I have a small pond on my property and recently I had access to a backhoe so I decided that I would do some cleanup in it while it was dry. I came across some layers of soil that were white and when dug up it would break into chunks. Any idea what this might have been. It is not limestone to my knowledge. It was almost like a mold of some kind.

It could be chalk which is a type of limestone and is often found around other limestone deposits. See these links. Most states have geologic maps of surface soils and deposits by location.

http://en.wikipedia.org/wiki/Chalk

http://en.wikipedia.org/wiki/Chalk_Formation

Might be an old layer of bentonite.

Mercury,

We have the same thing, I think. I found some of it when renovating an old pond. I believe it is limestone in a "cake" form. There is a vein of this stuff nearby which the County digs and uses for our road surface. It's good stuff if that is what you have.