dealing with soft (caving in) edges of a pond

My pond is about 8' by 20' and up to 5' deep, but it suffers from soft
edges.
The soil is very loose and soft and so my edges keep crumbling. This makes
putting edging stone around the pond a real challenge.

Any suggestions?

I've thought about putting a ring of concrete around the edge of the pond
and then putting the slate edging stones on top of the concrete. I'm
thinking I'd need to put a layer 2-3 inches thick and have it extend about
two feet from the edge of the pond out into the yard so as to create a
stable edge.

Suggestions welcome. I have a nice big pond with a wonderful ecosystem _in_
it...it just doesn't look all the great in the yard. Thanks.

Brian Tarbox
in new england