I have an established pond that has been around for more than 15 years. The dam is a straight 90 degree drop down to the water and over the last 2 years larger chunks of dirt/clay have fallen causing the reducing the buffer of dirt/clay I have for my dam. If this continue in certain areas the dam will be completely be gone. My dam is pretty lengthy (maybe 75 yards). On top my dam I have grass established but it is not holding the dirt/clay from falling. Although there is a 90 degree drop to the water the water follows roughly a 3/1 drop.

Now to my question. I believe my best course of action is to bring in some stone to line to dam, but I'd like to get your take on which stone is the best to use and also which ones to avoid. I have heard that certain limestones become very mossy which is something I would like to avoid. Thoughts?

I know in this case pictures would help so I'll work on trying get those.