Landscape
Drift: Quartzite Pebbles
"From many an ancient river..." or a load of old cobbles. OGG President, Phil Powell, writes about these well travelled pebbles.
If you walk on Cumnor Hurst or on the hill to the east near the beacon you may have noticed underfoot brown cobbles like the ones in the picture (below). They are heavy, smoothly rounded and are made of a dense kind of sandstone called quartzite. On the Hurst these stones, mixed with other pebbles and sand, form a deposit of gravel which fills what remains of an ancient river channel.
Similar patches of the same sort of gravel occur on the high ground across Oxfordshire on a north-west to south-east line between Wychwood Forest and the Sinodun Hills. The localities include Ramsden, Combe, North Leigh and Boars Hill. This pattern of distribution indicates the existence of a large river ancestral to the Evenlode and Thames. Other geological evidence shows that the river was active 500 k years ago
That the head of this river reached far beyond that of the modern Evenlode into the Midlands and perhaps North Wales is evident from the wide variety of rocks found in the gravels. The quartzite pebbles are especially abundant and noticeable and they can be traced back to their sources the Pebble Beds of the West Midlands.
The pebble beds themselves were formed around 250 million years ago (MYA) from the sediments dumped by another ancient river. The river flowed northwards from the area of Brittany where it was eroding sandstones that were themselves already 200 million years old. On its way north it gathered up other rock types in its path. The Pebble Beds therefore contain pieces of quartz from Cornwall as well as slates, lavas, limestones and sandstones. Millions of years later, these rocks were eroded out of the Pebble Beds and carried by the ice-age Evenlode into Oxfordshire and beyond.
LOWER GREENSAND.
deposited c. 115 million years ago
GRAVEL with QUARTZITE PEBBLES deposited c. 600 thousand years ago
GRAVEL with QUARTZITE PEBBLES deposited c. 600 thousand years ago
GAULT CLAY
deposited c. 110 million years ago
KIMMERIDGE CLAY deposited c. 110 million years ago
former river channel
Section of strata at Chawley Brick Pit, Cumnor as seen circa 1920
Quartzite Pebbles [longitudinal dimension = 10 cm]
In fact, the quartzite bearing gravels can be traced through the Goring Gap into the London Basin. From Maidenhead they pass north-eastwards towards Watford and continue on through the Vale of St Albans into East Anglia. This old route of the Thames was blocked by ice near Maidenhead about 470,000 years ago, forcing the river to turn south-eastwards into its more or less modern channel.
The ordinary-looking quartzite cobbles have experienced many changes of scene in their long history from ancient rivers and desert plains to glacier and tundra. Because they are so durable they are likely to be around much longer yet, perhaps much longer than the human race.