Geology of the Berry Islands:
The Berry Islands, including Great Stirrup Cay, are part of the Bahamian carbonate platform, a geologic system dominated by shallow, warm marine environments that promote the formation of limestone. These islands are composed primarily of carbonate rocks, especially limestone formed from the accumulation of calcium carbonate derived from marine organisms such as corals, mollusks, and algae. Over time, wave action, biological activity, and chemical processes break down these materials into fine carbonate sediments, which form the white sandy beaches characteristic of the region. In addition to skeletal fragments, some sediments are oolitic, meaning they are small, spherical grains formed by the precipitation of calcium carbonate around a nucleus in shallow, agitated water. The lack of major river systems in the Bahamas limits the input of terrigenous (land-derived) sediments, making biogenic carbonate processes the dominant force shaping the islands’ geology and coastal environments.
Biogenic vs. Terrigenous Sand:
Beach sand can originate from different sources, broadly categorized as biogenic or lithogenic. Biogenic sand is derived from biological material, including coral fragments, shells, and other skeletal remains of marine organisms, which have been broken down into smaller particles. These grains are typically composed of calcium carbonate. In contrast, terrigenous sand originates from the physical and chemical weathering of rocks on land, such as volcanic or sedimentary rocks, and is transported to the coast by wind, water, or gravity. Terrigenous grains are usually mineral-based (such as quartz or feldspar) and may differ in color, density, and texture from biogenic material.
Logging Tasks:
- Walk along the shoreline and closely examine the sediment that washes up. Look at the grains in your hand or near the waterline. Do you observe mostly mineral grains (such as clear or glassy quartz, or angular feldspar)? Or do you see coral and shell fragments?
- Describe the dominant sediment type and any variation you notice across the beach. Based on your observations at the GZ and the information provided above, determine whether the beach is biogenic, terrigenous (lithogenic), or a mixture of both.
- Are there signs of erosion on Great Stirrup Cay? Describe what you see.
- As of June 2019, earthcaches contain required photo logging tasks. Please provide a photo of yourself, your GPSr, or a personal item that proves that you have visited this site. Please post this in your log.
Sources:
https://onlinelibrary.wiley.com/doi/10.1111/j.1365-3091.2010.01155.x
https://sandatlas.org/biogenic-sand/
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