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
Chirk Tunnel was let in January 1796, and its construction was completed in 1801. The of canal eastwards to Frankton was opened immediately, because of the valuable trade in limestone from the Fron Quarries. The section through the tunnel, which also passed through Whitehouse Tunnel to reach Pontcysyllte was opened in June 1802. The tunnel is claimed to be the first in Britain to have a towpath running through it (along with its shorter neighbour - Whitehouse Tunnel). However, Berwick Tunnel on the Shrewsbury Canal was open by 1797 and included a towpath throughout its length. As the tunnel at Chirk is