River Brent - Geology, Topography and Natural History

Geology, Topography and Natural History

The River Thames can first be identified as a discrete drainage line as early as 58 million years ago, in the Thanetian stage of the late Palaeocene epoch. Until around half a million years ago, the Thames flowed on its existing course through what is now Oxfordshire, before turning to the north east through Hertfordshire and East Anglia to reach the North Sea near Ipswich. At this time the river system headwaters lay in the English West Midlands and may, at times, have received drainage from the North Wales Berwyn Mountains. The river Brent and its valley's formation was the result of glacial action during the ice age which had started some 500,000 years ago; in that period of the early neogene, the major drainage channel for this part of England, the proto-Thames, was 25 miles (40 km) north of Brentford and travelled east via the St Albans depression. The River Brent and adjacent tributaries the Colne Brook and those downstream such as the River Lea either flowed in to this more northern Thames or formed the early course of the present day river Thames.

The arrival of an ice sheet in the Quaternary Ice Age, about 450,000 years ago, dammed the river in Hertfordshire causing large ice lakes which eventually burst their banks and caused the river to be diverted onto its present course through London. Progressively in this Ice Age, the northern channel was pushed south to form a lake, now the St Albans depression, by the repeated advances of the ice sheet. This created pressure to form the Goring Gap and a new river route through Berkshire, joining with the Kennet that formed the early southern headwater and on into London after which the river rejoined its original course in southern Essex, near the present River Blackwater estuary. Here it entered a substantial freshwater lake in the southern North Sea basin. A torrent produced by the rupture of this lake (now a sea) was a major cause of the formation of the Dover Straits or Pas-de-Calais gap between Britain and France. Subsequent development led to the continuation of the course which the river follows at the present day.

Most of the bed rock of the Vale of Aylesbury is largely made up of clay and chalk that was formed at the end of the ice age and at one time was under the Proto-Thames, creating vast underground reserves of water that make the water table higher than average in the Vale of Aylesbury from Thame to Hemel Hempstead were created.

The last advance from that Scandinavian ice flow to have reached this far south, covered much of north west Greater London and finally forced the proto-Thames to take roughly its present course. At the height of the last ice age around 10,000 BCE, Britain was connected to mainland Europe by a large marshy expanse of land known as Doggerland in the southern North Sea basin. This forced flow southwards from the eastern Essex coast where it met the Rhine, the Meuse and the Scheldt flowing from what are now the Netherlands and Belgium. These rivers formed a single river—the Channel River (Fleuve Manche) that passed through the Dover Strait and drained into the Atlantic Ocean in the western English Channel.

The ice sheet which stopped near Finchley deposited Boulder clay to form Dollis Hill and Hanger Hill. Its torrent of meltwater gushed through the Finchley Gap and south towards the new course of the Thames, and proceeded to carve out the Brent Valley in the process. Upon the valley sides there can be seen other terraces of Brickearth; laid over and sometimes interlayered with the clays. These deposits were brought in by the winds during the periglacial periods, suggesting that wide flat marshes were then part of the landscape, which the new river Brent proceeded to cut down. The steepness of the valley sides is witness to the very much lower mean sea levels caused by glaciation locking up so much water on land masses, the potential energy causing the river water to flow rapidly seaward and so erode its bed quickly downwards.

The original land surface was some 350 to 350 to 400 ft (110 to 120 m) above the current sea level. The surface had sandy deposits from an ancient sea, laid over sedimentary clay (this is the Blue London Clay). All the erosion down from this higher land surface and sorting action by these changes of water flow and direction, formed what is known as the Thames River Gravel Terraces. Since Roman times and perhaps earlier, however, the isostatic rebound from the weight of previous ice sheets, and its interplay with the eustatic change in sea level, means that the old valley of river Brent, together with that of the Thames, has been silting up again. Thus along much of the Brent's present day course one can make out the water meadows of rich alluvium, which is augmented by frequent floods.