...what to line a pond with but not butyl

[friend of stouby]

 

We complete on our new French home in a week or so and are still at the stage of dreaming before the reality hits. Our first major outdoor project is to dig a pond, about 15 x 10m x 1m max deep.

The soil type is unknown at the moment but have seen some mole hills in the field and it looks like they are pushing up seed compost!

Also there is some form of limestone? rock not too far under the surface in at least one position as we have a large dip in the ground which one side has a rock face about 3 meters high. Whether this is a single piece of rock or not isn't known.

Nearly there....Assuming that there is no clay at about 60 - 100cm down, what is the best material for making a pond  relatively watertight but not using a butyl liner? Importing clay??

I will post this in 'gardening' also as not too sure where this question belongs.

Thanks in advance for any tips, as in this we are not sure where to begin.

Jamie