Sea levels began to rise at the beginning of the Silurian Period in response to the melting of the southern hemisphere ice sheets. Welsh sea levels gradually rose and spread eastwards over the eroded Precambrian landsurface forming a shallow sea. The Malvern Hills lay on the edge of a Silurian coastline which extended northwards towards Church Stretton in Shropshire (Figure Seven). These early Silurian rocks are represented by The Upper Llandovery rocks, the Cowleigh Park and Wyche Formations (known collectively as the May Hill Sandstone Group). The Cowleigh Park Formation forms a narrow, prominent ridge of mainly sandstones and conglomerates, extending from West Malvern (SO760464) to Cowleigh Park (SO761472). The Cowleigh Park Formation was deposited by sediment-laden streams flowing across an early Silurian beach. The Wyche Formation forms a narrow belt of steeply dipping mudstones, siltstones and sandstones, located on the western side of the Hills. (Figure Eight)
The Wyche Formation is exposed at Gullet Quarry The nature of the contact between the Silurian and the Precambrian rocks has provided considerable debate amongst the geologists who have studied it. Early geologists such as Groom, 1899 and Phipps and Reeve, 1969 believed that the boundary between the Silurian and the Precambrian is a fault formed by tectonic activity. However, later research has suggested that the contact represents an angular unconformity (Box Three). This was formed when the conglomerate marking the boundary between the Silurian and Precambrian rocks was deposited against an eroded cliff shoreline (Photograph Four).
A Parallel unconformity
The lower and upper series of beds dip at the same angle and in the same direction. The gap in the rock sequence between the younger and the older rocks may represent a period of non-deposition. Or it represents a period of erosion, which has been followed by a period of renewed deposition.
An Angular unconformity
The lower, older series of rocks dip at a different angle to the younger, upper series. A likely sequence of events would be as follows:-
- deposition of the older rocks
- period of uplift accompanied by folding or tilting
- period of erosion
- renewed deposition on the erosion surface
A Heterolithic unconformity
This type of unconformity occurs when sediment is deposited on top of an igneous intrusion or metamorphosed rocks, which have been exposed at the surface by erosion.
Box 4. Coral Reefs
A coral reef is a wave-resistant structure, resulting from cementation processes acting on the skeletal remains of organisms. A typical coral reef consists of thousands of coral communities and the skeletal remains of other marine organisms such as brachiopods.
The best known type of reef is the barrier reef. However the Much Wenlock Limestone Formation represents patch reefs (bioherms), which are isolated concentrations of corals in shallow waters.
Modern day reefs are only found up to 27 degrees north and south of the Equator since corals require high temperatures to grow. Therefore, during Wenlock times, the Malvern Hills experienced a warm tropical environment.
The shallow tropical seas gradually began to deepen and limestone formation was succeeded by the deposition of the shales and mudstones representing the Coalbrookdale Formation. The deep water conditions were gradually replaced by a shallow sea, resulting in the deposition of the Much Wenlock Formation (formerly the Wenlock Limestone), Lower Ludlow Shales, Aymestry Limestone Formation and the Upper Ludlow Shales. The relatively calm, shallow, tropical conditions allowed the growth of small patch reefs which have been compared to the modern day patch reefs found in the Caribbean sea. The Aymestry Limestone forms a gently curving wooded ridge on the western side of the Hills, which is known locally as the Ridgeway. The Lower Ludlow Shales form the relatively low-lying area dividing the prominent limestone ridges of the Much Wenlock Limestone Formation and the Aymestry Limestone Formation located between the Wyche Cutting and Brockhill Coppice (Figure Nine).
Towards the end of the Silurian Period saw the final closure of the Iapetus Ocean which had been slowly closing since the Ordovician Period. England and Scotland formed a large continent called the “Old Red Sandstone Continent”. Sea levels gradually dropped and the relatively quiet seas were replaced by a storm driven environment, depositing the sandstones of the Downton Castle Formation.
The end of the Silurian Period, marked by a change from marine to continental deposition, was as a result of a period of uplift associated with the final closure of the Iapetus Ocean. The Raglan Mudstone formation was deposited by sinuous streams migrating across an alluvial plain. These streams emerged from the Caledonian mountain belt which was forming in the collision zone between what is now England and Scotland (Figure Ten).
