If you have followed my writings for a while, you probably know that one can use fluid equations to describe the movement of mountains on geological timescales.
If you haven’t, then yes, I just wrote that.
However, the fluid model loses its usefulness when we want to look at stones falling down the mountainside and staying put somewhere down below. The problem: Time scales. Things happen too fast, and pieces of the fluid break away.
I just wrote that, too. 🙂