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The Sandman Reefs are a group of islets, reefs, and wave washed rocks between the Pavlof Islands and Deer Island in the north, and extending south almost to the Sanak Islands, Alaska. The reefs were named in 1882 after Captain Sandman of the Alaska Commercial Company, by William Healey Dall, of the U.S. Coast and Geodetic Survey. The islands are part of the Alaska Maritime National Wildlife Refuge and important habitat for seabirds such as Ancient Murrelets.
The Sandman Islands are mostly composed of igneous columnar basalts that are particularly strong and resistant to erosion creating steep sea cliffs and reefs. The columns are formed during the cooling of thick lava flows. While a flow can shrink in the vertical dimension without fracturing, it can't easily accommodate shrinking in the horizontal direction unless cracks form. The extensive fracture network that develops results in the formation of columns. The columns are predominantly hexagonal in cross-section, and the size of the columns depends on the rate of cooling.
The sea cliffs erode primarily by thermal expansion of freezing ocean spray and rainwater that seeps into cracks and crevices. The broken rock particles tumble into the sea and accumulate as talus at the base of the sea cliff. Ocean waves and currents act only to remove the finer particles from the debris pile. Read more here and here. Click here to download the CoastView app and explore the Sandman Islands.