Geology (cont. 10)


The land bounced upwards frequently because of movement along the San Andreas, Rose Canyon, and other faults, raising old beaches high above sea level. Today, the old beaches lie elevated above the present beach. We can see them from the trail along the lagoon. They are the flat tops of the surrounding hills, and look like gigantic steps down to the present beach. They can be seen many miles inland: the “Mesas” of San Diego are actually old wave-cut terraces. They have a reddish sediment deposited on them. The red color is an iron oxide (rust). This means the sediment was deposited in very shallow water where oxygen was plentiful, and the iron in some of the minerals in the sediment could combine with oxygen, making iron oxides, or rust. (Iron does not rust much in deep water, because there is little free oxygen available). The youngest terraces are close to the ocean, and are the lowest.


Page 10

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