I am making an animation where the character is shimming across a ledge, and the ledge starts to crumble. I would have to delete faces of the ledge to make them crumble. How can I do that?
You could use a Mask modifier if you really want to delete certain faces on a certain frame, however I'm not sure this is really what you want to do.
If you really want to delete a specific face (or faces) on a certain frame, then you can use the Mask modifier.
Select the faces you want to hide and assign them to a vertex group in Properties > Object data > Vertex groups
Add a mask modifier and select your vertex group and enable Invert by clicking the button next to the vertex group selector:
Keyframe the visibility of the modifier by hovering over the eye icon and pressing I.
If you want more variation, try using a reversed Build modifier. This will delete all faces in the object over time, either randomly or in order.
If you are trying to make a more complex/realistic You might also want to try the Cell fracture addon (see this post):
Or the explode modifier.