Offshore shallow seismic, bathymetric and geological surveys are interpreted and, together with other geophysical data and onshore geology, used to produce a description of Quaternary and Pre -Quaternary geology. An attempt is then made to trace the geological evolution of the Sea of the Hebrides from the Late Palaeozoic to the present.
A major unconformity separates consolidated Pre -Quaternary rocks from unconsolidated Quaternary sediment. The Pre - Quaternary geology is controlled by three principal north -easterly faults which form the western margin of asymmetric troughs floored by Pre -Cambrian and probably Palaeozoic rocks and filled with Permo- Triassic to Cretaceous sediments. A third asymmetric trough has been detected on the outer continental shelf.
Tertiary igneous activity and north -westerly faulting are important secondary controls on the geology.
The Pre -Quaternary unconformity on the inner continental shelf has a characteristically glaciated morphology and patterns of ice movement can be deduced.
Quaternary sequence, locally exceeding 250m in thickness, is divided into four sedimentary formations. These are interpreted as glacial moraine, glacial- marine sediment, peri- glacial marine sediment and modern sediment. The sequence was deposited
diachronously by the retreating ice of the last (Devensian) ice sheet. Concentration of glacial- marine sediment along the paths of major ice -sheets is considered to be responsible for the major variations in Quaternary sediment thickness. The complex morphology of the present sea floor reflects the Pre -Quaternary geology, glacial sedimentation and the present marine regime.
The complexity of the geology is such that this study must be regarded as a first reconnaissance.