Threshold concepts in geology - the poll

I'm curious how many people agree with some of the threshold concepts in geology suggested by commenters (and by members of the UK group). So I figured out how to add a poll to Blogger. Please vote - I'm curious how many people have similar experiences.

(Edit: I've borrowed the idea of "threshold concepts" from a Journal of Geoscience Ed article, and it seems as though the idea has been floating around other science education communities for some time. Perhaps such things don't exist, and there are simply things that some students find difficult. But it they do exist, they might help me choose which things to spend more time on.)