The progressive onlap of the Lower Lias (Lower Jurassic) onto the London Platform is analysed using cores and geophysical logs obtained from numerous boreholes in the south Midlands. The earlier beds of the Lias (including the pre–planorbis Beds) were regressive in relation to earlier Rhaetian strata. The Liassic transgression was first across Rhaetian strata, but the Upper Sinemurian overstepped onto the Palaeozoic basement of the Platform. In the Hettangian and early Sinemurian the periods of greatest transgression coincided with the deposition of more argillaceous units, with regressive or stillstand conditions more characteristic of the calcareous units.

The London Platform remained a comparatively stable area of very low relief during this time. It was subject to intermittent relative subsidence so that the transgression extended some 60 km onto its northern flank and resulted in an extension of over 14,000 km2 in the area of deposition by the end of the Lower Pliensbachian Substage. The thickness of Lower Lias accumulated increases away from the Platform, where concurrent major downwarping in the Severn–Worcester, Cheshire, and Cardigan Bay basins allowed much greater amounts to accumulate.

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