The water at the Pawtucket Falls is a mix of freshwater from the Blackstone River and the brackish waters of Narragansett Bay. The Pawtucket Falls is the last waterfall on the Blackstone River, and below the falls, the river becomes the brackish Seekonk River due to tidal influence from the bay.
The Blackstone River Valley was formed by glacial action during the last Ice Age, when a massive glacier carved a deep, U-shaped valley. As this glacier receded, meltwater created the Blackstone River and deposited sediments that would later form rocks.
- Glacial carving: The immense weight and movement of the glacier scoured the landscape, carving out the U-shaped valley where the Blackstone River now flows.
- Glacial erratics: Large, isolated boulders, sometimes of different rock types than the underlying bedrock, were plucked by the glacier and deposited "erratically" across the landscape as the ice melted. The retreating glacier left behind large boulders, known as glacial erratics, that can still be found throughout the Blackstone Valley.
- Glacial till: This is an unsorted mixture of clay, silt, sand, and large rocks deposited by the glacier.
- Ancient bedrock: The glacial action stripped away soft, younger sediments and exposed ancient quartzite bedrock that is over 1.5 billion years old.
- Polished bedrock: The continuous, concentrated flow of water over the dam, along with the passage of rocks and debris, has scoured and polished the underlying black bedrock.
- Rock Formation: Over time, these sediments were compressed and turned into sedimentary rocks, such as sandstones, which were deposited in areas with faster-moving water.
- Waterfall carving: As the glacier melted, the immense volume of rushing water flowed over the newly exposed, durable bedrock. Over thousands of years, this water carved its way down, creating the distinct waterfalls that define the area.
The Pawtucket Falls are created by the Blackstone River flowing over a resistant outcrop of metamorphic rock belonging to the Blackstone Group. The specific rock unit is likely an undifferentiated metasedimentary rock, which includes various types of gneisses and schists.
Key characteristics of the bedrock at Pawtucket Falls:
Highly resistant metamorphic rock: The rock of the Blackstone Group is extremely dense and durable, making it resistant to erosion. This is why a waterfall exists at this specific location, as the river has a harder time eroding it away compared to the surrounding terrain.
Origin as sediment: The rock in the Blackstone Group was originally formed from sediments like muds and sand on the floor of an ancient sea over 600 million years ago.
Metamorphosed over time: These sediments were then deeply buried and subjected to immense heat and pressure during geologic events that formed the Avalonian terrane, a large block of continental crust in the region. This metamorphism transformed the sedimentary layers into hard rock.
Gneiss and schist: The specific rock type is described as containing different types of gneisses and schists, which are characteristic of the Blackstone Group. These rocks are exposed at the surface today due to tectonic uplift and erosion.
To get credit for finding this Earthcache message with answers to the following questions:
1. What evidence do you see of the glacier receding?
2. Why are the large rocks so black in color?
3. What type of rock is found here and how did the waterfall itself form while these rocks did not wash away?
4. Post a picture of yourself or geocaching item with the falls in the background.
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