Bucks Earth Heritage Group

Portland

There are few in-situ exposures of Portlandian strata in Bucks, although this limestone and the fossils it contains can be seen over much of mid-Buckinghamshire, where it has been the local choice of building stone. Without doubt the most famous Portlandian fossil is a very large ammonite called Titanites giganteus. Examples were built into the walls of Hartwell House estate in the 19th Century and similar uses are widespread, as far afield as Buckingham town.

The Portlandian rocks were deposited during the latest Stage of the Jurassic Period. They represent a time when warm, tropical seas marine conditions to the south, extended north as far as Buckinghamshire, some 144 million years ago. The famous Portland Limestone from Dorset (after which these rocks are named) is a thick, massive series of rock, whereas, in and around Aylesbury, we find just the marginal remnants from this seaway, which terminated somewhere around the Bedfordshire border. Farther to the north-east, in East Anglia to Yorkshire, is a series of clays and sands of the same age, but part of a huge shelf sea stretching across Northern Europe to Russia.

The Portlandian rocks of Aylesbury can be summarised as follows:

Creamy Limestone

Crendon Sand

Aylesbury Limestone

Glauconitic Bed

Upper Lydite Bed

The Creamy Limestone is cream to pale grey and very fine-grained, with small gastropods particularly abundant. The Crendon Sand is favoured by badgers for digging their setts, and can be mapped out on hillsides in this way. The Aylesbury Limestone, which caps the top of the hill at Aylesbury, is pale orange-grey and grotty, due to the amount of shell material, It yields many fossils of bivalves, gastropods and ammonites, Beneath the Aylesbury Limestone, a soft, mustard-yellow weathering sand (Glauconitic Bed) passes down in turn into a thin rib of black and then green sandstone , with a few small, black chert pebbles. This is the Upper Lydite Bed. It also contains occasional remnants of fossil ammonites derived from a zone of the underlying Kimmeridge Clay which is no longer represented around Buckinghamshire, proving that the earliest Portlandian stage was one of slight erosion in our area. In the early 20th Century, pits in Portlandian rock included that at Warren Farm, Stewkley, where a small, preserved, exposure still exists. Photographs of this pit, in its heyday, can be seen below, courtesy of the British Geological Survey collection:

Hover cursor over photograph for label.

Many fossils from the Portland Limestone weather up to the surface where this rock outcrops (on top of the hill on which old Aylesbury is built and around Walton in particular). Some illustrations of these are added below.

 

As well as its use for general walling stone, the Portland Limestone has attracted other uses...

The gypsy kings gravestone, at the bottom of Pitchcott hill. Legend has it that when gypsy kings die a stone is rolled down a hill and where it comes to rest is to be the location of the grave.

A farm wall at Pitchcott. Here the Portland Limestone was sourced not far away from a small quarry between Oving and Pitchcott. Fossils (bivalves) can be seen in the central block and in the close-up below.