The Pleistocene epoch, which is more commonly referred to as the Ice Age, actually consisted of several periods of advancing and then receding ice. During the several major glacial periods, sea levels dropped all over the world and the San Francisco Bay existed as the river valley. The combined Sacramento and San Joaquin rivers which flowed through it, joined the sea around the Farallon Islands, some 15 miles outside the present day Golden Gate. Mastodons, giant ground sloths and other life forms uncommon in the county today and more like those preserved in Los Angeles' La Brea Tar Pits (mural, 158K) grazed on the margins of the river flowing through the valleys now occupied by San Francisco Bay.
When the ice began to melt, some 18,000 years ago, the river valley and tributaries were flooded. It is said that when the Spaniards first came to California in the 1600-1700's, the local native mythology included reference to a great flood which drowned the lands of the ancestors outside the Golden Gate.
By approximately 6,000 years ago, as the bay waters lapped at the edges of Ring Mountain, sediments carried by the major rivers and the smaller tributary streams began to build at its base. The large meadow just inside and to the left of the Corte Madera entrance gate to the Ring Mountain Preserve was an inlet early on.
The muddy sediments which have built up at the base of the mountain appear solid enough. Developers have figured all they need to do is add a little dirt fill and they have prime building sites (see Bay Area Land Use Information Page). Unfortunately, that has not proved to be the case as seen in this photo of an apartment complex where the level of the water in the pool differs from one end to another and the slab upon which the recreation building sits is now elevated above the pool deck. Here's a developer temporarily out of action and showing just how soft these sediments really are (photo). One certainly needs to be aware of the potentials of damage to structures and loss of life that can occur from amplified shaking in similar types of ground. This amplification was dramatically demonstrated in both our own San Andreas system's Loma Prieta quake (October 17, 1989) and the comparable Kobe, Japan earthquake (January 17, 1995 ).
While the lowlands have their problems, landslides are a common upland problem in Marin County as illustrated by landsliding activity that covers part of a cul-de-sac carved into the Serpentine covered ridge of Corte Madera (1980 aerial view) and the 1997 observations of a housing development being impacted by landsliding on the slopes of Mt. Burdell.. A listing of the Burdell landslide photos. It should be noted that neither of these were triggered by earthquakes.
Material continues to slide and wash down from the slopes of Ring Mountain; fine sediments from many sources continue to build at its base, and, if undisturbed, will fill in the Bay in a short time geologically. The material that is so unstable now may eventually turn to stone, only to be eroded away to some other locations. The cycle continues.