How can I identify a Mario Bros character (Big gray stone that falls)

I was looking for images of this and can't find because I'm lacking a term.

I would like to know the name (or how would you would call) a character from (that I know of) from the old Mario's (NES and such).

I'm talking about the big grey rock with a bad/mean face on it, that stood still until you were near and fell quickly.

Do you know what I'm talking about? The normal google images searches you would expect to work are not giving me any good results.

Thanks in advance!

Please comment if the question is not clear enough or would like any clarifications.

Ah, you must mean Thwomp.

Thwomp has a number of incarnations through old Mario games, and he's more than adequately covered on the Super Mario Wiki. Here's one example from Super Mario World though: