Just over half of Coos Bay's 12,380 acres consist of tidal flats. These areas, exposed to air during low tide, are composed of mud, silt, sand, and clay. Content of organic matter varies, but can be high, and sediments are compacted in some places. The North Spit mainly deflation plain and beach sand with areas of both unstable and stable sand. Where natural or introduced vegetation thrives, the sand has been stabilized.
Sediments there are mostly alluvial deposits of sand, silt, clay, and mud.
Most of the sediment deposits in the estuary are from the Holocene or Recent Epoch, which dates from 10,000 years ago to present. Much of what borders the bay along the south shore, from North Bend to the ocean, however, is made up of ancient marine deposits of sand, silt, and gravel, from the Pleistocene Epoch, dating back two million years. On the east side of the bay, along East Bay Drive, are older deposits of deltaic sandstone, siltstone, and beds of coal, covered by sandy or silty loam or loamy sand. Human activity also greatly affects geological processes in the bay. Manmade structures, for example, can alter or impede the flow of fresh or salt water, or can influence the way water circulates through the estuary and how sediments are deposited.