J. D. Ritchie & K. Hitchen write: Flinn (1992) provides a comprehensive review of the published information on the location and history of the Walls Boundary Fault and its relationship to the Caledonian Front, which he equates with the Moine Thrust. However there are serious flaws and omissions in his arguments which invalidate his conclusions about the location of the Walls Boundary fault and Moine thrust north of Shetland.

Walls Boundary fault.Flinn (1992) proposes that the fault is associated with Bouguer gravity anomaly ‘lows’ and magnetic negative anomaly ‘valleys’ and hence, north of Shetland, must trend northeastwards. This argument is unsound for the following reasons.

(1) Gravity ‘lows’ do not necessarily mark the position of faults. If rocks of equal density are juxtaposed across a fault then no anomaly will be caused.

(2) If, according to Flinn’s logic, the fault is aligned through gravity ‘lows’, why does this not apply south of Shetland where his fig. 2A shows the fault to miss completely a large –20 mgal low west of the southern tip of the Shetland mainland? This low is actually caused by the West Fair Isle Basin, whose eastern margin is the Walls Boundary fault (BGS Shetland solid geology map).

(3) Flinn (1992, his fig. 2A) uses the published British Geological Survey (BGS) Bouguer gravity anomaly map, with selectively chosen contours, to trace the Walls Boundary fault north of Shetland. However, the ‘lows’ which Flinn claims to mark the location of the fault are actually caused by the

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