Recent sand dunes cover the Colma Formation and the Franciscan Complex over large areas of San Francisco. These dunes are composed of sand that has blown up and over the hills from Ocean Beach and Baker Beach.
The sand originated on the broad coastal plain of the Sacramento/San Joaquin River system, which extended from the Golden Gate to the Farallon Islands during the last glacial period, when sea level was as much as 300 feet lower than present. Sea level rose rapidly between 10,000 and 5,000 years ago, transporting the sand from this plain onto the beaches, from where it then blew over the coastal hills.
Last updated: February 28, 2015