Model For Formation Of Natural Bridge

This is an amateur geologist's idea of how the natural bridge could have formed. If you know of a more plausible scenario, please let me know.

Original cross-section through natural bridge area, looking along the bridge. The cross-section is through the middle of the bridge.

A weak rock layer is found under alluvium and a strong rock layer. A strong rock layer under the weak rock layer is not shown.

The weak rock layer bears water, and creates a spring where it meets the surface at bottom left.

Some erosion due to the spring has now moved the surface back to expose the strong rock layer.
Two possible scenarios:

1. Water flowing through the weak rock layer has now removed the top of the weak rock layer to the point to the right of the strong rock layer.

2. Humans desiring more water from the spring "tunnel" the spring, breaking through to the alluvium.

In either case, with the alluvium in contact with the water table, erosion proceeds quickly and a cavity is formed in the alluvium.

The cavity expands rapidly, either due to seep of water through the alluvium from above, or from flow toward the spring from below.
The cavity has now broken through to the surface, and stream runoff now enters from above as well.
The stream runoff from above erodes the uphill side rapidly. The downhill side of the cavity erodes at a lesser rate, leaving a more vertical wall.
Two possible scenarios, corresponding to the two cases above:

1. The water level now recedes, perhaps because the climate of the San Gabriel Mountains has become drier in the last few centuries or millennia. The recession leaves a tunnel through the rock, supported by the strong rock layer on either side of this cross-section. The gray rock is the weak rock layer without water.

2. No recession of the water level is necessary, if humans drilled the tunnel into the alluvium. However, if the water level had been higher before the tunnel was dug, the current water level would be lower above the bridge due to the creation of the tunnel by humans.

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Copyright © 1999 by Tom Chester.
Permission is freely granted to reproduce any or all of this page as long as credit is given to me at this source:
Comments and feedback: Tom Chester
Updated 8 December 1999.