Monument MPM1557 - Medieval cistern, Oyster Street

The remains of a medieval fresh water cistern were revealed during excavations in Oyster Street in 1968-71. The cistern underwent several phases of construction during the 13th/14th and 15th centuries until it became redundant and was backfilled. First phase consisted of a sheer-sided rectangular pit 4m long x 3m wide. Sides of the pit revetted with tree-trunk piling and branches, ends of the piles driven 5m below the contemporary ground surface. This phase was loosely dated to the late-14th century, but it may have been later in date. It may have been an independent structure, or may have formed part of the construction pit for the two later phases. The second phase was the lowest of the three structures in the archaeological record. Rough construction. Heavy oak framework 3m square x 2m deep, set c. 7m into the clay from the contemporary ground level. Frame timbers had an outside cladding of horizontal planks. A few upright pegs and interwoven twigs belonging to a sparse wattle arrangement found at the base. Fragments of early-mid 16th century German salt-glazed stoneware recovered from the lower silt. It was not clear whether this represented a separate construction in use between the first and final phases, or whether it formed the foundation for the latter phase mounted squarely above. The final phase formed the upper part of the structue. Sturdy interlocking framework of timbers 3.5m x 3m x 2m deep, clad with heavy planks of uniform size. Upper frame covered by planks with c.10cm gaps to allow the water to rise and fall. Centre left open, probably to allow water to be drawn. The cistern had been backfilled with loose gravel when it fell out of use. A few sherds of mid-16th century local coarseware pottery and German salt-glazed stoneware were recovered from the fill. A well (W4) was built into the backfill of the cistern during the 16th century. A wooden barrel liner, 87cm high, was found at the bottom of the shaft. Sherds of local coarseware pottery and a piece of Iberian oil jar were recovered from the primary fill of the well shaft. The well was sealed during the late-16th to early-17th century. [1, p46-50]