Basement Quoyelsh Felsite

Before burial by the Devonian sediments of Lake Orcadie, the basement rocks formed a range of fault bounded steep-sided hills running north to northwest. These hills eventually stood out as islands within the lake and were surrounded by locally derived beach deposits of sands, conglomerates and breccias.

An interesting outcrop of pre-middle Devonian age "felsite" is found at Quoyelsh, east of Stromness. The rock is quartz porphyry with phenocrysts of quartz in a microcrystalline groundmass and exhibits marked flow banding. It is overlaid by a sandy breccia containing blocks of the felsite passing upwards into the sands and mud of the lake sediments. The age of this felsite is not known but may be related to the Lower Devonian igneous event in Shetland and the intrusion of the compound dyke rock at Garthna Geo, Yesnaby rather than the much older granites and gneiss.

the location is approximately 1 km East of Stromness town centre separated from the main Stromness granite and breccias by a flat lying sequence of lower Stromness flagstones (10 -- 15 m)

Detailed mapping shows that the flagstones are differentially compacted over this basement knoll on the up thrown side of the Borwick-Navershaw Fault (down to the East normal fault, about 650 m throw).

 

The felsite surface is overlain by large loose breccia blocks where the sea has eroded out the contact which is clearly seen at low tide.