Patriot's Rock EarthCache

The Battle of Setauket was a significant event during the American Revolutionary War, which took place in Setauket, Long Island. In the early morning hours of August 22, 1777, Patriot forces, led by General Samuel Holden Parsons, crossed from Connecticut to nearby Cranes Neck to launch a bold attack on the Loyalist fortification. Fighting alongside the raiding party were Setauket residents Caleb Brewster, Zachariah Greene and Jonathan Dickerson. The Patriot forces established their fighting position at the Rock. For hours the air was filled with the thunder and smoke of both musket and cannon fire. Finally, with neither side bowed and knowing British reinforcements would soon arrive, General Parsons ordered his forces to stop firing and to return to their whaleboats. As the raiding party sailed back to Connecticut, each American Patriot knew, that although they did not dislodge the British on this day, they let local residents know they were not abandoned. The Patriots demonstrated the fighting strength that would someday bring both freedom and democracy to a new nation.

Geologically, Patriot's Rock is a glacial erratic. Glacial erratics are rocks that were transported by glaciers and deposited in areas far from their original source. During the last Ice Age, massive glaciers advanced and retreated across the landscape, picking up rocks of all sizes and carrying them along with the flow of the ice. As glaciers melted and receded, these rocks were left behind in their new locations. These rocks are often different in size, shape, and composition from the rocks found in the surrounding area, making them easily identifiable as glacial erratics. Many of the glacial erratics that are found on Long Island are part of the moraine deposits. These boulders are most abundant on the terminal moraines, which is an accumulation of glacial till that was carried by glaciers or ice sheets, although many occur in the thin ground moraine layer deposited on the surface north of the terminal moraines as the ice melted.

To figure out where the glacial erratic likely originated from, it is important to understand the glacial history of the area. The map above shows the limit of glacial advance. As you can see, the furthest south that the glaciers extended was roughly the center of Long Island. That is visible by a terminal moraine. As you can see in the map, the glacier traveled south, so it is likely that the glacial erratic erratic was picked up somewhere in Connecticut and transported to this location where it currently rests. I reached out to the Long Island Geolgists, which is run by Stony Brook University Department of Geosciences and they confirmed that the glacial erratic did originate in Connecticut.

We can further narrow down the location where this glacial erratic originated by studying its composition and comparing it to the bedrock found throughout Connecticut. There are three main types of rocks: igneous, sedimentary, and metamorphic. Igneous rocks are formed from cooled and solidified magma or lava, and can be identified by their interlocking crystals and glassy texture. Some examples of igneous rocks include granite, basalt, and obsidian. Sedimentary rocks are formed from the accumulation and cementation of small particles such as sand, clay, and organic matter. These rocks often have visible layers or strata and may contain fossils. Examples of sedimentary rocks include sandstone, shale, and limestone. Metamorphic rocks are formed from the transformation of existing rocks through heat, pressure, and chemical changes. They often have a banded or striped appearance and can be identified by their distorted or stretched texture. Examples of metamorphic rocks include marble, slate, and gneiss. The image above shows a generalized bedrock geology map of Connecticut. The tan colored area in the center of Connecticut is sedimentary rock and the red areas mixed in are igneous intrusions. The areas to the east of the pink line are metamorphic rock. The area to the west of the sedimentary rocks in the central part of the state are metamorphic rock as well, and the dark pink areas near the northwest part of the state are igneous intrusions.

1. Examine Patriot's Rock. Describe the color, texture, and grain size. Of the three main types of rocks listed (igneous, sedimentary, and metamorphic), which do you think it is, and why?
2. Using the generalized bedrock geology map of Connecticut, which part of the state do you think this glacial erratic originated in and why?
3. Glaciers move on average between 0.1 to 2 meters per day. Assuming that the glacier traveled at 1 meter/day, which is roughly 0.22 miles/year, how long do you think it was from when the boulder was first picked up to when it got deposited here? The Long Island Sound is roughly 20 miles wide in this area, and you can use the scale found on the map.
4. Examine the ground at the base of Patriot's Rock. Does it appear to rest of bedrock, or sandy soil?
5. Upload a photo with Patriot's Rock. You don't have to be in the photo, though it is strongly encouraged.