How do you create and import custom terrain brushes?

I am having a lot of fun creating maps for DreamCarRacing 3D that mostly involves importing height maps and terrain sculpting and painting.

I would like to create a few custom brushes for height editing that would create a series of smooth indentations and ridges.

I have not been able to find any documentation for brushes like the default ones in the terrain height edit tools.

Can anyone give me the specifics or point to a good resource that explains them.