Ancestral Thames

The extensive terrace deposits laid down by the Ancestral Thames over the past two million years or so have provided a rich source of material for studies in geology, geomorphology, palaeontology and archaeology.

As the land emerged from beneath the Cretaceous sea, precursors of some of today's major drainage systems of central, eastern and southern England developed.

Thus, from the early Tertiary period a number of major consequent rivers flowed approximately NW-SE down the tilted emergent Chalk surface towards what later became southern England.

South-eastern England was low-lying and, from the late Palaeocene onwards (from about 55 million years ago), it was inundated periodically by marine transgressions from the east.

But that watercourse flowed parallel to the proto-Thames towards the North Sea basin, and it did not form a confluence with the Thames until later on, in the Pleistocene period.

[6] At times during the Pliocene period (around 5-3 million years ago), the North Sea coastline extended westwards, depositing various early Crag formations in an area now occupied by parts of East Anglia.

Subsequent Pleistocene uplift caused a relative displacement of c.180 metres between the western London Basin and the Suffolk coast, and remnants of those fossiliferous sands are now found at up to that altitude in the North Downs and Chiltern Hills.

[10] There is evidence that, for a time during the Early and possibly Middle Pleistocene period (c.2-0.5 million years ago), the headwaters of the River Thames extended north-westwards as far as north Wales.

[14]) The deposits laid down by the Thames in East Anglia are also said to contain material from "Carboniferous and Devonian sandstones from southern Wales".

[15] This suggests that the headwaters of the River Kennet (which today has its source in Wiltshire and meets the Thames at Reading), extended back into south Wales during the Early and Middle Pleistocene periods.

During the Early and Middle Pleistocene, prior to the Anglian glaciation, the Thames in the Chilterns and in East Anglia moved progressively south-eastwards.

The Darent thus ceased to be a right-bank tributary of the Ancestral Thames, and instead turned east somewhere just north of Dartford, and flowed into the Medway.

This was because of the formation and overflow southwards of a large pro-glacial lake which was fed by the Thames, Rhine and other major north-west European rivers and which was blocked to the north by the coalesced ice sheets.

[26] This process may have played a significant role in the formation of the relatively deep and box-shaped Lobourg Channel in the bed of what is now the Dover Strait.

[33] [34] [35] The onset of colder climatic conditions at the beginning of the Pleistocene period led to frequent downcutting by rivers in the Thames catchment area (and in north-west Europe in general).

[44] - Extensive spreads of wind-blown deposits, laid down in extremely cold, dry conditions during the most recent (Devensian) glaciation, are found in lower stretches of the Thames and Lea valleys.

Some of them have also been dated, and, through often minute scientific investigation of the deposits, it has been possible to trace in detail the Pleistocene history of the Thames as it changed course and continually cut down into underlying geological formations.

The oldest identified Thames deposits in the London Basin are those at Nettlebed (possibly) and Stoke Row in the Oxfordshire Chilterns, at altitudes today of 200 and 175 metres respectively.

[48] Tributaries of the Thames, such as the Evenlode, Kennet, Lea, Mole, Wey and Darent, have also left behind spreads of fluvial deposits.

The oldest of such tributary deposits appear to include spreads of "Pebble Gravel", now found on plateaus and hilltops in north London, Hertfordshire and Essex.

[50] It has been suggested that the Stanmore Pebble Gravel at Harrow Weald Common, for example, was laid down at about the same time as the Thames terrace at Stoke Row (see above) - that is, nearly two million years ago.

This Stanmore Gravel is seen as representing a south-bank tributary (perhaps a precursor of the Mole-Wey drainage system) of the Early Pleistocene Thames.