u/Dull_Usual3916

What type of material would be used to repair the gap between the rocks in this waterfall? You can probably see the crack between the levels.

It looks like something is sagging or broken but in reality the cement or mortar or whatever in between them is what finally deteriorated leaving this gap.

What kind of material would this have been in order to remain waterproof and look decent?

Currently water is leaking through here gradually and presumably going into the ground.