Is it Gneiss? Or is it Schist?

I did a spot of time travelling this morning. In physical terms, the distance I travelled was about 40 miles, but it turns out I actually travelled 130 million years in time…geological time, that is.

It was my morning commute. Normally a depressingly bland rail journey from Tunbridge Wells to Central London but this morning I saw it with new eyes. The train line cuts right through the Weald of Kent, known as the Garden of England but also home to a sequence of rock formations that is so diverse it helped found the science of Geology in the 19th century.



These rocks were laid down in the Cretaceous era, just after the age of reptiles in the Jurassic and just before the age of mammals in the Paleogene. Alternating layers of clay and sandstone were deposited in turn and then topped off with a layer of chalk built up from the tiny calcified skeletons of marine microorganisms.

This perfect layer cake of sediments was then disrupted by the Alpine Orogeny. The continents of Africa and India, drifting slowly northwards, collided with the Eurasian tectonic plate and threw up a series of spectacular mountain ranges as a result: the Alps, the Himalayas, the High Pamirs and the heavenly Tian Shan that look down upon the Silk Route in Western China. In addition, the pressures of this continental collision also caused the Wealden rocks to buckle upwards so a flat plain became an enormous chalk topped dome which would have been one and a half kilometers high, or roughly the height of Ben Nevis.




Ben Nevis is made of hard granite, but this chalk dome was easily eroded. After 70m years of wind and weathering, all that is left of its original majesty is the outcrops of the North Downs and the South Downs and the famous white Cliffs of Dover. So to travel across the face of the Weald is to travel in time across the striations of a chronological sequence of exposed rocks; from the 135 million year old Tunbridge Wells sandstone, through the 70m year old North Downs to the relatively recent alluvial mud of the Thames at London.



If you run your fingers over the local sandstone blocks of the station building in Tunbridge Wells, you are touching the ancient past; a tactile time tunnel to 135 million years ago. If you half close your eyes and open your imagination, you can summon up a vision of a dozen semi-evolved scaly reptiles blinking in the weak sun of a Cretaceous dawn. Not so different, in fact, from the morning commuters on the platform today…


How you see a landscape depends upon your training; you see what you are trained to see. To a geologist, the train trip from Tunbridge Wells to London is a trip through the Cretaceous Era. Scanning the rock formations, he is the one who can distinguish between what is gneiss and what is schist. A painter looking at a sunlit valley may see an exquisite balance of colour, light and shade. A military commander, looking at the same valley, may see ridges to shield advancing infantry and escarpments for setting gun emplacements. It all depends on your frame of mind and what you have been taught.