Walking in London. Peter Aylmer

      The Capital Ring is a well-signposted London trail

      This book asks you to invert your view of London – to see it not as a city for humans, but as a range of habitats for wildlife – and this is incontrovertibly best done on foot. A corollary of London’s greenness is that there are remarkable opportunities for the walker – one National Trail, six regional trails, and many more local ones, all taking advantage of over 600 miles of signed footpaths and countless extra miles of informal paths. You will from time to time encounter roads and houses – but on every one of these 25 walks, you will often wonder where all of these have gone.

      That said, London is clearly a city that the hand of man has shaped in extreme ways, dating back now over two millennia. It would be foolish to say that even the off-the-beaten-track stretches, which you might visit with this book, are immune. Even the verdant open landscapes of the Lake District are highly artificial in their way, the result of centuries of tree-clearance for sheep pasture which, if mute economics were allowed to take its run, would soon become afforested again. Perhaps a better way of looking at things is to accept that no landscape of Britain – from the great hillscapes of the Scottish Highlands to the long level fields of Fenland – is free from human influence. The question is, where is the line drawn between influence and overt domination?

      In the case of London, it is a question without easy answer, bound up in the approaches of Londoners and their authorities (regal and mercantile, state and municipal) to the needs of humans in the city. And that, in turn, depends in part on its geology, and the very particular circumstances of an invading force of Romans in the first century AD.

The geology of London

      If there were no city, there would be a great tidal flood plain, as the Thames made its way to the sea. It would be maybe five times the width of the current river. One of the meanings of the word ‘strand’ is ‘bank of a river’; the central London thoroughfare known as the Strand, now 200 metres from the river, was named in 1002 as ‘Strondway’ because then the Thames lapped its edges.

      The Roman army that Aulus Plautius commanded in AD43 landed in Kent and soon had a beachhead on the south bank of the Thames opposite what is now Westminster. A ford was practical here (it was then, roughly, at the tidal limit) and the army advanced to its first capital in England, Camulodonum, now Colchester in Essex, where it took over a Celtic fortified town. It was soon apparent, however, that the Thames would have to be bridged if supply lines were to be effective. A pontoon bridge in the vicinity of what is now London Bridge was replaced by AD55 by a permanent structure, and on its north bank the Romans started to create a new town from scratch, which by AD120 was known as Londinium.

      The north bank was more favourable than the south as three little hills, now Ludgate Hill, Tower Hill and Cornhill, each rising barely 15 metres from the river, afforded some protection against flooding and perhaps some relief from insect life, which is why London’s core is where it is. To gain some idea of what the territory must have been like, look at the marshes around Tollesbury, just south of Colchester, a warren of mud, channels and islands through which progress is difficult to this day.

      Then, as now, the Thames, rising 215 miles away in the Cotswold hills, drains much of south-east England, and is the longest river entirely within England. For the last quarter of its length, it runs across a flat plain of clay laid down around 50 million years ago and so specific of its type that it is known as London Clay. Bricks made from it are yellow, and easy to distinguish throughout the capital. But for agriculture London Clay mostly gives rise to poor, alkaline soils, and in prehistoric times the flood plain supported fishing and rough pasture but little in the way of crops.

      On a wider scale this clay – which covers most of Essex to London’s east as well as much of modern London – is encircled by the chalklands of the foothills of the Chiltern Hills to the north and the North Downs to the south. In the south London boroughs of Croydon and Bromley there are examples of downland that could easily be mistaken for the South Downs of Hampshire and Sussex.

      But there is a smaller scale, too, the most significant of which are the gravel beds and terraces. In south London, the sandy Lambeth beds are associated with heathland and acid soils. The Bagshot beds, named for the Surrey town, spread into London’s south-west and also cap some of north London’s higher parts, such as Harrow and Hampstead; they reappear too in Epping Forest, on London’s north-east boundary with Essex.