crevice

隙間(すきま)	A crack between two different objects. Between the cabinet and the wall. Between two planks in a log cabin.

裂目(さけめ)	A crack inside of an object:a cracked brick or a rift in a glacier. Your butt. Also, typically すきま is used of 2-dimensional objects, while さけめ is used about 3-dimensional ones.