The first documented use of watermills was in the first century BC and the technology spread quite quickly across the world. Commercial mills were in use in Roman Britain and by the time of the Doomsday Book in the late 11th Century there were more than 6,000 watermills in England. By the 16th Century waterpower was the most important source of motive power in Britain and Europe. The number of watermills probably peaked at more than 20,000 mills by the 19th Century.......
Humankind has not woven the web of life. We are but one thread within it. Whatever we do to the web, we do to ourselves. All things are bound together. All things connect. ~Chief Seattle
Here's a picture of the Higher Bebington windmill, close to Storeton quarry. This was the quarry to the East of Mount Road which was filled with the waste from the Wallasey tunnel. This is what it looks like today.
If you do a search for "Wirral windmills pictures" you'll find loads of pictures, both photos and paintings, although there are some which aren't Wirral.
I suspect that it was back filled with the surplus material excavated from the 2nd bore,the material from the 1st being used in embankments and general fill areas.Bidston Moss had a rather large capacity for absorbing things!!!