Aligning sand bottom to glass side of tank?

Hi all. I’m creating a terrarium and ran into this problem: The ground I am using sticks out of the tank when I apply fractal subdivision. I suppose I could just use proportional editing and drag vertices around manually, but the fractal produces nice results. Any tips on keeping the edges aligned to the glass sides or fixing it afterwards?

When you use the subdivide with fractal noise, in the subdivide options change the ‘Along Normal’ value from 0 to 1