At the west end is a tall tower with a saddleback roof.
St Mary's parish church was built to replace an earlier church nearby which had been damaged by subsidence, It is constructed in red sandstone in Perpendicular style.
It is the last survivor of a system of seven vents whose purpose was to remove the effluent from chemical manufacture.
Other than the bridges and the lock, the building materials used are brick or local red sandstone.
Listed buildings are categorised in three grades: Grade I consists of buildings of outstanding architectural or historical interest; Grade II* includes particularly significant buildings of more than local interest; Grade II consists of buildings of special architectural or historical interest.
Buildings in England are listed by the Secretary of State for Culture, Media and Sport on recommendations provided by English Heritage, which also determines the grading.
In 1833 a canal and a railway reached the area; the Sankey Canal was extended to a point on the River Mersey to the east of Runcorn Gap and the St Helens and Runcorn Gap Railway established a terminus adjacent to the canal.
Widnes Dock, the world's first railway dock, was established at the new terminal, The town's listed buildings reflect its history.
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Between the chapels is a tower with an octagonal steeple.