Background
This large boulder in Southeastern Park is not local to Indianapolis. The natural bedrock here is mostly limestone and shale, formed in warm, shallow seas about 350 million years ago. Limestone is light gray and can contain fossils, while shale is a softer, gray-blackish color, a layered rock that breaks into thin, flat pieces.
This boulder, however, is different. It contains pink bands of feldspar, gray/white quartz, and dark streaks of biotite. These features identify it as gneiss, a metamorphic rock that forms when granite is squeezed and heated deep within the Earth until its minerals realign into stripes or bands.
Because gneiss is not part of Indiana’s native geology, the only way it got here is by glacial transport. During the last Ice Age, glaciers carried rocks from far to the north and dropped them here when the ice melted. Rocks like this are called glacial erratics — travelers from hundreds of miles away.
Local Bedrock
Shale and limestone are two of the most common bedrock types found in Indiana, and both formed hundreds of millions of years ago when the state was covered by shallow seas. Limestone developed as marine organisms such as corals, brachiopods, and other shelled creatures died and accumulated on the sea floor, their calcium-rich shells and skeletons compacting into solid rock. Shale, on the other hand, is formed from fine mud, clay, and silt that settled in quieter waters, gradually compacting into thin, layered rock.
Simple Rock Terms
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Glacial Erratic: A rock carried and dropped by a glacier, far from its home.
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Feldspar: Pink mineral, often forming stripes.
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Quartz: Clear, white, or gray glassy-looking mineral.
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Biotite/Hornblende: Dark black or brown streaks or flakes.
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Granite: A speckled igneous rock with random minerals (cookie-like).
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Gneiss: A metamorphic rock with minerals arranged in stripes (layered cake-like).
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Shale: Soft, gray sedimentary rock that splits into thin flakes or sheets.
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Limestone: White to gray colors, usually a sandy color, represent a type of sedimentary rock primarily composed of calcium carbonate.
Granite vs. Gneiss – How to Tell Them Apart
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Granite – random, speckled crystals, no stripes (random spots).
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Gneiss – light and dark minerals arranged in stripes (layered).
Logging Tasks
To claim this EarthCache, please answer these three questions at the site:
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Mineral Colors & Features: Describe the colors and minerals you see in the boulder. Can you spot any large bands along with any white/gray or darker minerals?
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Rock Type: Does the boulder look more like granite (speckled, random minerals like a cookie) or gneiss (striped bands like a layered cake)? Explain your choice.
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Local Bedrock Comparison: Compare a limestone or shale to the boulder. How are they different from the boulder?
(Optional: Take a non-spoiler photo of yourself, your GPS, or another item with the rock.)