Jurassic and Cretaceous 205-135 Ma ( Arran at 40-45 degrees North)

The Jurassic and Cretaceous were a time of extreme raised sea levels. Indeed, nearly all of the British Isles was covered by extensive tropical seas which deposited thick sequences of mudstones and limestones.

However, these rocks have since been removed by the errosive power of glaciers during the Quaternary, and the only representative rocks on Arran from these periods are foundered xenoliths of muds and chalk found in the Central Ring Complex. Fossils preserved in these xenoliths have helped to identify them as Lower Lias marine clays and muds, and as Upper Cretaceous Chalk.

These rocks correspond with the succession of Jurassic and Cretaceous seen on the Antrim coast of Ireland, where there exists only Lower Lias and Upper Chalk. Foundered blocks of Lower Lias shale can be found on Windmill Hill (NR983351), and have yielded ammonites of Sinemurian age. Weakly metamorphosed chalk, which was once quarried, can be found south of Glenloig Farm on the other side of the String Road (NR946531).