Hi, we've just finished the open day tours for Aberdeen, Glasgow, Edinburgh and Bristol for Geology/ Earth Sciences, they all said that it is very difficult to get geology related work experience and so they don't expect it on a ps. Perhaps put down your other work experience to show that you're motivated and not just sitting around doing nothing :-)
Oh and wider reading - The Map that changed the World by Winchester (specifically mentioned by Aberdeen lecturer), The Planet in a Pebble by Zalasiewicz.
The head of Bristol earth sciences said basically not to fret too much about the PS, they would be less concerned about it than the applicant, it's all about getting good grades and not wasting precious revising hours on a PS!.