Harrow and Hillingdon Geological Society

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Reconstructing Rivers: Pebble Counting in the British Quaternary

Investigation of the development of rivers of the quaternary period in Britain has led to a better understanding of their subsequent structure and enabled attempts to draw up stratigraphies. By cutting down into the landscape episodically, rivers created terraces such as Boyne Hill and Taplow on the Thames. In the mid 1920s there were problems in understanding the morphology of the river valley and these terraces on the River Thames. Various techniques were used, glacio-eustasy or the glacial effect on valleys, and sedimentology of terrace deposits in the 1960s. Pebble counting in conceptual change was developed in addition to the mechanics of the glacial effect and these were examined in conjunction to temperature and climate variation.

Neotectonics or regional uplift seen in Southern Britain occurred also in other parts of Europe. Most of the rivers in these areas are very complex in structure and are flanked by terraces derived from quaternary activity. The terrace deposits were classified in units, now >18mm. This is done by passing the material through a series of sieves of known pore size. It was difficult to compare the various results as different scientists used different standards.

Examination of the gravels of the great Ouse showed them to be peri-glacial in origin while others were harder to interpret. It had been a matter of conjecture as to how the smaller Blue stones of Stonehenge had been transported from their source in the Preseli Mountains of South Wales to Salisbury Plain. Were they taken by man or on an ice sheet during movement from Wales into the Bristol Channel? The Sarson stones came from Marlborough Downs and the River Avon and its tributaries, which feeds Salisbury Plain has, a set of terraces. If ice had reached Salisbury Plain, there should be outwash but there were no glacially derived pebbles. It was concluded therefore, that the stones of Stonehenge, were transported by man not ice and hence the area was not glaciated.

The higher terraces of the Avon contained more quartz showing that a tertiary outcrop had been progressively eroded during the quaternary. On the river Thames flint pebbles are positioned on the higher terraces. With the rivers Axe and Otter, ice from the Bristol Channel may have flowed into the Axe. These gravels are poorly structured and contain both sand and palaeolithic material but there is no exotic glacial material in either the Axe or the Otter, however there are a lot of outliers, which do contain exotic material. Purbeck limestone has been found in east Devon probably from marine gravels during a marine transgression, at the time of the London clay, during the tertiary.

Much work has been done on the terraces of the Thames, which are both pre-Anglian and post-Anglian glaciation in origin.

Thames-Hertford, Watford, St Albans-Pre-Anglian, the pebble gravel is the oldest in the Thames system and is mainly flint. Westland, Beaconsfield, Gerards Cross-Anglian glacial gravels lower and upper. Colne Terraces are gravels occurring in three broad sweeps, they are mainly quartz and quartzite from the Midlands.

Volcanic rocks can also bet raced to the Ordovician in N.W.Wales although these constitute a very small amount, but material is widespread, eg High Wycombe, Beaconsfield, Gerrards Cross etc can be traced out into Essex, Suffolk and Norfolk. Rhaxella chert was spread by the Anglian ice and distinguishes between glacial and non-glacial deposits. Glacial gravels could also be traced eastwards. Lower greenstone chert occurs in clusters. These line up exactly with rivers which drain the Weald and therefore these right bank tributaries of the Thames have influenced their deposition.

Pebble counting yields an enormous amount of data and by comparing the ratios of the rock deposits it was possible to determine the sources of those deposits.

This was a very interesting and enjoyable talk but with rather too much information to absorb in the time allotted. The speaker was clear and obviously enthusiastic and used 31 slides, which illustrated his lecture very well.

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