Short answer: You want to create a vertex group and put that in for particle density.
If you assign the particle emission to to the land you'll get something like this:
Obviously we don't want the icospheres all over the plane, just a select portion. We can do this we two methods, weight pain or creating a vertex group. Weight pain will give you more control, but I'll show how to assign a vertex group because it's a little bit quicker.
So tab into edit mode. Hit "c" and paint the vertices you wish to be affected.
Now invert the selection with Ctrl "i" and go to vertex groups on the right and assign a group.
Now go back to the particle system, scroll down to vertex groups and select the group. I would recommend naming the group.