One of the best ways to really start understanding the geology of an area is to look at the geologic map in Google Earth. Of course, unless you are interested in San Francisco or some other top notch place, you will not find the geologic map available in kmz or kml format (although you can get the whole US geological map here, and they are working on covering most of the globe).
Before that happens however, with a little patience it is possible to draw your own maps in Google Earth. You can use an image overlay as a starting point, and draw polygons on it after you managed to position it properly. As a quick test (well, actually it took me almost one day to do it), I created a small map that covers part of the southern East Carpathians in Romania, an area where I did some work for my thesis. It is based on the Geologic Map of Romania, 1:50000 scale, that is, one sheet from the series, edited by Murgeanu et al. and published in 1968. Old stuff, but good stuff. And a lot of work.