Welcome to my Earthcache! An Earthcache is a special type of geocache where you won’t find a container or logbook. Instead, your goal is to learn about a unique geological feature at this location. To claim a find, you’ll make observations, answer questions, and take a photo at the site.
The goal of this Earthcache is to teach you about gneiss, a type of metamorphic rock that forms deep within the Earth (that likely made its way here from Canada during the last ice ice). You’ll learn how it differs from other rocks, what causes its banded appearance, and how it forms under extreme heat and pressure.
All of the observations for this EarthCache can be made from the nearby trail - there is no need to leave the trail and trample the nearby vegetation.
EarthCache Requirements
As with all of my EarthCaches, I’m not looking for a scientific paper or a PhD-level answer — just take some time to look closely, think about what you’re seeing, and learn something new. If you are caching with friends or family, please send one set of answers that includes all caching names. There’s no need for separate messages. You can log your find once you’ve sent your answers and included the required photo in your log. You don’t need to wait for a response unless there’s a problem.
Observational Task
Along the Scioto River, you'll find a glacial erratic made of a unique type of granite called GNEISS - this erratic has likely been here for the better part of 15,000 years! You'll need to look at the composition and crystal formation within this erratic to answer the questions below:
Questions to Answer
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Look closely at the erratic. What textures, patterns, or colors do you see? Do you notice light and dark bands or stripes (a sign of gneiss)? Or does it have a shiny, flaky texture (more like schist)? Describe the minerals you can see. Are there small, sparkly crystals, or larger, interlocking ones? What colors stand out?
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Using your observations and information below, do you think this is a good example of a gneiss? Why or why not?
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Glacial erratics are transported by ice. Where do you think this rock came from originally — nearby Ohio bedrock or farther north? Have you seen any other rocks of this type in the area, or is most of the surrounding bedrock limestone?
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Mandatory Photo: Include a photo of yourself, your GPS, or a personal item with the erratic.You do not need to show your face in the photo, but your photo must be unique to you. If you are caching with a group you can use the same photo, but each log must upload a photo.
The Geology of Gneiss
What is a Glacial Erratic?
A glacial erratic is a rock that was carried far from its original location by glaciers during the Ice Age. When the glaciers melted about 12,000 to 15,000 years ago, they left behind these “out-of-place” boulders scattered across the landscape.
Ohio was once covered by a thick sheet of ice more than a mile deep, flowing south from the Canadian Shield — an ancient area of hard crystalline rock in northern Canada and Ontario. As the glacier moved, it scraped up pieces of bedrock and carried them for hundreds of miles. When the ice melted, these rocks were dropped, sometimes in fields, forests, or — like this one — along riverbanks.
If you find a boulder made of granite, gneiss, or schist in central Ohio, it probably didn’t form here. Instead, it’s a traveler from Canada, deposited by a melting glacier long ago!
What is Gneiss?
Gneiss (pronounced “nice”) is a metamorphic rock — meaning it formed when an older rock was changed by heat and pressure deep underground. It often begins as granite, but when that granite is squeezed and heated over millions of years, its minerals are rearranged into visible bands or stripes.
Gneiss is usually made up of light-colored minerals like quartz and feldspar, and darker minerals like biotite or hornblende. The alternating layers of these minerals give gneiss its distinctive striped or banded look.
Because gneiss forms under very high pressures and temperatures, it’s tough and resistant to weathering — which is why it can survive long journeys inside glaciers!
This rock is formed under pressures ranging from 2 to 15 kbar, sometimes even more, and temperatures over 300 °C (572 °F). Gneiss nearly always shows a banded texture characterized by alternating darker and lighter colored bands and without a distinct cleavage. In other words, it is a metamorphic rock composed of mineral grains easily seen with the naked eye, which form obvious compositional layers, but which has only a weak tendency to fracture along these layers
- How it forms:
- Gneiss forms when rocks are heated and cooled, causing changes to the chemical composition of the rock
- It needs a lot of pressure and heat to change, and may be changed by single indicents (such as volcanos) or over many millions of years
How It Differs from Schist and Granite
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Granite is an igneous rock, formed when molten magma cools slowly underground. It has large, interlocking crystals but no banding.
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Schist is a metamorphic rock formed from shale or mudstone. It’s shiny and flaky, with minerals like mica that align in thin layers.
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Gneiss, in contrast, is more solid and shows distinct light and dark bands. It forms under higher pressure and heat than schist, and it doesn’t split easily.
Gneissic Banding
The minerals in gneiss are arranged into layers that appear as bands in cross section. The darker bands have relatively more mafic minerals (such as magnesium and iron). The lighter bands contain relatively more felsic minerals (such as feldspar or quartz, which contain more of the lighter elements, such as aluminium, sodium, and potassium).
The banding is developed at high temperature when the rock is more strongly compressed in one direction than in other directions (nonhydrostatic stress). The bands develop perpendicular to the direction of greatest compression, also called the shortening direction.
A common cause of nonhydrodynamic stress is the subjection of the protolith (the original rock material that undergoes metamorphism) to extreme shearing force, a sliding force similar to the pushing of the top of a deck of cards in one direction, and the bottom of the deck in the other direction. These forces stretch out the rock like a plastic, and the original material is spread out into sheets.
Some banding is formed from original rock material (protolith) that is subjected to extreme temperature and pressure and is composed of alternating layers of sandstone (lighter) and shale (darker), which is metamorphosed into bands of quartzite and mica.
AI Content Disclosure
Some of the descriptive text and/or images on this page were created with the assistance of artificial intelligence tools. All information has been reviewed, verified, and edited by the cache owner for accuracy and clarity.
This cache was placed by a PROUD Platinum Earthcache Master.
