This could be any natural surface such as rock, sand, or dust.

What outdoor sport isn't better on a natural surface?

They also state that 'the natural surface shall not be significantly disturbed.'

The maximum height of the earthen dam is 56 feet above the natural surface.

These are sitting on a wooden surface, much better light, natural, I think.

He is, however, just learning how to field on a natural surface.

Pittsburgh's ownership stated that the decision was up to the players, who once again defended the natural surface.

The rest of the trail is natural surface used primarily in the winter months for snowmobiling.

The form is constructed using often unusual materials which are generally left with natural surfaces.

During the 2011-12 football season the natural surface underwent significant wear and damage.