worn quartz sandstone. This is apparent from the grain size of the rock and its neutral reaction to hydrochloric acid. Quartz is also a mineral most resistant to weathering, and makes a logical mineral type for the composition of rock at the top of a mountain. How does the quartz sandstone of Waggoner’s Gap fit in with the geologic picture of the Carlisle area illustrated by the prior stops for this lab? As I mentioned in the observation section, banding was apparent in the rock. This banding is not all vertical, but tilted in different directions. This is indicative of a phenomenon called cross-beading. Cross-bedding occurs in dunes, where layers of sand blow over each other. These layers can harden into rock. The cross-bedding in the rocks of Waggoner’s Gap indicate that the area may once have been ancient sand dunes, perhaps even shoreline.
