Having steep sides on a pond?

Hi,

I'm in the process of building a pond and am currently digging the
hole. A side view of what I have dug is below (I hope the formatting
is comes out OK):

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So basically, it's 2 metres/8 feet wide, 2 metres/8 feet long, half at
a depth of 0.8 metres/2.5 feet and the other half with a depth of 0.4
metres/1.2 feet. The pond's edges are dug at roughly an 80 degree
angle.

My questions:

1. Will the steep sides be a problem and cause too much pressure on
the pond liner? The sides are a sandy/soily material while the bottom
of both the shallow and deep ends are sandstone. I'm planning to get
the Firestone 45mil pond liner.

2. With the bottom of the pond being sandstone, will this cause any
problems? How thick a layer of sand would you recommend lining the
hole with before putting in the pond liner?

Thanks in advance,

Han.