How to get the character to walk along an uneven surface, like sand dunes

I am playing with the “first person examples map” and I added a Sand dune terrain geo, so the ground is no longer flat, instead, it is uneven like sand dunes. It appears by default the collision is seen as a bounding box, so my character walks along the plane of the bounding box. Instead, I want the character to walk along the uneven surface. Can anyone advise how I can do this?