Shaft A was created to facilitate the construction of Blea Moor Tunnel.
As mentioned in the following extract from the October 21st, 1871 edition of the Lancaster Guardian, this site was originally intended to be the location of the tunnel's South Portal:
Shaft A, sunk at the proposed entrance to the south end of the tunnel, is 35 yards deep.
The same source provides brief details of the construction and use of this shaft during this early period:
About 100 yards have been driven or tunnelled northwards. The lining of the arch with brickwork varies from 1ft 6 inch to 2ft 3 inch in thickness completed. At this shaft, a 12-inch winding engine is employed, which also works an 8 inch pump and a Blow George to supply the men below with air.
See also 'A change of plan for the south end of Blea Moor Tunnel'.
After construction work had finished, Shaft A was not needed for either access or ventilation purposes, so it was capped. However, decades later, the area around the old shaft began to suffer a significant amount of subsidence, so a fence has been erected to ensure public safety.