Sounds like a spot where an above ground pool was installed, not a
garden... without more information I can't advise what you can/canot
do... but naturally if it's your property you can do whatever you like
so long as it's legal. How large an area are you talking about? How
high above grade is this area? How is this area situated in relation
to the house or other buildings; distance and elevation? A few photos
would help a lot.[/quote]

Depth 1 ft 3 inch
Length 19 ft 4
Width 11 ft 4

The sheet is located approx 4 inches down and you can see it as a white colour on the picture. The sheet has also been found by the shed which is 14 ft 6 away from the corner nearest the property.