The Daly City Dunes

The last remnant of an ancient dune formation 

iron-stained sands of the Colma Formation with cascading deerweed (Lotus scoparius) © 2012 SBMW

Most people who live in the San Francisco Bay Area are familiar with the coastal dunes along the Great Highway at Ocean Beach and Fort Funston. They may even be aware of the dunes in the Presidio and Baker Beach at the northernmost part of the peninsula.  What you see there are surviving fragments of a once vast coastal dune system on the northern San Francisco Peninsula – now covered with homes and lost forever to development decades ago. These Holocene Sand Dunes were most likely formed from sand brought down from the Sierra by the Sacramento/San Joaquin River system, forming a broad coastal plain. However, these coastal dunes are youngsters - formed during a period a mere 18,000 and 5,000 years ago.

Chamisso Bush Lupine (Lupinus chamissonis) © 2012 SBMWOn the western end of San Bruno Mountain you can find much more ancient dunes.  The Daly City Dunes in Hillside Park, on the lower slopes of Reservoir Hill, loom above the houses on Bonnie Street. These are the remaining remnants of truly ancient dunes that were formed from 80,000 to 125,000 years ago as part of the Pleistocene Colma Formation. This deposition occurred during an interglacial period when the sea level was higher than today. This higher water level from melted glaciers made the northern San Francisco Peninsula an island - separated from the southern peninsula by a narrow stretch of water connecting the ocean to the bay.

When ocean waters reached the base of an ancient San Bruno Mountain, the deposit of material from several environments, over thousands of years, created the sands of the Colma Formation. Contributions from shallow tidal lagoons and silt from valley slopes gave this sand more soil-like properties and its characteristic iron-stained brownish coloring, contrasted to the gray sands of the coastal dunes. The unique sand of the Daly City Dunes now hosts the last of a diversified and rare inland dune plant community (see Dune Plants).