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
Whilton Locks who were brought in to advise. Low level headings were cut to drain the main workings of water, and the tunnel opened on 25 March 1805, the last part of the canal to be completed. The Grand Junction Canal became part of the Grand Union Canal in 1929, following amalgamation with the Regents Canal, the Warwick and Birmingham Canal and the Warwich and Napton Canal. Below the bottom lock was a long level pound stretching to beyond Blisworth Tunnel. The seven locks raised the level of the canal by to a fairly short summit pound, which passed through Braunston Tunnel