A Pleistocene Ecosystem
by Wesley Gordon
page 7


The Pink Granite Problem 

A few of the water worn Miocene shells have been found in the sediments of the lower hills. This means that some of the Irvington sediments were produced by erosion of the older Mission Peak strata. But the source of the other Irvington rocks rounded pebbles and cobbles of pink and granite, for example has not been determined. That is, no primary source for these rocks is known in the San Francisco Bay area.

Wes Gordon pointing out cross-bedding structures at Bell Quarry

There are exposed granite about 60 miles south-southeast of Irvington that could have provided the pink pebbles. The material could have been carried the 60-mile distance and deposited at Irvington by waters flowing north. There is strong evidence to support this possibility: First, Irvington deposits are cross-bedded. “Crossbedded” means that small structures within the sedimentary layers are inclined at various angels. The angles are measured from the horizontal line. The angels help determine the direction of flow of the waters which deposited the sediments. If sediments are deposited by northwesterly-flowing waters, they exhibit cross-bedding inclined toward the northwest. This is the condition at Irvington. As stated earlier, Irvington sedimentary beds dip 20 degree to 25 degrees northeast because of faulting, This dipping took place after the cross-bedding had been established.

There, is, however, another possible source for the pink granite pebbles, as well as for other varieties of Irvington rocks. They could have come from old conglomerates (consolidated gravels) that lie along the base of an extension of Mission Peak about 12 miles south-southeast. The argument against this possibility is that most of the rocks in the fossil-bearing Irvington hills are much too rounded and polished to have been carried such a short distance. An explanation for the physical condition of the rocks one that could support the 12 mile hypothesis is that they were shaped before they became part of the bed of conglomerate. (The geologists would say these rocks might have been shaped during a “previous cycle of erosion.”) More accurately, the rolling trip for 12 miles to final deposition at Irvington would have merely added to their roundness and polish, rather than having been wholly responsible for it.

View of sediments in the Bell Quarry.  Irvington fault on South Creek back of T2 1961

Compared with the Miocene deposits of Mission Peak and the Pleistocene deposits at Irvington, the pebbles that compose the conglomerates are indeed ancient. They were deposited about 130 million years ago. But many of the pebbles and cobbles at Irvington the chert and quartzites, for example are thought to be even older. This means that they probably were shaped during a much earlier cycle of erosion. The extreme hardness of these particular rocks probably enabled them to survive many cycles of burial and reburial.

Obviously, fossils cannot be the only concern of a paleontologist (or pale ecologist). The reconstruction of an ancient ecosystem requires knowledge of physical as well as of biological factors. Knowing the source of rocks in a bed of sediment containing fossils can lead to knowledge about where the ancient streams aids in reconstructing the extinct landscapes, and reconstructing the landscapes helps to determine the environment of plants and animals that lived at that time.

How would you proceed to develop methods for solving the problem of the pink granites?


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