This prefab is a lake with rocky walls. The Brushes used are all flat faced.
Prefab available at: http://files.filefront.com/rock+lakezip/;12193256;/fileinfo.html
Screenshots:





Prefab available at: http://files.filefront.com/rock+lakezip/;12193256;/fileinfo.html
In the folder includes 2 map files, one of them is from screenshot 5 and the other is the map for i used to compile the map so that i could take screenshots. Screenshots included in file aswell.

it has to be triangles and not squares