Letton Formation / Folly Sandstone Formation
Age: 439-436 million years (Silurian, Llandovery)
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 the two counties.
The Silurian Period in Herefordshire is mostly represented by rocks in the north-west part the county, between Presteigne and Ludlow (Shropshire). Isolated outcrops occur along the western flanks of the Malvern Hills and in the Woolhope Dome area.
Two Formations in the Llandovery Series are the Letton Formation and Folly Sandstone Formation. There is little information surrounding these two units, but they have been correlated to the lower Llandovery and pre-date the May Hill Sandstone Group.
The Letton Formation is found near Pedwardine resting with clear unconformity on the Ordovician Shineton Shale Formation. It consists of well-jointed bands of yellowish brown sandy conglomerate separated by shaly bands.
The Folly Sandstone is a fossiliferous sandstone found in northwest Herefordshire on Nash Scar Ridge along the line of the Church Stretton Fault. Some layers are coarse conglomerates with quartz pebbles.
Nash Quarry, Herefordshire (Folly Sandstone Formation)