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
Blisworth Tunnel backs pushing the boats with their feet (legging). From 1871 steam tugs were used to pull boats through, and extra ventilation shafts were installed. Boaters wishing to avoid the extra cost involved in using the tugs could still leg their boats through, but by the 1930s motor boats (towing a butty boat) were common so the practice would have been unusual especially by the time the film Painted Boats was filmed there in 1944. Due to changes in the shape of the tunnel over time, the tunnel became unnavigable. There was major rebuilding of the tunnel in the 1980s, with