Interpretation

Facts do not speak for themselves they must be interpreted. Each scientist is obligated to interpret his facts (data), though two or more scientist may use the same data to arrive at different conclusions. For example, two explanations have been given for the origin of San Francisco Bay, and each explanation is based on identical data.

An overview of the Irvington Site with Mission Peak in the background (1950).

The following account of geologic events at Irvington is an interpretation of Savage’s data. How well do you follow his reasoning? Can you visualize the changing scenes?

1. Deposition of the mid-Pleistocene clay, sand, gravel, and bones of mammals and of other forms of life by streams or by the wandering branches of one stream flowing from the south.
2. Lowering of the elevation of the drainage basin, which reduced the speed of running waters, resulting in the deposition of finer sediments.
3. Mild earth movements which tilted the beds 20 degrees to 25 degrees northeastward.
4. Lowering of the entire Irvington area to base level by the stream or streams that previously deposited the beds.
5. Movement along the Hayward fault, causing the beds on the west, toward the present Bay, to be lowered at least 300 feet.
6. Building up of Mission Creek, resulting from the erosion of the earlier beds the mid-Pleistocene beds by the increasing stream flow. Beginning of stream erosion into the fault scarp.
7. Lowering to base level and filling in of the San Francisco Bay plain. Continued erosion of the fault scarp by the small streams that began their work in stage 6.
8. Later movements along the Hayward Fault, perhaps both vertical and horizontal are causing small faults in the Irvington beds.
9. Small present-day streams continue to remove sediments from the Irvington hills and deposit them on the plain to the west. The streams are aided in this process by man, who removes sediments with machinery.