How can we know how the ground's elevation has changed?

V

Viktor Bout

New member
There's an image of land subsidence in San Jaoquin, California, that shows that the ground used to be 30 feet higher than it is now. We've sucked away so much of the underground water that used to help support the land that the land itself is sinking.

Here's the picture, from the US Geological Survey: http://gallery.usgs.gov/photos/09_15_2010_b2Vi84Kxx6_09_15_2010_0

How can we know how high the ground used to be?
 
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