Ice Ages in Herefordshire and Worcestershire
Professor Dick Bryant
Since his retirement from Worcester University, Dick has been focusing on the Quaternary Period in the two counties. Looking at the local Worcester geology map there are 4 aspects – alluvial deposits of the main rivers, their terraces, glacial till from ice sheets and head. On the Redditch map vast amounts of till from 500,000 BP are shown while in Herefordshire the edge of the Devensian ice sheet of about 18000 BP is prominent. Dating using ocean core deposits to differentiate the proportions of oxygen isotopes 16 and 18 has enabled accurate plotting of climatic changes in the last 2.6 million years. This in turn has enabled glacial and interglacial periods to be numbered and used for correlations between locations.
For Malvern area there is agreement that there have been 2 periods with local ice sheets – the Anglian 500,000 BP, which reached the Scilly Isles and North London and the Devensian around 20000 BP reaching to 10k west of Hereford. No clear agreement on any UK glaciation between these 2. During the Anglian the Malvern Hills may have been a nunatak above the 2 ice movements down the Severn valley and from Wales. From maps and photos Dick showed the evidence of the till deposits in the Colwall area.
The Devensian [or MI stage 2] was at a maximum about 18,000 BP, stopping just short of Hereford and Leominster. It wrapped around the Black Mountains as a ‘Piedmont’ glacier spreading out on the surrounding plains. The River Lugg was untouched so its older terraces were not affected. Features like moraines, kames and kettle holes were described together with melt water channels off the Hergist ridge. Castlemorton Common is covered with head deposits locally derived from the Malvern Hills, angular and with no stratification. These can be seen in the stream banks. Likewise head [moved by gelifluction ie soil flow in periglacial conditions] is quite extensive in the Colwall area from the flanks of the Hills to Cradley Brook.
Dick now turned to local river diversions and in turn summarised what had happened to the Mathon, Severn, Teme and Lugg rivers. In each case after the original course of the river had been dammed by ice causing a lake to form, the river cut a new route via a gorge – at the Knapp and Papermill, Ironbridge, Downton and Kinsham. He described the evidence in the Mathon sands and gravels for how the original south flowing larger Mathon river had been blocked by a tongue of Welsh ice around Pitches Brook causing a lake to form and as a result the river, in the form of the much smaller Cradley and Leigh Brook, reversed its flow via the Knapp. Reference to how the much smaller proto Severn river became a high flow braided river around Holt Heath once it was reinforced with the additional waters surging through Ironbridge and to the role of isostatic uplift in the generation of river terraces.
Mountain glaciation in the Black and Red Daren areas of west Herefordshire has been Dick’s focus recently and he gave us a copy of his paper. With the aid of maps, diagrams & photos, he described the features and processes involved in generating land slips, ridges and an embayment containing a small glacier.
He concluded by relating work done on understanding long term climatic changes to predicting future ice ages. The work on oxygen isotopes now matches well with the work done [but not generally then accepted] by Milankovitch in the 1930s on the 3 cycles [the Eccentricity, Precession and Axial Tilt cycles] to predict these changes – a topic in itself for another talk. Dick’s view is that the current increase in carbon dioxide concentrates does not have a clear natural cause.
A final thought was how far Plate Tectonics may influence longer term impacts like snowball earth – for example the movement of Antarctica over the South Pole to give a land base for glaciers, or the future movement of the American plate northwards into the Arctic.
[Please note that this is my impression of the ground covered by Dick, so any errors or omissions are mine!]
Jim Handley
“The glaciations of Wales and adjacent areas”, Colin A. Lewis and Andrew E. Richards