Blisworth Limestone Formation today in the seas off the Bahamas (Kirkaldy p. 59). Much of it also contains marine shells, notably of oyster. Subsequently, the site was submerged less deeply so that the overlying Blisworth Clay was deposited in brackish conditions. (Gallois p. 20) The stratum is known as Blisworth Limestone because it was first studied at Blisworth in Northamptonshire when the Blisworth tunnel was being dug for what is now called the Grand Union Canal. The tunnel finally opened in 1805. Blisworth limestone is a generally thin but widely spread deposit in eastern England. It becomes thicker as one progresses southward from
Chesterfield Canal Norwood Tunnel. At the time of construction, Norwood Tunnel was the second longest canal tunnel in Britain, exceeded only by another of Brindley's tunnels, that at Harecastle on the Trent and Mersey Canal. The canal was a typical Brindley contour canal, following the contours to avoid costly cuttings and embankments, which resulted in a less than direct route in places. In late 1775, the company had decided to build a branch, which left the canal between Renishaw and Staveley, and ran generally southwards for about to the turnpike road at Norbriggs. This may have been to help with transport to