A summary of previous gravity and magnetic field interpretations is presented along with a review of the LOWNET and LISPB models. Recently acquired seismological data are compared with the LISPB upper crustal model. The LISPB Layer 1 is found to represent an average of lower velocity (VP = 3·0–3·7 km s−1) Carboniferous and Upper Old Red Sandstone sediments, and higher velocity (VP = 4·0–5·5 km s−1) Lower Old Red Sandstone and Lower Palaeozoic sediments.
On the basis of high P-wave velocities along strike (VP = 60·6–1 km s−1), the LISPB Layer 2 S of Glasgow and Edinburgh is re-interpreted as a quartzofeldspathic crystalline layer of igneous or metamorphic origin, which is also seen to cross the surface of the Southern Uplands fault at around 2·5 km depth.
This interpretation implies that within the southern part of the Midland Valley, the Lower Palaeozoic sediments are nowhere thicker than about 3 km, as postulated under the Silurian inliers, and generally much less.
