You can put a variety of materials under a retaining wall, including gravel, sand, crushed stone, or even bricks. The main purpose of the material is to help stabilize the wall and prevent it from eroding.

Retaining walls are often used to hold soil in place, so the most common material to put under a retaining wall is more soil. However, you can also use rocks or other materials. If you use rocks, make sure they are stable and won't move over time. You can also use concrete, asphalt, or other hard surfaces. Make sure the surface is level and firm before installing your retaining wall.