Like other great tholeiite sheets of the world, the Midland Valley Sill and Whin Sill were emplaced in thick accumulations of sediments which, at the time of intrusion, were not much older nor had been orogenically deformed. However, the Carboniferous sediments of northern Britain had been deposited under the influence of continuous differential subsidence, so that they were already disposed in syn-sedimentary basins and swells when intrusion began. By drawing palinspastic contours of the sills relative to a late Westphalian marine band (approximating to the surface at the time of emplacement) it is established that the dolerites mainly follow bedding planes, dipping at angles of as much as 5° down to the bottoms of the basins. Isopachs of the sills indicate a further relationship, for the intrusions are thickest at the bottoms of the basins.
These facts are not explained by any intrusion mechanism previously postulated, though there seems to be broad agreement as to the processes which have given rise to thinning and en echelon lensing at leading edges as well as to step-and-stair transgression of sediment bedding. It is proposed that emplacement must have been controlled in part by gravitational flow down dip from feeder dykes which extended to between 0.5 and 1.0 km below the contemporary surface. Thereafter hydrostatic equilibrium was attained first by accumulation at the bottom of the basins and then by further advance up-dip under pressure of head. The sheets are so similar in aspect to flood basalts that it must be assumed that the sediments offered only low frictional resistance to splitting and flow.