The medieval and modern city of Worcester lies on the east bank of the Severn, on a gravel terrace overlying the Keuper Marl at a point where the river, meandering within the 500-metre-wide floodplain, cuts into the terrace, making its banks directly accessible from the high, well-drained ground. Another broader terrace lies beyond the alluvium on the west bank and is the site of the transpontine suburb of St John’s. The city site itself is a naturally defensible south-facing promontory defined by the river on the west and the Frog Brook to the east. The brook, now canalized and culverted, joined the Severn at Diglis about 500 metres south of the cathedral, and was flanked by its own narrow belt of alluvium. The medieval (and earlier) High Street and Foregate Street follow the north–south spine of the promontory, rising gradually from about 23.8 metres above Ordnance Datum on the northern city boundary to a peak of about 25.9 metres AOD around St Helen’s church, falling gently southwards towards the cathedral before dropping sharply down to the Frog Brook alluvium at Diglis.