The Mohegan Bluffs are located on the south shore of Block Island and considered by many to be the most beautiful spot on the island. The 200 foot tall Mohegan Bluffs harbor a rocky, secluded beach. The bluff is composed of a combination of clay and glacial till and offer one of the most dramatic views of the Atlantic in all of Rhode Island. On a clear day, you can see Montauk, the tip of Long Island. The Mohegan Bluffs got their name from a Native American battle that took place in 1590, where 40 members of a Mohegan raiding party were said to have been driven off the cliffs to their deaths by the Manisseans, the victorious natives that lived on the island.
The three main types of particles that make up soil are sand, silt, and clay. You may be wondering, how can you observe soil here if you are standing on a beach? Soil is defined as the layers of weathered particles of earth material that contain organic matter and can support vegetation. While it may not be obvious at first, sand is a type of soil. Sand, silt, and clay are distinguished by the size of their particles, clay is the smallest (<0.002 mm), next is silt (0.05 to 0.002mm), and then there is sand (2 to 0.05 mm). Anything larger than sand for the purpose of this earthcache we can consider gravel. An easy way to distinguish them is that sandy soils feel gritty when rubbed between your fingers. Silts feel smooth and a little like flour. Most clays are sticky and moldable. Where the ground water has a high concentration of iron, the clay appears red. The clays of the southern end of the island tend to the darker, bluer part of the spectrum. The clays of Clay Head are lighter; in some places, white outcroppings of ancient cretaceous clay can be seen.
The most dramatic natural force at work here is weathering and erosion. Erosion is defined as the movement of pieces of soil and rock. It can be caused by: gravity, wind, rain, rivers, oceans, and glaciers. Gravitational erosion transports rocks and material downhill due to gravity. Wind erosion takes place when the wind carries tiny particles of dust and sand, and over the course of thousands of years, the sand slowly breaks up whatever surfaces it encounters. Rain works to erode the land on which it falls through rain-splash erosion, which is rain splashing down on the land and dislodging weathered material such as pebbles or soil. Rivers can cause erosion since when there is a bend, the water flows slightly faster on the outside bend and the banks will be eroded. Even something as simple as waves breaking on a beach can move particles of sand and rock up and down the beach resulting in erosion. Glaciers are also worth mentioning since Block Island was formed by glaciers! As glaciers move, they pick up whatever material is in their way and can transport it for many miles!
Weathering, in contrast, is the process by which rocks, soil, and minerals are broken down through contact with Earth's atmosphere, water, and biologic resources. There are three types of weathering, physical, chemical, and biological. Physical weathering is primarily caused through frost wedging, when water seeps into cracks within the rock, and when the water freezes, it expands. This process continues until eventually a piece of the rock breaks away. Another example of physical weathering is exfoliation. This occurs when cracks develop parallel to land surfaces, and it almost looks like "layers" of the rock are being broken apart. Chemical weathering takes place when rain water reacts with minerals within the rock. In most cases, the water is slightly acidic. Lastly, biological weathering takes place when tree roots grow in between cracks within rocks, and as the root grows, it breaks apart the rock. Algae and lichen that live on rock may also produce chemicals that break down the rock so they can obtain the nutrients needed to survive.
All the observations can be made from the beach.
Logging Requirements:
- What type of soil predominantly makes up the base of the bluffs? Based on your observations, what are some of its properties?
- You may notice that the upper section of the bluffs appear to be eroding at a faster rate. What type of soil is it composed of? Why do you think this soil erodes faster than the one found at the base of the bluffs?
- Are you able to see any evidence of weathering or erosion here? How do you think this will shape the landscape in the future?
- Upload a photo taken from the beach at the base of the Mohegan Bluffs. You do not need to be in the photo, though it is strongly encouraged. Additional photos from the overlook at the top are also welcomed!