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*cracks knuckles* I'm a geolgist so I'll try to give you some links and tips.
www.terraserver.microsoft.com Has topo maps that are free to look through, you'll need at least a town to be able to find the location.
Google "Whichever State (ie, Oklahoma, New York, wherever you are) + geologic map" and you might turn up a geologic map that way.
Google "About + geology + State" and it will pull up a list of the state geologic maps. Be warned, not all states go into great detail on what they will release on the internet. Some states I can get right down to the nitty gritty, others...not so much.
If you have a university near you, check if they have a geology department/professor. They might be willing to let you look at their department maps.
Try the state geologic survey. Google "State + geologic survey". You might have to pay for maps this way or they may have a map in the office you can go look at.
Those should at least get you started on tracking down the geology, so long as you know what Formation you are targetting. Now, if you are expecting to find a map that says "Find this rock, gold here!", that isn't going to happen. The map will list the formation, not what is in it.