The large magnetic anomaly which runs along the Highland Boundary Fault of Scotland immediately to the NW of Blairgowrie has been studied by both a ground-based magnetic survey and a palaeomagnetic investigation. A newly developed optimisation program, making use of a simplex algorithm, was used to model the long wavelength component of this effectively two-dimensional anomaly. The consequent model consists of a vertical, rectangular body, 3 km wide and 13 km deep, whose top surface is 2 km below ground level, with a magnetisation directed vertically downwards. The composition of this body is most likely to be that of an ultra-basic, metamorphic complex which has been brought up to its present position between two of the many near-vertical faults in the area. The medium wavelength structure of the anomaly was modelled using the common technique of trial and error, and can be interpreted in terms of a pair of Devonian andesitic lavas and a small extension of the main ultrabasic block. Subsequent palaeomagnetic remanence measurements confirmed that the magnetisation of the two lavas recorded a Devonian polarity reversal. The magnetisation of the ultrabasics is directed vertically downwards, exactly as deduced from the earlier modelling work.