FRANCISCAN MELANGE

Just above the comparatively orderly graywackes and shales at Ring Mountain's base, a highly disrupted mass of rocks, large and small, lie embedded in a crushed, fine-grained material. Known as a melange (from the French word for "mixture"), this jumble is clearly a product of the grinding and mixing of rocks, and perhaps the result of a continent and ocean floor collision like that drilled by the Ocean Drilling Program during Leg 146.

Melange landscapes in Marin contain a wide variety of rock types which formed before faulting brought them together. Greywacke, chert, serpentinite and greenstone. . . are typical of the rocks found in the melanges of Marin.

Additionally, the Ring Mountain melange contains blueschist, which is featured in the next page and is a rare and somewhat perplexing mineral which is formed under extremely high pressure and a relatively low temperature. These high pressure but low temperature conditions are also necessary to the formation of jade and lawsonite, other rare minerals found in Franciscan melanges. Geologists believe that these minerals formed when the Franciscan sediments were carried between 9 to 18 miles down into the subduction zone where they remained for perhaps 100 million years until the subduction action ceased. How these minerals could have remained sufficiently cool at such depths for so long a period is unknown.

The sheared and crushed material of the melange is weak and easily eroded. At the same time, large outcroppings of erosionally-resistant rock masses stand monument-like, projecting out of otherwise smooth, grassy slopes (photo), a common landscape of Marin and shown here near Lake Nicasio. These erosionally-resistant rock masses tend to be massive greywackes or cherts.

Because the crushed material is so quick to erode, it is difficult to find an intact unit to view except where the land has been recently dissected by a road cut. The Highway 101 cut between Mill Valley and Corte Madera shows a relatively recent roadcut through a melange unit, while the cut on east side of Highway 101 between central San Rafael and the Civic Center shows a much older cut, where there has been substantial erosion of the loose material and the resistant masses are quite marked.

Erosion is further hastened by the manner in which melange weathers: its clay-rich expansive soils swell up when wet and shrink upon drying. Seasonal wetting and drying can lead to slow yet persistent downslope movements, while saturation by very heavy rainfall can lead to landslides. Another important characteristic of the melange: after the first few rains of the season, the wet, swollen clays of the melange form a relatively impermeable barrier, resulting in increased runoff. However, if the melange lies beneath a fractured and thus permeable rock, it allows the rock to retain considerable amounts of water. Consequently, large rock masses, particularly when located at the crests of ridges, act as natural water storage tanks. They serve as the source for many of the springs located high on Marin County's ridges.

The serpentine of Ring Mountain, which is described on a susequent discussion, lies on top of a melange unit and the contact can be seen in this aerial view of the mountain, just above Endeavor Cove Drive in the foreground. Ground water lubricates the serpentine landslide around the bend of the cul-de-sac and can be seen draining from the base of the slide near the crest of Ring Mountain (photo).

Such springs can be seen at several sites at Ring Mountain where they might last for months after the last spring rains and can even persist throughout the long dry summers of Marin County.


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