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Ice Age to Middle Ages

Prehistory

London is located largely around a shallow basin - 'the London Basin' - through which flows the Thames with ridges of hills to the north and south, and the floodplains widening as the Thames broadens eastwards to the North Sea.
 
This basin is underlain by a broad seam of chalk laid down over 70 million years ago, exposed at the North Downs and the Chilterns, overlain by thick glutinous London Clay and, at certain high points (for example Totteridge, Hampstead Heath and Sydenham Hill) covered by pebbly Claygate Beds laid down in shallow seas some 50 million years ago.
 
The broader flood plains alongside the Thames and her tributaries are covered in alluvial gravels and soils. This geology has helped shape and continues to influence London's nature today.
 
The city’s nature developed around the wetlands bordering the Thames, which used to be very much broader, muddier and slower-flowing.
 
Following the retreat of glacial ice sheets some 7,000 years ago, and warmer temperatures, grasslands and then woodlands started to colonise the area.
 
By the time London was first colonised by people, it would have been covered by oak and hornbeam woodland, punctuated with open glades maintained by large herbivores - aurochs, elk and deer - with broad swathes of wet marshes, willow woodland, reedbeds, and mudflats by the rivers.
 

Early clearances

As London first became settled, especially after the Roman invasion of AD43, efforts were made to clear woodland to create pasture for grazing livestock and fields for crops, and embank and control the rivers.
 
The Norman Conquest changed land ownership through the Manorial system, and establishing forests - hunting grounds for the aristocracy - across much of southern England including those of Epping and Middlesex.
 
Common land, owned by the Lords of the Manors, allowed certain local tenants (known as 'commoners') to use the land at times for grazing and the collection of firewood.
 
Many would have surrounded London, surviving largely intact until the enclosure acts of the late 18th century, and still inform land patterns in the capital today.
 
London quickly grew from the 11th century, and her surrounding countryside became more and more tamed to feed and support the growing population.
 
By the 15th century, large tracts of woodland were managed to provide timber, charcoal, and oak bark (for leather-making) - Epping Forest, the Great North Wood (around Sydenham), and around Ruislip.
 
The river, used for fishing and transport, slowly became more polluted, so much so that Acts were put in place to conserve fish stocks.
 
Some birds of prey, especially red kite and raven, were protected from persecution due to their role in scavenging from rubbish tips outside the city walls and keeping vermin down.
 
Elsewhere, however, persecution led to the extinction of species such as wolf, and by the 17th century wild boar, from the area.