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Bolinas: Geo-logic

About 200 million years ago–almost all the land on earth was clustered together in a giant supercontinent that geologists call Pangaea (pronounced pan-g e-a). Surrounding Pangaea was a single ocean covering the rest of the earth. Two hundred million years ago is not long in geologic terms –about 95% of the earth’s history had already happened by then.

At the time of Pangaea, there was no Bolinas Lagoon. None of the land surrounding the lagoon was here either. There was only deep water. This story tells how the land got here over millions of years, much of it from hundreds or thousands of miles away.

The lagoon is very young, geologically. Many things had to happen before it could come into existence. The story begins with plate tectonics: sea floor spreading, moving plates, subduction, and plate collisions. It continues with other processes: rocks being formed and eroded away, earthquakes, sea level changes, land being uplifted and subsiding. The last chapter is about how sand and silt first created the lagoon and then immediately started to destroy it.

The story will unfold in the following steps:

  1. Pangaea breaks apart and the Atlantic Ocean opens
  2. Land coming in from the sea extends the edge of North America
  3. The rocks wear down and create new rocks
  4. The plates start sliding past each other
  5. The Point Reyes peninsula moves north
  6. The rest of Marin County forms a different way
  7. Land moves along the San Andreas fault
  8. Sea level rises and falls
  9. Finally, the Lagoon is formed

All of this activity reduced the amount of estuarine habitat as mud flats changed to salt marsh and salt marsh to meadows. The lagoon is much shallower today than 150 years ago. If sedimentation continues at the current rate, there may not be enough tidal circulation to keep the channel open to the sea. In the absence of a major earthquake to lower the floor of the lagoon, the channel could close intermittently by 2040. The 1960s saw a great burst of development proposals that would have ended any possibility of preserving the lagoon as a rich estuary habitat. The plans included making Highway 1 into a freeway, dredging a small boat marina, and piling fill on Kent Island to create land for hotels, stores, and houses. A four-lane highway was proposed for the top of Bolinas Ridge. These plans were defeated only with great effort by the founders of Audubon Canyon Ranch and many others. The Future Today, many people are working to keep Bolinas Lagoon alive.

source: A SHORT GEOLOGIC HISTORY OF BOLINAS LAGOON (Geologic processes at work on a small coastal lagoon on the San Andreas Fault near San Francisco, California) by Diana King.



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