Lias Group

Typical exposure of Lias Group mudrocks on the Dorset coast

Download the pdf of Engineering geology of British rocks and soils: Lias Group

The Lias Group encompasses some of the most important geological formations in Britain. The Lias has been involved in a number of major engineering projects, explored for industrial use and investigated in scientific research over the last two centuries.

The Lias Group consists mainly of clays, mudstones and limestones of Late Triassic and Early Jurassic age, deposited between 180 and 205 million years ago.

The outcrop of the Lias extends in a continuous band from the south coast of Dorset to Yorkshire, with some outlying areas in Somerset and South Wales.

Significant outcrops are found in the high sea-cliffs on the coasts of Dorset, South Wales, and Yorkshire.

The main study areas across the Lias Group outcrop in England and Wales

Geological Hazards and Geotechnical Problems

  • Much of the Lias has high sulphate content and is responsible for high levels of thaumasite concrete attack.
  • Lias clays contain the clay mineral smectite, and are hence prone to swelling and shrinking. The smectite content of the Lias Group is variable, and whilst the Lias overall has a ‘medium’ volume change potential rating, some formations contain smectite-rich layers which have a ‘high’ rating.
  • The Lias Group rocks are recorded as having the highest incidence of landsliding in the UK. The Upper Lias (Whitby Mudstone Formation) having as many as 42 landslides per 100 km² of outcrop.

Plot of undrained cohesion against depth

Photograph of Dunn Caan landslide, Raasay, Inverness-shire

Report download

The Engineering geology of British rocks and soils: Lias Group 11 MB pdf provides an in-depth study of the Lias Group using data held in the National Geotechnical Database.

The geological, lithological, geotechnical, and mineralogical features of the major formations within the Lias Group have been described in detail, including the processes and effects of weathering.


Contact Marcus Dobbs for more information