The Ancestral Thames is the geologically ancient precursor to the present day River Thames. The river has its origins in the emergence of Britain from a Cretaceous sea over 60 million years ago. Parts of the river's course were profoundly modified by the Anglian (or Elsterian) glaciation some 450,000 years ago. 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.Shreve, D., 2011 ("Shreve 2011"), The Thames Through Time, Geological Society, online at www.geolsoc.org.uk.
Cretaceous to Miocene periods During the latter part of the Cretaceous period, around 80 million years ago, sea level is thought to have been over 150 metres higher than today. Cretaceous sea levels were 550 feet higher than today, at mongabay.com. Much of the land which was later to form Britain was covered by sea. In this marine environment, thick deposits of Chalk were laid down.
From the early Paleocene (from around 65 million years ago), much of what is now Britain emerged above sea level. The maximum uplift occurred in the north-west, with a regional tilt towards the east and south-east. The North Sea basin and the western English Channel basin also developed. Gibbard, P.L. & Lewin, J., 2003 ("Gibbard & Lewin 2003"), The history of the major rivers of southern Britain during the Tertiary, Journal of the Geological Society, 160, pages 829-845. Tectonic map and Palaeocene sections. Online at www.qpg.geog.cam.ac.uk.
The drainage of much of England was thus aligned to the south-east. 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. One of those watercourses was a precursor of the River Thames.
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. Substantial deposits of marine, lacustrine and deltaic origin were laid down across the region. In particular, over 150 metres of London Clay were laid down.
And, as the Palaeocene period progressed, subsidence in this region led to the formation of the London Basin, with its approximately west-east axis. Clements, Diana, The Geology of London (Introduction), Geologists' Association Guide 68, 2011. Online at geologistsassociation.org.uk. The Ancestral Thames thus flowed from the north-west, into this basin.
By the Oligocene period, around 30 million years ago, the coastline had retreated to the east of the London Basin. It is believed that, at that time, the Ancestral Thames was still flowing from the north-west, into the London Basin. From there, the river flowed broadly eastwards towards the North Sea basin of that period. Gibbard & Lewin 2003, Oligocene section.
And, also by the Oligocene period, the Wealden (or Weald-Artois) Anticline, to the south of the London Basin, was uplifted as a result of the Alpine orogeny. Gibbard & Lewin 2003, Neogene (Miocene and Pliocene) section. Rivers which were draining from the northern flank of that anticline - precursors of watercourses like today's Mole, Wey and Darent rivers - would have begun feeding into the Thames from the south. Further east, a proto-Medway river also flowed northwards from the Weald. 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.
There were periodic marine transgressions towards the London Basin during the Miocene period (around 10-20 million years ago), but it is thought that these did not significantly alter pre-existing drainage systems.
Pliocene period to early Middle Pleistocene period 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. Suffolk, Natural England. Holt-Wilson, T.D., 2012, Geodiversity Suffolk an Introductory Excursion, in Dixon, R.G.D. (ed): A Celebration of Suffolk Geology, GeoSuffolk 10th Anniversary Volume (GeoSuffolk, Ipswich). Online at www.academia.edu. But, again, this appears not to have had any significant effect on the Thames drainage system.
However, in the Late Pliocene and early Pleistocene periods (c.3-2 million years ago), the London Basin became submerged. Fossiliferous sands were deposited. 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. As the sea retreated eastwards, the River Thames system occupied the basin vacated by the sea. Gibbard & Lewin 2003, Neogene (Miocene and Pliocene), and Late Pliocene - Earliest Pleistocene sections.
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. Pebbles from formations of that area have been found in deposits laid down by the Thames in East Anglia. It has also been established that, during that time, the Thames, after entering the London Basin somewhere around the Goring Gap, then followed a course north-eastwards and northwards into North Norfolk, on a course towards the North Sea Basin. Bridgland, D.R. (1994, 2012) ("Bridgland 1994"), Quaternary of the Thames, Geological Conservation Review Series, Springer Science & Business Media, ISBN 978-94-010-4303-8, ISBN 978-94-011-0705-1 (eBook) (originally published by Chapman and Hall). "...there is abundant evidence to show that the modern Thames is a mere shadow of its Pleistocene forebear. Not only did it once flow from the London Basin out across East Anglia to north Norfolk, but there are indications that its headwaters may once have drained a large part of the West Midlands and even North Wales" (from chapter 1). At times, "the southern North Sea was occupied by the huge delta complex of the North German Rivers, the Rhine, Thames, Meuse and Scheldt". Gibbard, P., North West European Rivers - 4. Late Cromerian Complex Stage, Quaternary Palaeoenvironments Group. Online at www.qpg.geog.cam.ac.uk.
The Early and Middle Pleistocene gravel which was deposited by the Thames in East Anglia also contains chert from Lower Greensand beds to the south of London, and this "can only have been transported northwards by south-bank tributaries of the Thames and then the Thames itself". Bridgland D.R. and Gibbard P.L, 1997 ("Bridgland & Gibbard 1997"), Quaternary River Diversions in the London Basin and the Eastern English Channel, Géographie physique et Quaternaire, vol. 51, n° 3, 1997, p. 337-346. Online at www.erudit.org. Thus, ancestral versions of today's Mole, Wey and Darent rivers were feeding into the Thames from the Weald at that time. (The Mole and Wey joined to form one river in south London, and that Mole-Wey watercourse flowed north-eastwards across London and south Hertfordshire before joining the Thames. Gibbard, P.L., 1979, Middle Pleistocene drainage in the Thames Valley, Geological Magazine Volume 116, Issue 1.)
The deposits laid down by the Thames in East Anglia are also said to contain material from "Carboniferous and Devonian sandstones from southern Wales". Rose, J., Whiteman, C.A., Allen, P., Kemp, R.A., 1999 ("Rose & others 1999"), The Kesgrave Sands and Gravels: 'pre-glacial' Quaternary deposits of the River Thames in East Anglia and the Thames valley, Proceedings of the Geologists' Association 110, page 99. 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. In the Chilterns, it moved south-eastwards to a line approximately along the Vale of St Albans. In East Anglia, it moved down to around Colchester. East of that, around Clacton, the Thames finally formed a confluence with the River Medway. Bridgland & Gibbard 1997, Figure 1. Rose & others 1999, Fig. 2.
It seems likely that, at some time prior to the Anglian glaciation, the River Darent was captured by a left-bank tributary of the Medway. 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. As noted below, it is thought that, when the Thames was diverted by the Anglian glaciation, it followed this lower section of the Darent into the Medway.
Middle and Late Pleistocene periods
During the Anglian glaciation, some 450,000 years ago, ice sheets of a significantly greater amplitude than that of any ice sheets which had previously developed in the Pleistocene moved down from Scandinavia and the north of Britain. They coalesced and reached as far south as the southern part of the North Sea basin, East Anglia, London, the Cotswolds and South Wales.Gibbard, P., North West European Rivers - 5. Elsterian/Anglian Stage, Quaternary Palaeoenvironments Group. Online at www.qpg.geog.cam.ac.uk.
One result of this was that the Thames lost those of its headwaters which had previously extended back into the Midlands and Wales.Gibbard, P.L., 1985 ("Gibbard 1985"), Pleistocene history of the Middle Thames Valley, University Press: Cambridge. See page 135.It has been suggested that the Thames may have lost its north Wales headwaters before the Anglian glaciation (see: Rose & others 1999). This may have been effected by a pre-Anglian glaciation (see: Whiteman, C.A., and Rose, J., 1997, Early-Middle Pleistocene Beheading of the River Thames, Géographie physique et Quaternaire, vol. 51, n° 3, pages 327-336; online at www.erudit.org) or, according to another hypothesis, through river capture by a supposed pre-Anglian "Bytham River", flowing SW-NE across the East Midlands then eastwards across East Anglia (see: Shreve 2011). However, that river capture hypothesis does not appear to be widely supported now, and in fact the existence of such a pre-Anglian Bytham River has since been called into question (see: Gibbard P.L., Turner C., West R.G., 2013, The Bytham river reconsidered, Quaternary International 292, pages 15-32).
The Anglian glaciation had the effect of causing a breach of the land bridge between Britain and north-west Europe, across the Weald-Artois anticline. 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. The lake drained, with immense force, into the Channel River to the south-west, thus carving out what was later to become the Dover Strait. Gibbard, P., 2019, The Quaternary Evolution of the North Sea, University of Cambridge. Online at www.spri.cam.ac.uk.