Ancestral Thames

The Ancestral Thames is the name given to the geologically ancient precursor to the present day River Thames.

Relationships with other rivers

In central and southern Britain, during the Early and Middle Pleistocene were two main rivers of more than 150 miles (240 km) the Bytham and the Ancestral Thames. For most of Early Pleistocene the latter was the main river with, at its maximum extent, a catchment area that extended into Wales, alongside the Chiltern Hills, through southern East Anglia and finally lowlands in what is now the North Sea, to join the ancestral Rhine. In detail, for instance in this early Ice Age the Thames flowed along a line similar to the River Thame, then through parts of Bedfordshire and Hertfordshire to the north of Greater London, through north-west Essex and took a course resembling that of the River Waveney on the Suffolk/Norfolk border. Initially the more northern Bytham River was a tributary of the Thames but as temperatures overall warmed it progressively extended its catchment. During the Anglian Stage the Bytham river more or less disappeared and the Thames was diverted to its present route through London.^[1]

Vestiges

At that time, this river and its tributaries formed a river system draining the Welsh mountains bringing some of their characteristic volcanic rocks into this area. The evidence for this being a substantial thickness of what is called Kesgrave Sands and gravels which represent the bed of the river. These old Thames gravels contain a variety of distinctive pebbles from as far away as North Wales, evidence of the ancient drainage catchment.^[2]

These gravels also contain large boulders of puddingstone and sarsens, which are very hard conglomerates and sandstones respectively. They are believed to be derived from pebble and sand seams in the Reading Beds, and which have subsequently become cemented by quartz. They have been put to use by man as ancient way markers at road junctions. The gravels have great commercial value and are worked in numerous gravel pits between Harlow, Chelmsford and Colchester which was a course until at least 600,000 years ago.^[3] During this time the River Medway flowed north across east Essex to join the Thames near Clacton, leaving behind a ribbon of distinctive gravel which can be found between Burnham-on-Crouch and Bradwell-on-Sea. There were also other northward-flowing tributaries of the early Thames. Evidence of these are the patches of gravel that are found near the tops of the modest hills in south Essex, principally the Langdon Hills, Warley and High Beach in Epping Forest.^[4]

References