Caved in caves

Simple idea: in a cave, a "caved in" cave would spawn about as rarely as geodes

Have cobblestone or gravel (or both) fill a cave for 10-20 blocks, with a piled effect on the end (aka they aren't just big straight walls of cobblestone). 

This could apply to only small caves, but maybe also to large ones too.

You could also have a different block other than cobblestone fill caves in the deepslate area.