A student spreads modeling clay in a flat, rectangular tray. He presses small pebbles and nails into the clay and puts the tray in the freezer. In a similar tray, he pours wax and allows it to harden. After a while, he takes the clay out of the freezer and places it on top of the wax. When he applies pressure with his palm and drags the clay over the top of the wax, deep impressions are made. Which geologic process is the student trying to model?