I thought this link may be of interest to anyone thinking of a Geology major. Provided is a link to SDSU course requirements. As you can see, a lot of math, two or more semesters of chemistry, and a couple physics. So HS students would be advised to prep on these courses, but even if you have not, this won't be a roadblock, it only helps. Requirements will vary from one UC to another, but this should give a you a rough idea.
http://coursecat.sdsu.edu/catalog/GEOL.pdf