Post Anglian Deposits

Post Anglian Deposits

Age: 2.5 million years – present (Quaternary, Pleistocene/Holocene)

Overview

There is a hiatus of around 168 million years in Herefordshire and Worcestershire between the Jurassic and the Quaternary periods, and no rocks are found at the surface dating to this time. After about 56 million years ago, there was a gradual cooling of the Earth’s climate, reflecting the migration of the Antarctic continent into the south polar region, and the partial surrounding of the Arctic ocean by drifting northern hemisphere continents. The cooling became particularly pronounced 2.5 million years ago, marking the beginning of the Quaternary Period. This was characterised by repeated alternations of warm and cold episodes, driven by variations in the amount of solar radiation received by the Earth’s surface. Some of the cold periods resulted in extensive glaciations (ice ages), separated by relatively warm interglacials.

The complex nature of glacial and interglacial phases means that they destroy and rework previous deposits. This has created a fragmented yet extensive, network of material of various ages and depositional environments within Quaternary time. In the two counties firm evidence is only preserved of two major glaciations. The first took place in the Anglian Stage, which occurred about 500,000 years ago, and is now considered to include both Anglian and Wolstonian glacial deposits. The second, much more recent Devensian glaciation, took place about 25,000 years ago. Glacigenic deposits from this latter stage are largely confined to the western part of Herefordshire (the Herefordshire Formation). Apart from these deposits, the majority of sediments in the two counties deposited after the Anglian Glaciation represent terraces formed on the major drainage systems. These river systems are the Severn, Avon, Lugg, Wye and Teme.

Lithology

Alluvial Deposits

This class contains the most abundant type of superficial deposit within Herefordshire and Worcestershire. Alluvial is the generic term for all unconsolidated clay, silt, sand and gravel deposited by a flowing water body.  There are two categories of alluvial deposits; fluvial deposits and alluvial fan deposits.

Fluvial Deposits

The major rivers in Herefordshire and Worcestershire have a series of relatively flat terrace features associated with their respective valleys. These are the dissected remnants of previous floodplains, glacial meltwater and river deposits. They formed from a complex combination of sea level change, variations in river discharge and amount of sediment in the water, and the natural rebound of the land at the end of the final glaciation. This has resulted in a series of stepped terraces being formed, which can be divided up based on their present height and material composition. Each terrace has been partially destroyed by subsequent terrace development, so that today only fragments of the oldest terraces remain. The younger terraces, which are lower in height and closer to the modern day rivers are the best preserved.

All river terrace deposits comprise varying quantities of clay, silt, sand and gravel. They occasionally contain derived fossil content, which gives information on the sediment source. For example, the Wasperton Member is dominated by sand and gravel throughout its 4.6m thickness, but in addition contains Jurassic fossils and oolitic limestone, suggesting a source from the Jurassic limestones that today form the Cotswolds.

Sites

Sutton Walls Hill Fort, Herefordshire (Sutton Walls Member)

Beckford Gravel Pit, Worcestershire (Wasperton Member)

Tarmac sites, Worcestershire (Worcester Member)

Alluvial Fans and other alluvial Deposits

Alluvial fans are outspread, gently sloping or relatively flat deposits shaped like a fan or cone. They are deposited by fast flowing streams at the mouth of a tributary where it meets a large flat valley. As the stream meets the main valley, the stream gradient flattens. This leads to the water slowing and being unable to carry much of its sediment load. As a result, most of the sediment is deposited. The deposits were formed largely in a periglacial environment, contemporaneous with the last (Devensian) Glacial Stage.  Exposures of alluvial fans are very rare, as they mainly form the lower ground in valleys. The most extensive deposits in Worcestershire are located on the flanks of the River Arrow from Alvechurch to Redditch. Exposures have also been recorded in the Habberley Valley. The former workings at Chadwick Farm Quarry contain the best exposure, as their relationship with the underlying bedrock is clearly displayed.

In Herefordshire, up to 4m of alluvial gravel have been laid down by a cold stage braided river during the transition between the Late Devensian and the Holocene. These gravels have been worked at Bodenham and are still being worked at Wellington.

Mass Movement Deposits

Mass movement deposits are locally derived, poorly sorted angular rock fragments and soil, deposited by  solifluction (slow viscous downward flow of waterlogged soil), geliflucation (slow flow of fluidised material during the thaw of seasonally frozen ground) or hillwash (soil washed down by the overland flow of water). It differs from landslipped material in that is formed by a slow, almost continuous movement downslope.

The most extensive deposits occur on the eastern flank of the Malvern Hills, where they can be traced almost as far as the River Severn. Here, the deposits contain clasts of local Pre-Cambrian and Lower Palaeozoic rock that washed down from the hills after the last glaciation. They have been termed the Malvern Gravels. Similarly, the slopes around Bredon Hill and the Cotswold escarpment have extensive deposits containing local Jurassic limestone.

Bredon Hill is also host to a very local and peculiar kind of deposit called “Gull Rock”. It was formed by frost shattered limestone fragments falling into joints or “gulls” in the bedrock. The clasts were cemented together by calcite (tufa) which was precipitated as lime rich water ran down through the joints of the bedrock.

Head deposits may have been locally worked for gravel, but the poorly sorted nature of the deposits makes large scale workings improbable.

Sites

Upper Lyde Quarry, Herefordshire

Shobdon Quarry, Herefordshire