How to close the sides and bottom of the heightmapground

i create a ground by heightmap, but the side of the ground is not closed, This doesn’t look very good, is that a way to close the sides?

just like bellow:

I changed the edges of the image to 1px black, and the edges of the ground were closed. However, I want to set the edges to pure color. What should I do

I think you will have to create a new mesh by hand for the border, something like:


Great, it solved my problem. Thank you