Upper Ludlow Shales Group

Upper Ludlow Shales Group

Age: 421-419 million years (Silurian, Ludlow)

Overview

Silurian rocks in Herefordshire and Worcestershire are used as markers for this period of earth history throughout the world. They mark a time when the world’s great oceans were in the process of closing, leading to the formation of marine sandstones, mudstones and limestones. The rocks are also famous for their diversity and richness of fossils, and this has led to accurate sub-divisions of the period being devised: the Llandovery (443-428Ma), Wenlock (428-423Ma), Ludlow (423-419Ma) and the Pridoli (419-416Ma). All four of these time frames are represented in the two counties.

In Worcestershire the outcrop of Silurian rocks generally follows a north-south linear trend, beginning just north of the Malvern Hills and continuing along the central and eastern half of the Teme Valley, before splitting into two outcrops around Abberley. The Upper Ludlow Shales form the sloping ground to the west of the Aymestry Limestone ridge, including the NW slope of the Abberley Hills. In Gorsley in southern Herefordshire, the unit lies unconformably on the Much Wenlock Limestone, but is conformable with the Aymestry Limestone in the north of the county.

The Upper Ludlow Shales Group is part of the Mid-Late Silurian Ludlow Series.

Lithology

Marking a return to deeper water conditions the unit consists of olive-grey calcareous siltstones and shaley mudstones with limestone nodules at the base and more sandy beds at the top. A large diversity of fossils are common at the base, reducing in quantity towards the top of the succession. The Group has been divided into three Members in the Malverns area; the Mocktree Shale Member, Woodbury Shale Member and Whitcliffe Flags Member. In the Ludlow type area, the Mocktree and Woodbury Shales correspond to the Lower and Upper Leintwardine Formation respectively. The Whitcliffe Flags Member is correlated with the Lower and Upper Whitcliffe Formation.

Mocktree Shale Member/Lower Leintwardine Formation

Consists of light olive grey calcareous siltstones and shaley mudstones. The unit is quite calcareous towards its base with thin bands of nodular limestone and common lens of shell fragments. Brachiopods and bivalves dominate a rich and varied fauna, with cephalopods, gastropods and ostracods. In northern Herefordshire the Lower Leintwardine Formation was deposited in an active tectonic environment, suggested by the development of submarine channels and many manifestations of soft sediment deformation. The submarine canyons cut into the underlying beds, at their greatest depth to the level of the Coalbrookdale Formation. At the base of the Lower Leintwardine south of Wigmore Rolls and in the Mocktree area are boulder beds in these channels.

Woodbury Shale Member/Upper Leintwardine Formation

This is lithologically similar to the Mocktree Shale, but is characterised by the superabundance of particular brachiopods, trilobites and gastropods.

Whitcliffe Flags Member/Lower and Upper Whitcliffe Formation

Comprises flaggy, calcareous siltstones with lenses of shell material commonly found. These have typically been leached and weathered to a greyish brown “rottenstone”. Bands of nodular limestone occur but are much less common than in the Mocktree Shale. The top 3-4m are usually very sandy and micaceous. These upper beds display a transition to the Downton Castle Sandstone Formation.

Sites

Woodbury Quarry, Worcestershire

Frith Barn Quarry, Herefordshire

Bircher Common – Welshman’s Lane, Herefordshire

Brockhill Farm Quarry, Herefordshire

Leinthall Earls Quarry, Herefordshire

Linton Quarry, Herefordshire