Portland quarries and stone - the geologists' view
Jane Francis (University of Leeds) and Tim Palmer (University of Wales)
The layers of rock in Jordan Quarry tell a unique story of ancient environmental change in the Portland region about 150 million years ago during the Jurassic period. By detailed study of the components of each bed of rock - the microscopic particles that form the body of the rock, the plant and animal fossils, the chemistry of the stone, and the subtle changes in thickness and composition across a quarry face, geologists can piece together reconstructions of how the rocks formed and what this area was like millions of years ago.
These rocks formed about 150 million years ago at a time when Britain was located much closer to the equator - the latitude of Portland at that time would have been about 30°N, comparable to the present position of Morocco and Tunisia on the Mediterranean coast of North Africa. The climate of the Portland region then would have been similar to the Mediterranean region and indeed evidence from rocks and fossils tells geologists that it was a warm and sunny place with warm wet winters and hot dry summers.
Fossilised beach in Tout Quarry
The rock layers near the base of the quarry, known as Portland stone, are limestones that formed in clear, warm shallow carbonate-rich seas, similar to those in the Bahamas today. The individual grains of the rock are called ooliths, and consist of minute spheres of lime (calcium carbonate) that grew by precipitation of lime in the sea-water, rather like lime-scale grows in a kettle. Seen with a microscope, an oolith looks like a tiny gobstopper, made up of many concentric layers growing around a central nucleus. Layers of fossil shells in these rocks show that shell banks were common, often composed of large flat oysters, clams or long pointy snails.
These warm seas were also the home of the giant ammonites, a squid-like creature that had a large coiled shell, now found as large fossils all over Portland, both in the quarries, in the museum and in many islanders' garden walls. Locally, reefs of hardened rock made by shellfish and lime-secreting bacteria stuck up as rocky outcrops on the sea-bed.
The rock layers in the upper part of the quarry above the building stone are called the Purbeck Beds. They are also limestones but they are different types because they formed in a different environment. About 5 million years after the limestones of the Portland Stone were formed in the shallow seas, the sea level in this region dropped and a large enclosed lagoon was formed. The water supply to the lagoon was restricted and in the hot summer months the lagoon water often evaporated and salt crystals formed. Impressions of cubic salt crystals can often be found in the layers of slatt in the beds at the top of Jordan Quarry.
Tiny fossil insects can also be seen in these rocks; these are the remains of insects which died as they fell into the salty water. Algae thrived on the shores of the lagoon and the sticky mats of algae trapped mud and built up large mounds, called stromatolites. These mounds form the main part of the beds called the Caps by the quarrymen.
On the land behind the lagoon, forests of conifers and the palm-like cycads grew in deep rich soils. These conifers were a special type of Cupressus which is now extinct, but they were able to tolerate these hot arid conditions. The tree stumps, trunks and branches of these trees became fossilised and can still be found today in the quarries in the fossil soils, called Dirt Beds by the quarrymen. The fossil wood is so well preserved that the original woody cells can be seen with a microscope, and in cross-sections of the trunks the concentric tree rings can be seen.
These rings hold a unique record of the climate conditions millions of years ago that affected the trees when they were growing. The rings in the Purbeck fossil trees on Portland tell us that the climate was hot (average temperatures of about 20°C) and very seasonal with light winter rainfall (about 800 mm per year) and hot dry summers. Some of the forests became periodically flooded, and the stromatolites quickly grew on the dead trunks, helped by lime-secreting bacteria. When the trunks and fallen branches subsequently rotted away, holes were left in the limestones - these 'Chaff Holes' are common in the thick limestone known as the Hard Cap, near the base of the Purbeck Beds.
Aptyxiella portlandica ("Portland Screws") from the Roach Bed.
The fossilised squat stems of the cycad plants are much rarer (some can be seen in Portland Museum) and were thought to be fossilised birds' nests by quarrymen because they were always found alongside fossilised trees. Years ago when the quarries were worked by hand, and the strata exposed layer by layer, it was possible to see spectacular numbers of fossil trees laying in their original positions on the Dirt Beds. In 1897 Thomas Hardy, in his book 'The Well-Beloved', noted:
"To find trees between Pebblebank and Beal it was necessary to recede a little in time - to dig down to a loose stratum of the underlying stone beds where a forest of conifers lay as petrifications, their heads blown down by the gale in the Secondary geologic epoch."
Early on, the limestones that had started off as soft piles of sediment in a shallow sea became hardened into rock. This hardening, which geologists call lithification, happened as a result of tiny crystals of the lime mineral calcite growing into the minute spaces between the sediment grains, acting as a natural cement. Since the formation of the Portland Stone and Purbeck Beds during the Jurassic this region had been uplifted above the present sea level. The surrounding rocks have been eroded away, leaving Portland as an island containing a unique record of what life was like in this area millions of years ago.