Geology
Composed primarily of Eocene sandstone and shale, much of the island also has marine terrace deposits of Pleistocene age, indicating that it was probably completely submerged at that time. The entire western part of the island is covered with reddish-brown eolian dune deposits laid down during the early Holocene. In some places these deposits are more than 10 meters deep. Small quantities of volcanic rocks (primarily andesite) exist on the southeast end of the island.
Stone available to natives for tool making on San Nicolas Island was largely limited to metavolcanic (including porphyritic metavolcanic) and metasedimentary (mainly quartzite) rock. The metavolcanics are found in the form of cobbles within conglomerates and cobble-bearing mudstones. This material is dense and not easily workable.
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