hmm not sure what elevation interval you been looking at on topo maps, but they will definitely show a 50ft drop, crevace maybe not. although you would have to be reading your map to see it, and may then miss it. my gps has the topo maps with 3m intervals, so if ya know how to read the thing you should see the drop off, again a guy may miss it, a white out is a dangerous situation to be in. i bent a brake lever in half in one, i'll sit and wait next time.
To be honest with you Im not that great with my GPS...lol Dont use it that much. But if you can spot a 50ft cliff with yours then wow thats pretty impressive.