Seismic sections in North Sea Blocks 28/25, 28/30, 29/21, 29/26 and 29/27 on the northern flank of the Mid North Sea High show many mound-like features in an interval just above the base of the Zechstein. The top of the interval showing these anomalies is defined by a strong, widely traceable seismic reflection beneath thick salt. The mounds have a relief of up to 120 ms TWT and are up to 4 km across. Superficially, they have the appearance of build-ups in the carbonates of Z1 or Z2, possibly reefs. However, examination of all the anomalies reveals features which are not easily explicable by the reef hypothesis. In a few, for instance, the reflector defining their top is broken and one side is markedly displaced vertically from the other. Virtually all are situated below former pillows or diapirs in the overlying salt, and many are associated with faults that cut the Rotliegend. These facts, together with the low position of a unit interpreted as the basinal Plattendolomit in all nearby wells, suggest that the mounds might consist of swells in otherwise thin Z2 evaporites, sandwiched between basinal (i.e. condensed) representatives of the Z3 Plattendolomit, and the carbonates and anhydrites of the first two cycles. The abandoned Amerada Hess Well 29/27-1, drilled on one such mound, supports this explanation. The heart of the mound, which has no hydrocarbon potential, contains the low density magnesium chloride mineral tachyhydrite (rare in marine evaporites) rather than porous carbonates. This result downgrades similar anomalies as drilling objectives. The evaporite swellings appear to have formed in response to whatever factor initiated movement in the overlying salt; an association with basement faults is possible. Known igneous activity nearby may have been a factor, either in mobilizing the evaporites or stimulating their diagenesis. The absence in this area of thick Z2 halite such as is usual in the Southern Salt Basin is characteristic of much of the Northern Salt Basin; it appears to be a depositional feature related to regional differences in subsidence, possibly linked to an early phase of rifting in the Central Graben. The place of the Z2 halite, as the chief active component forming salt structures, is taken north of the Mid North Sea High by thick Z3 salts.