Editing Box Tunnel (section)

===Geology===
While a tunnel had been included in the 1835 Great Western Railway Act, contemporary engineers considered the construction of Box Tunnel to be an impossibility at worst and a dangerous undertaking at best. The challenge posed was not only its length but the difficult underlying strata it would have to pass through. The rocks through which it passes comprise [[Great Oolite]] overlying [[fuller's earth]], and [[Inferior Oolite]] and [[Bridport Sand Formation|Bridport Sand]] beneath, a combination with which tunnellers were familiar.

The Great Oolite limestone, known as ''[[Bath Stone]]'', is easily worked and had been used for construction since [[Roman Britain|Roman]] times. In the 17th and 18th centuries it was extracted by the [[room and pillar]] method and used for many buildings in [[Bath, Somerset]].<ref>{{cite web |title=Combe Down Stone Mines Land Stabilisation Project |work=BANES |url=http://www.bathnes.gov.uk/BathNES/councilinformation/areainformation/combedownstonemines/default.htm |access-date=13 July 2006 |archive-url=https://web.archive.org/web/20060117002821/http://www.bathnes.gov.uk/BathNES/councilinformation/areainformation/combedownstonemines/default.htm |archive-date=17 January 2006 |url-status=dead }}</ref>
To assess the geology more accurately, between 1836 and 1837, Brunel sank eight shafts at intervals along the tunnel's projected alignment.<ref name=NetRailBox>{{cite web |url=http://www.networkrail.co.uk/VirtualArchive/box-tunnel/ |title=Box Tunnel |publisher=[[Network Rail]] |access-date=2 March 2013 |archive-url=https://web.archive.org/web/20120719071845/http://www.networkrail.co.uk/VirtualArchive/box-tunnel/ |archive-date=19 July 2012 |url-status=dead }}</ref>