The Longmyndian Supergroup is a c.6000m thick sequence of rocks that outcrop between the Pontesford-Linley Fault System and the Church Stretton

Church Stretton

Church Stretton is a small town and civil parish in Shropshire, England. The population of the town was recorded as 2,789 in 2001, whilst the population of the wider parish was recorded as 4,186...

 Fault System in the Welsh Borderland. The supergroup consists of two major geological groups, the Stretton Group and the overlying Wentnor Group. The rocks are a generally regressive sequence from basinal facies to clastic sedimentation

Sedimentation

Sedimentation is the tendency for particles in suspension to settle out of the fluid in which they are entrained, and come to rest against a barrier. This is due to their motion through the fluid in response to the forces acting on them: these forces can be due to gravity, centrifugal acceleration...

. The rocks have since been folded due to fault movements and plunge gently to the south.

Wentnor Group

The Wentnor Group is made up of the fluvial deposits of the Bridges Formation and is the topmost of the supergroup. This is underlain by the braided deposits of the Bayston-Oakwood Formation which completes the Wentnor Group.

Stretton Group

The topmost of the Stretton Group, the Portway Formation, consists of braided fluvial deposits with marine incursions. The underlying Lighspout Formation is interpreted as a fluvio-deltaic environment, this is succeeded by a similar facies in the Synalds Formation. Turbidite facies are prevalent in the Burway Formation and the deep marine basinal environments continue toward the base of the supergroup within the Stretton Shale Formation. The Ragleth Tuff Formation completes the Stretton Group and the supergroup sequence.

Dating

Within the strata are bentonite

Bentonite

Bentonite is an absorbent aluminium phyllosilicate, essentially impure clay consisting mostly of montmorillonite. There are different types of bentonite, each named after the respective dominant element, such as potassium , sodium , calcium , and aluminum . Experts debate a number of nomenclatorial...

 and lapilli

Lapilli

Lapilli is a size classification term for tephra, which is material that falls out of the air during a volcanic eruption or during some meteorite impacts. Lapilli means "little stones" in Latin. They are in some senses similar to ooids or pisoids in calcareous sediments.By definition lapilli range...

 tuff

Tuff

Tuff is a type of rock consisting of consolidated volcanic ash ejected from vents during a volcanic eruption. Tuff is sometimes called tufa, particularly when used as construction material, although tufa also refers to a quite different rock. Rock that contains greater than 50% tuff is considered...

 horizons that allow for zircon U-Pb dating

Uranium-lead dating

Uranium-lead is one of the oldest and most refined of the radiometric dating schemes, with a routine age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1-1 percent range...

and noted within the Batch Volcanics of the Synalds Formation, toward the base of the Stretton Group, a date is returned of 566+/-2.9Ma. The Lightspout Formation toward the top of the Stretton Group returned a date of 555.9+/-3.5Ma.