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
Southampton Tunnel partial construction of the Salisbury and Southampton Canal tunnel, which the railway tunnel crosses at an acute angle and slightly higher. Because the canal tunnel was lined with clay "puddle" it collects water which ends up in the rail tunnel making the latter very wet. In the 1960s an access was cut into the canal tunnel in order to examine it, although this was later sealed behind a new segmental wall structure when, between 1983 and 1985, the tunnel underwent extensive engineering works and the line through it was worked as a single line for long periods while remedial work