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The geological significance of magnetic anomalies in the region of Islay

Synopsis Upward continuation of the aeromagnetic map for the Islay region has been used to clarify the major anomaly caused by Lewisian rocks which are more strongly magnetized than the overlying metasediments. Interpretation of the magnetic anomaly indicates that the interface between the Lewisian and overlying rocks dips SE and flattens out under the plunge depression of the Loch Awe Syncline of SW Argyllshire, where the maximum present day vertical thickness of metasediments above the interface is estimated to be of the order of 15 km. This interface has played an important part in controlling the structure of the Dalradian rocks in this area, and it is likely that at depth the Loch Skerrols thrust runs along it. A small prominent anomaly persisting in the upward continued aeromagnetic map may be attributed to a major Dalradian volcanic or igneous complex between Jura and the mainland. The Loch Gruinart Fault, passing through the area occupied by the main anomaly, cannot be the continuation of the Great Glen Fault and probably has a net sinistral strike-slip component of only a few kilometres.