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London's Geology
Posted on May 24, 2003 - 12:08 PM by Bill McCann

The modern landscape of London has been sculpted by two major influences. Geological processes, that occurred over a period of millions of years, and historical activity that has adapted and altered that ancient landscape during the past 1,960 years. This article is intended as a brief overview of the geological factors which influenced the development of the landscape in the London region.

A General Section through the Geology beneath London

The Thames Valley
The oldest, and therefore the deepest, rocks beneath London are from the Silurian period and are about 425 million years old. These form a central core with younger beds lying to the north and south of it. This arrangement is known as an anticline, a folding in the geological deposits which has an arched structure. The way in which the younger deposits are wedged against the core suggest that it may have been a shallow ridge or even dry land. It was overwhelmed by the Cretaceous seas about 90 million years ago and which deposited the chalk escarpment which defines the large geological feature known as the Thames Basin.

London lies close to the heart of this Basin. It is formed by a depression in the chalk levels. Its rims are formed by the North Downs to the south and the Chilterns to the north. The Chalk formation displays a striking change in the lithology from bottom to top. The lower layers merge imperceptibly into the underlying Gault clay and Upper Greensand. Moving up through the formation the mixture of clay and sand is gradually replaced by calcareous matter. At the top it has become the soft white limestone known as chalk. It has a different appearance to other limestones because it is porous and earthy whilst they are compact and crystalline.

It is widely believed that chalk is almost entirely composed of microscopic fossils. However, this is not true. The proportion of these fossils, mostly foraminafera, varies but is never more than ten per cent of the rock. The most abundant constituent is an exceedingly fine calcareous matter which can account for up to ninety per cent of the whole. It is probable that this has an organic origin in the disintegration of planktonic algae. In the Lower Chalk and in the lower region of the Middle Chalk there are abundant shell fragments, especially Inoceramus. In places these form the greater part of the rock but they decrease in amount upwards in the succession. Flints are abundant in the Upper Chalk. They occur either as irregular masses, in definite layers, or as isolated nodules scattered through the rock. The also occur as thin tabular strips filling cracks in the rock.

The Basin formed by the depression in the Chalk was filled by the younger deposits of the Tertiary era, in particular marine sands and clays such as the Reading/Woolwich sand beds, Thanet sands and the London Clay. These strata were deposited around 60-50 million years ago during the Eocene Period. The fossil evidence within them clearly demonstrates that southern England lay beneath a warm tropical sea when they were deposited. The sands contain both estuarine and freshwater species and there is evidence of animals boring into the underlying chalk. Leaves and the remains of vertebrates are also found.

The London Clay, deposited as a sediment in the tropical sea, is a stiff dark or bluish-grey clay which weathers to brown. One characteristic of the stratum are the septaria. These are concretions of argillaceous limestone and occur as layers of nodules. They have sometimes been found to contain numerous fossils. They are commonly known as cement stones as they were used by the inventor of Portland Cement, Parker, in 1791. Thee is considerable variation in thickness. The maximum of 130 metres (430 feet) is seen at Wimbledon, Esher and Brentwood and on the north side it is about 106 metres (350 feet) at Highgate. In the centre, there has been considerable erosion so that it is between 26 and 40 metres (85 and 130 feet) in the City and only 19 metres (63 feet) in Tottenham Court Road.

The London Clay contains numerous plant remains and hundreds of specimens, belonging to more than fifty species, have been collected. Large blocks of fossil wood are known (mainly from Sheppey but the great majority are fruits and seeds. One of the most striking plants represented is a palm which is very similar to the modern Nipa palm found in the Sunderbunds at the mouth of the Ganges. Remains of the Sabal also occur. Other vegetation included magnolia, mangroves, laurels, acacia-like plants and numerous vines.

The mammals are even more interesting. These included crocodiles and Coryphodon, a primitive hippopotamus. They had feet shaped something like those of an elephant and had five toes. They also had strong canine teeth and were omnivorous feeders. The group of ungulates to which it belonged has no living descendants but was at one time the most prominent group of ungulates, the Amblypoda. Another mammal was the Hyracotherium, about the size of a fox but the ancestor of the modern tapir, rhinoceros and horse. It belonged to the order Perissodactyla, ungulates with an odd number of toes, and had four toes on the front and three toes on the hind feet, covered by hooves.

Birds are represented by two main species. The Odontopteryx was a sea bird the size of a gannet. It had strongly serrated jaws which were probably covered by horn. These probably enabled the bird to seize the fish which constituted its diet. The other is the Dasornis. Little is known about it apart from the fact that it was large and may have resembled the ostrich.

The ancient tropical Sea that covered London 50 million years ago. A reconstruction of what the area may have looked like by E Marsden Wilson, 1909.

This tropical sea gradually disappeared and was succeeded by an ancient river system around 40 million years ago. Its waters rose in the south-west of England and deposited its sediments in a broad floodplain which extended over what are now Middlesex and northern Surrey. These deposits consist of fine sandy deposits known as the Bagshot Sands but have been all but eroded by subsequent geological activity so that only small cappings remain on some of the hills in the region. Another consequence of this later erosion, which was at its most severe during the last ice advance, was to expose large tracts of the London Clay across the region.

The river system underwent gradual geological evolution until about 2-3 million years ago when a somewhat steady state was achieved. In this, a forerunner of the Thames followed a course some distance to the north of the present river system. Its waters rose out to the west, in Wessex, and it flowed through what is now the vale of St. Albans and the Mid-Essex Depression on its eastward journey to the North sea. It was joined by tributaries which rose on the chalk slopes to the north and south. This equilibrium system lasted until the onset of the last Ice Age around 500,000 years ago.

The Ice Age totally disrupted the drainage system in the proto-Thames Valley. The first advance of the Anglian ice-sheet blocked the Mid-Essex Depression and forced the proto-Thames and the Wey-Mole systems to overflow across the Hampstead-Epping ridge to the Romford river. During the subsequent retreat the proto-Thames continued to utilise the Lower Lea-Romford river route. However, the re-advance of the Anglian ice-sheet blocked the proto-Thames route in the Vale of St Albans and the Finchley Depression. The river was then forced into its present route.

The final and gradual retreat of the ice-sheet caused great fluctuations in the volume of river flow when the supply from the melting ice varied. As a result, the Thames was, at times, as large as the modern Ganges whilst at others it shrank to no more than a fraction of this. Naturally, the distribution of deposited alluvial material varied accordingly and the exposed London clay was blanketed with this material over a wide area.

These glacial fluctuations interrupted the normal fluvial sequence. This is the evolutionary cycle of erosion and deposition which gradually shapes a river valley, the final form of which is reached at the equilibrium or steady state condition in which these complementary processes are balanced. Such interruptions in the cycle give rise to a revival in the erosive capability of a river so that the river is said to become rejuvenated.

The Thames Terraces

A rejuvenated river begins a new cycle of erosion and deposition and thus superimposes new features on the landscape already sculpted by earlier river action. This has two effects on the cross-profile of a river system:

  1. It produces, by both lateral and vertical erosion a series of terraces on either side of the channel;
  2. The channel itself becomes more deeply incised into the surface although the course of the original meanders is maintained.

Both of these effects are exhibited in the Thames Valley.

The development of the Thames Terraces was a highly complex process whose stratigraphic problems remain unresolved although the system has been studied for more than 150 years. One of the more superficial complications in the study of this system lies in the shifting terminology used to describe these Terraces. Not only has the terminology itself evolved with increasing research, but two different systems are used to describe the Terraces of the London Basin and the Oxford Clay Vale. The development of the nomenclature since 1912 has been summarised recently.

The concepts behind the formation of river terraces, however, remain relatively simple, When a river renews its down-cutting, it sinks its new channel into the former floodplain, The latter is then left well above the level of the new river and the remnants form terraces on either side, The new river valley is then gradually widened so that the terraces are further cut back by the resulting lateral erosion.

If renewed rejuvenation takes place, the process is repeated and another pair of terraces are created at a lower level than the first and any subsequent terrace creation will be at lower levels still. In the Thames Basin, successive periods of rejuvenation created three terraces. The original material deposited in the valley consisted of gravel and alluvium but the latter is easily removed during episodes of erosion with the result that most of the older terraces are covered with gravel sheets. These are the Thames gravel Terraces, the youngest of which descends below the present level of the Thames.

Each of these Terraces therefore consists of a sheet of gravel and sand with a deposit of alluvium over it. The gravels are composed mostly of flints derived from the chalk and flint-pebbles derived from the Eocene deposits. The matrix of sand varies in the degree of its coarseness and the whole deposit may be between 9 and 12 metres in original thickness. Where the alluvial capping has not been subsequently removed by erosion or human activity it can reach a thickness 6 metres or more and is a reddish-brown loamy clay. (This deposit was used in the Roman and subsequent historical periods as the raw material for brick making and has come to be known a "brickearth".)

The Thames Gravel Terraces

The oldest (and highest) of the Terraces ("I", in the figure) covers the least area because much of it has been eroded and incorporated into the later and lower terraces. Downwash from this to the next terrace makes a steady slope which makes differentiation particularly difficult. In many areas, however, they are separated by a strip of exposed underlying "solid" geology. Fragments of this Terrace are found south of Hillingdon, in Islington, Wanstead, Wandsworth, Clapham Common, Croydon, Dartford Heath and Swanscombe.

The middle ("II" in the figure and youngest ("III") Terraces are more extensive, and III extends below the level of the modern Thames. This is the result of the sea level changes discussed below. In Terrace III, beds containing arctic plants have been found and these have been accompanied by other fossils, including the mammoth Mammuthus primigenius, which suggest a cold climate.

There are continued problems with the chronological relationships between the three Terraces. Since the younger Terraces incorporate erosion material from the older they are often distinguishable only by their relative elevations (the elevation at the bottom of the deposit, rather than its surface, being taken for the measurement). The process of erosion and deposition, rejuvenation and alluvial build-up is repeated on a smaller scale in the tributaries of the main river and complicates matters further in the floodplains of these rivers. The Fleet river is a good example of this.

In the Ludgate Hill area, the geology is that of the second Thames gravel Terrace and the alluvial deposits of both the modern Thames and the Fleet river. The brickearth capping on Ludgate hill is now much reduced as a result of erosion and human activity. The Terrace gravels themselves may have been eroded by river action so that outcrops of the underlying London clay have been exposed along the course of the Fleet in its deep ravine at Holborn.

Changes in Sea Level

The melting of the ice sheets at the end of the last Ice Age will naturally have caused an increase in the global Mean Sea Level (MSL) relative to the land. In the area of the British Isles, Ireland was cut off at an early stage and between 8,000 and 7,000 years ago the Straits of Dover were breached. It is significant that this coincides with the onset of the present Atlantic Period of Climate and it has been suggested that the high rainfall associated with this Period can be attributed to the establishment of a full oceanic current around Britain.

This general increase in the MSL has continued to the present time but at a much slower rate. However, it is complicated by the phenomenon known as isostatic uplift - the recovery of the land at the end of glaciation. Put simply, the weight of a large ice-cap puts such a load on the underlying earth' s crust that the land is depressed during the period of glaciation. When the ice melts, the land mass slowly returns to a balanced altitude and the relative MSL therefore falls. This recovery has been slow and unevenly paced and is still a feature of eastern Scotland.

In those areas where the ice-cap was relatively small, such as Wales and the Lake district, there will be minimal depression during glaciation, whilst in areas, such as Southern England, which saw no glaciation, there will have been none at all. This has the very important consequence that these areas will suffer an isostatic depression, relative to the uplifting land and the Mean Sea Level, in the post-glacial period. Thus, the land mass forming the east of Scotland is currently rising whilst that in the Thames V alley is sinking.

It therefore follows that the isostatic uplift will tend to offset the general increase in the MSL whereas the isostatic depression will tend to aggravate it. At London Bridge, for example, the combined effect of the two phenomena has produced an estimated increase in the MSL of 0.73 metres per century since 1791. However, the process is far from regular and periods of Regression have occurred in which the increase in relative MSL has been reversed and an actual fall in levels achieved. A Regression can last for centuries and may be triggered by a drop in the global temperature which causes more ice to be locked up in the Polar ice-caps and ice-sheets.

The fall in temperature will also give rise to a concurrent change in climate, introducing a new weather pattern with increased rainfall. This, in turn, will lead to increased runoff from the land and, if prolonged, may initiate a rejuvenation of some river systems. Additionally, in tidal estuaries, the ratio of fresh to saline water will increase and this may lead to a change in the associated eco-system including improved vegetational growth at the margins of the estuary.

In a prolonged Regression there will be a change from saltmarsh to reedswamp conditions and, eventually, trees such as alder, birch or willow, will become established on new areas of dry land. The end of a Regression is accompanied by a marked increase in tide levels which can restore the original estuarine conditions and much increased erosion as a consequence. Such a cycle can leave a very complicated stratigraphy on the original geological landscape.

Further reading:

Clout, H (ed.) The Times London History Atlas, London, 1991.
Baker, C A & Jones D K C, , Glaciation of the London Basin and its Influence on the Drainage Pattern: a review and appraisal, in Jones, D K C, (ed.), The Shaping of Southern England, Institute of British Geographers, Special Publication, No. 11, 1980.
Monkhouse, F J, Principles of Physical Geography, University of London Press, 1970.
Green, C P & McGregor, D F M, Quaternary Evolution of the River Thames, in Jones, D.K.C., (ed.), The Shaping of Southern England, Institute of British Geographers, Special Publication, No. 11, 1980.
Sherlock, R L, British Regional Geology, London and the Thames Valley, HMSO, 1960.
Megaw, J V S and Simpson, D A A, Introduction to British Prehistory, 1979Sparks, B W, Geomorphology, 1972.
Everard, C E, On Sea-Level Changes, in Thompson, F H (Ed) Archaeology and Coastal Change, Royal Society of Antiquaries, Occasional Papers No.1, 1980.

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