A complex overlap of middle Old Red Sandstone occus in N. Caithness and Sutherland. The relief of the underlying surface of Moine metamorphics and intrusions is irregular. The facies variation of the sediments reflects lacustrine transgression and regression within the Orcadian Basin. When the lake level was high and the lake margin coincided with the basin margin, thick (c.4m) carbonates were deposited on the unconformity. Carbonate deposition was controlled by photosynthesizing algae. At such sites the depositional environment was oxygen-rich and the carbonates do not contain biotite, organic carbon or pyrite. The offshore facies was deposited in deeper stratified lakes, anoxic at depth, and comprises organically-enriched sediments (with pyrite and biotite). When the lake margin was not coincident with the basin margin alluvial sediments mantled the unconformity as rivers cut to a lower base level. At marginal sites compaction of the lacustrine sediments over the irregular basement during periods of low lake level, when no deposition occurred, produced local unconformities.