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.

The first phase consisted of a sheer-sided rectangular pit 4m long x 3m wide. The sides of the pit had been revetted with tree-trunk piling and branches, the ends of the piles driven into natural clay 5m below the contemporary ground surface. This phase was loosely dated to the late-14th century, but it may have been later in date. This may have been an independent structure, or may have formed part of the construction pit for the two later phases.

The second phase, the lowest of the three structures in the archaeological record, was a rough construction, possibly built in a hurry. It consisted of a heavy oak framework 3m square x 2m deep, set c. 7m into the clay from the contemporary ground level. The frame timbers had an outside cladding of horizontal planks. A few surviving upright pegs and interwoven twigs belonging to a sparse wattle arrangement were found at the base. Fragments of early-mid 16th century German salt-glazed stoneware were recovered from the lower silt. It was not clear whether this represented an intermediate construction, or whether it formed the foundation for the latter phase which had been mounted squarely above.

The final phase formed the upper part of the structure, and was a sturdy interlocking framework of specially cut and prepared timbers measuring 3.5m x 3m x 2m deep, clad with heavy planks of uniform size. The upper frame had been covered by planks with c.10cm gaps to allow the water to rise and fall. The centre had been left open, probably in order 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.

[1, p46-50]

<1> Fox, R & Barton, K, 1986, Excavations at Oyster Street, Portsmouth, Hampshire, 1968-71 (Bibliographic reference). SPM34.