What I see around here a lot, not sure if I have any pics on hand, is rock that has a lot of more or less parallel lines in it. This is rock that has been laid down in layers and then compressed into hard rock. Now over a few millions of years, these get broken up and tumbled all over the place and set at all sorts of odd angles. Sometimes some of those layers are softer than others, and water gets into them and erodes away the soft part, and then more water gets in and freezes, causing cracks. Which get wider and wider over time.