What are you getting the other half for Xmas?

[Griswold]

That's a well-demarcated topsoil layer.

Close to the sea??

The topsoil was brought in when the houses were built, the area used to be a sand quarry. About 60 miles from the sea!

The sand is incredibly compacted, so much so that drainage is very slow!

Here's the finished pond 10' x 7' x 2' deep