and Dagley identified the Highland Boundary Fault as coincident with a regional magnetic feature dividing a string of negative anomalies in the north from positive ones in the south. On discovering this, Hall and Dagley concluded that the observed trend, which followed the length of the Dalradian trough, transition from positive to negative anomalies. This linear feature of magnetic anomalies has since been referred to as the Fair Head–Clew Bay line. Features Tectonic controversy At present, it is believed that the Highland Boundary Fault was active during two main orogenic events associated with the Caledonian orogeny: the Grampian orogeny in the Early Ordovician and the Acadian orogeny in the Middle Devonian. The fault allowed the Midland Valley to descend as a major rift by up to and there was subsequent vertical movement. This earlier vertical movement was later replaced by a horizontal shear. A complementary fault, the Southern Uplands Fault, forms the southern boundary for the Central Lowlands. The age of the Highland Boundary Fault has been inferred to be between Ordovician to middle Devonian and through several generations it has been interpreted as a graben-bounding normal fault, a major sinistral strike-slip fault, a northwest-dipping reverse fault or terrane boundary. The reason the precise nature of the fault is still unknown is because there is little evidence of a continuous fault plane on the surface. More recently, seismic activities marking the fault line have been analysed to show that the 2003 Aberfoyle earthquake had a hypocentre at depth and was caused by an oblique sinistral strike-slip fault with normal movement. The fault plane was estimated to be dipping at 65° NW. Associated lithology To the north and west of the Highland Boundary Fault lie hard Precambrian and Cambrian metamorphic rocks: marine deposits metamorphosed to schists, phyllites and slates, namely the Dalradian Supergroup and the Highland Border Ophiolite suite. To the south and east are Old Red Sandstone conglomerates and sandstones: softer, sedimentary rocks of the Devonian and Carboniferous periods. Between these areas lie the quite different rocks of the Highland Border Complex (at one time called the Highland Boundary Complex), a weakly metamorphosed sedimentary sequence of sandstones, lavas, limestones, mudstones and conglomerates. These make up a zone which is found discontinuously along the line of the fault and which is up to in width. Dalradian Supergroup The Dalradian Supergroup consists of metasedimentary rocks which underwent polyphase deformation and metamorphism during