Buckinghamshire Geology

BUCKINGHAMSHIRE

Buckinghamshire connects the southern Cotswolds with the Chilterns. It has two river basins of note: the Great Ouse in the north and the Thames in the south.

The Cotswold Stone in the north is oolithic limestone from the Jurassic period. This is stone with an egg shaped grain, which is what oolithic means, it was laid down in a sea in the period roughly 200-130 MYA. The Chilterns which begin just south of the Thames run unbroken through Buckingham, parts of Hertfordshire and into Bedfordshire and fade into the chalk of North Essex and Suffolk. The range continues north under the Tertiary and Quaternary deposits of East Anglia to re appear in the Lincolnshire Wolds and East Yorkshire. The chalk is Cretaceous from 130-66 MYA, generally it dates from the 2nd half of that period. It is a marine zoic deposit which seems to have come to an abrupt end with the K Pg mass extinction event- allegedly caused by a meteorite impact off Yucatan. Probably the Jurassic rocks run under the chalk, for in the anticlines of North Buckingham and the Weald they reappear as the domed strata have been eroded. The Thames Basin displays a syncline of chalk overlaid by London Clay and other deposits- so if the geology of the home counties is seen in the large, a great wave formation is observed from anticline to syncline to anticline. These strata were laid down on flat ocean beds and their raising was a peripheral phenomenon of the Alpine orogeny (mountain building period) of 50 MYA in the Pleistocene. The Oolithic limestone of North Buckinghamshire runs west to fade away in the vicinity of Bristol and South Gloucestershire. Companion strata with the chalk are gault clay and greensand- both are deposits from the Lower Cretaceous and were laid down a little earlier than the chalk. The lower land of Buckinghamshire shows Oxford clays and Kimmeridge clays. In the south-east of the county are London clays and Reading beds from the Eocene. The Eocene is an epoch of the Tertiary period and follows the Cretaceous to about 45 MYA. Generally the chalk hills of the county are capped with later material from the Tertiary period. The endemic tree of the region is the beech and Burnham Beeches is a remnant of an ancient and extensive forest. The beech was much used by the chair industry of Buckinghamshire- particularly about High Wycombe- but, interestingly, early “Windsor Chairs” were fruit wood and yew not beech so it might be that there were formerly extensive orchards and yew woods which, when exhausted by that industry, were substituted with the faster growing and less expensive hardwood. All such articles had elm seats and so another now largely vanished tree is represented in them and must have been widespread in this environment.

The Thames which forms Buckinghamshire’s southern border, formerly struck north-east following the north and west facing escarpment of the Chilterns. It then crossed East Anglia in the vicinity of the modern Waverney River and thence north towards the Rhine and Scheldt. It was joined by another ancient river- the Bytham, which formerly ran through valleys now occupied by the Avon and Ouse; thus two courses of two pre-historic rivers are important in the south, east and north of the county. It is difficult to know how much of the county could have had glacial drift material on it. The Anglian Ice sheet of some 250,000 years ago extended south to the Crouch in Essex, eroded the Hertfordshire Chilterns but did not erode the chalk at Dunstable. This was the event which diverted the Thames and it would appear that the southern boundary of the ice was roughly on a south-east north-west line from the Thames Valley in Essex to the High Chilterns of Bedfordshire- but then where?

The second great ice event, the Wolstonian, stopped in the latitude of Coventry and so that would not have deposited drift material here. The last ice age- the Devensian- had a limit much further north (Glamorgan to Flamborough). Of course melt water rivers and lakes from each of these events may well have crossed the county. As the Anglian ice sheet blocked the ancient Thames no doubt a lake backed up across Buckinghamshire until there was sufficient depth and power to force through the Southern Chilterns in the vicinity of Reading, Cavisham and Cookham. Buckinghamshire has interesting isolates of rock strata: Portland Stone, Purbeck stone, Lower Greensand and gault knolls at Muswell and Brill Hills. The evidence of ancient people in Buckinghamshire is curiously scant- except for Paleolithic groups moving up the Thames. Even later, the Saxons did not populate the High Chalk in anything like the numbers seen on the Downs of the Weald and it has been speculated that residual British populations made up the majority of the peoples here. Another curious aspect of the geography is that the most ancient pre Roman routes- such as the Icknield Way, tend to avoid high ground. Elsewhere “High Ways” are the general rule- not “Low Ways”. In summary, from the Paleolithic to the Anglo Saxon settlement, there was something undefined which seems to have dissuaded people from settling certain tracts of Buckinghamshire- particularly the high lands of the Chiltern Hills.