There are several different ways to do this. It looks like you have three objects here - Terrain grass, a sidewalk plane and a street plane. You can clip the grass terrain slightly in the terrain editor (Clip min) to add some jagginess to the edges of the terrain against the sidewalk. For the sidewalk to street edge you could add a cylinder object and map it with a curb or stone texture. If you're feeling really ambitious you create the whole ground terrain including the sidewalk and the curb as a single terrain (or perhaps boolean them together) and then uses a distribution map with feathered edges to drive the location of the materials