I used "rico rock" on mine, and although they are "stone" looking, I did not want the traditional boulder look, rico rock has a pre- made waterfall that is more along the lines of a "ledge", this is what I used, the good thing is that it is light weight, yet strong enough to stand on and dive off of, I as well extended the overhang to allow for the "cave" behind the sheet of water. Also from what I understand, the company will custom form whatever you shape you desire:
http://www.ricorock.com/
Here is my website with pictures and movies (not for dial up!)
http://web.mac.com/gunkel04/iweb/Site1/Pool%202.html
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