When you refer to the lake in Tatton Park I know you
mean the one by the Knutsford Gate known as Tatton Mere
but there are actually two different lakes separated by the estate
Tatton Mere began life as a moderately deep valley after the ice
sheets retreated from Cheshire. A river ran down the valley and
was blocked at some time in the distant past to form a mere.
This was then also enhanced by landscaping in the park and to create
a water supply for mills further along the water course at some
point, probably in Medieval times.
The raised dyke that runs along the lakeside contains reused building
debris which may indicate that it was constructed by William Egerton
in 1791 when he demolished the village of Tatton Green which stood
opposite the Old Hall and undertook major landscaping. Finds from
the bank point to this period for its construction. It is a flood
barrier and probably relates in some way to the lower lake (Melchett
Mere) across the road opposite.
Legend has it that this lower lake was formed in fairly recent times
by a massive underground collapse of some kind. One day park land
the next a lake! This has happened elsewhere due to mining
activities and natural subsidence and this type of lake is often
termed bottomless. The collapse which formed the lower
lake probably related to water eroding the underlying Cheshire salt
deposits over thousands of years.
In reality I do not know the depth of either lake, but nearby Rostherne
is the deepest mere in Cheshire at over 160ft., and those at Tatton
probably not much shallower than this as they are basically part
of the same chain.
I hope this answers your questions.