From Earth Science On-Site
Dryhill Nature Reserve, Kent
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Background information for KS2 teachers
The story of Dryhill Nature Reserve is told by the evidence contained in its rocks. The evidence to explain the sequence of events that took place over millions of years is there for you and the children to find. The Teaching Trail Notes list the Key points to investigate:
Summary of the Geological history
The rocks of Dryhill consist of alternating layers of hard sandy limestone [called “rag” or “Kentish ragstone”] standing out from soft sandstone [called “hassock”], which is less resistant to weathering and erosion. Individual beds vary in thickness from a few centimetres up to a metre. They belong to the Hythe Formation, one of four sub-divisions of the Lower Greensand, deposited during the Cretaceous Period. The name “Greensand” was used by William Smith in 1815 as the rocks contain glauconite, a green mineral. The Greensand hills form distinctive scenery around the Weald.
Time Sequence Of Events
The geological history is summarised in the follow-up notes
Sequencing exercise on the story of Dryhill. This could be illustrated as a cartoon story.
The evidence for the events in The Rock Cycle can be “read” from the rocks in any exposure. However, some parts of the story are always missing, since geological evidence has many “gaps” in it caused by a combination of never having been deposited and preserved in the first place, loss by erosion, and the fact that much is still buried and unknown. This means it is important to remember that the “story” at any one site is but a short part of a single Earth Science story that has an invisible “prologue” and “epilogue” each millions of years long, but for which we cannot see the evidence at any one site, because it is not available to us.
At any one site it is helpful to think of interpreting the evidence in a recurring pattern of events:
This is the story at Dryhill, Kent, and the evidence for it.
Events Forming The Rock Cycle
Event a) Transport and Deposition
The layers of rock exposed in the Dryhill quarries are alternating beds of sandy limestones and sandstones. The fossils they contain indicate they were deposited near the beginning of the Cretaceous period. From other evidence we have an age of about 115 million years for this part of the geological period. These beds were laid down horizontally in a marine environment.
Beneath the oldest bed in these quarries (and therefore not visible here) are many layers of older beds, which are exposed in other parts of the south of England. These older beds contain fossils of a freshwater snail (Viviparus) indicating they were deposited in a freshwater lake area, widely known as “The Wealden Lake” with sediment being transported into it by rivers, as indicated by the cross bedding in the sandstones, and the plant fossils they contain. These beds also have fossils of the dinosaur Iguanodon, suggesting that the land area around the lake had a food chain, with vegetation, herbivores, like Iguanodon, living on it and carnivores, preying on them. There was also a fish eating dinosaur called Baryonyx walkeri. None of these rocks, or these fossils, are visible at Dryhill, since they are older, and buried below.
This lake area later became flooded by the sea to form an arm of shallow sea linking a northern sea with a deeper ocean to the south, across what is now southern Europe. This narrow area of shallow sea was swept by tidal currents from the northwest and southeast, as indicated by cross bedding in the rocks (not so easily seen at Dryhill). From evidence in other parts of Europe, reconstructions show our area as a narrow shallow sea, linking a deep ocean area, called Tethys, (in which the rocks now forming the Alps were deposited) with a northern sea. The area we now know as the North Atlantic was occupied by North East America, which had not yet separated from Europe. See Figure 1.
It was in this tide-swept sea area that the beds at Dryhill were deposited. The evidence for this is that they contain marine fossils, particularly echinoids and bivalves, made of the mineral calcite, as well as the mineral glauconite (a hydrous silicate of iron and potassium, although often with aluminium, calcium and magnesium) which forms in oxidising shallow marine environments, and gives the rocks a greenish colour, but only when fresh and un-weathered. This colouration gave rocks from this part of the geological column the name Lower Greensand, (although they are not all sands, and are brownish, from the iron, when weathered).
The rocks at Dryhill are alternating beds of sandy limestones, called “Ragstone” and sandstones, loosely cemented by calcite, called “Hassock”. These beds are each between 0.3 and 0.9 metres thick and form a total of 30 metres thickness, with about 56% being sandy limestone. Studies have suggested possible reasons for this alternating pair of rock types.
It would be natural to expect that these beds of alternating rock type formed when rivers flooded from adjacent land areas, brought in more sand, (i.e. a stormy, wet period) and the limestone formed when rivers provided less sand (i.e. a drier period) as calcite was precipitated from seawater, either by marine animals forming skeletons or evaporation. However, studies of these two rocks suggest that sand was always being transported into this sea area by rivers flowing over land areas to the north and east. The main differences between the two rock types are that, compared with the sandstones, the limestones have less quartz sand and more broken echinoid skeletons, being well cemented by calcite. The sandstones have more quartz and glauconite, and are only loosely cemented together by calcite. This suggests that sand was being deposited almost all of the time, and that the main difference is that periodically the amount of calcite, in the form, of broken calcite echinoid skeletons increased, and later recrystallised as calcite to form a limestone (with quite a bit of sand in it).
Possible explanations for the repeated increase in deposition of calcite in the form of broken echinoid skeletons include:
Younger rocks have been eroded from this area and are not available to us, but from other parts of Kent we know that more marine Cretaceous rocks were deposited on top, ending with the 300 metres thick Chalk deposits, now forming the Downs to the north. These are almost pure calcium carbonate, indicating marine deposition with very little material being transported from the land, or a position well away from the coast, and rivers bringing in sand and mud.
Event b) Deformation - Folding and Faulting
Periods of folding and faulting of the Earth’s crust are usually long drawn out and complicated events in detail. Here we will simplify things as if they are more or less a single event occurring after the deposition of the beds at Dryhill (and, of course, after the deposition of the Chalk). The main folding events are dated to approximately 24 million years ago, and are part of the crustal compressions and uplift that created the Alps in southern Europe from the sediments deposited in the Tethys Ocean. The Alpine area is interpreted as the site of a destructive plate margin, closing the Tethys Ocean, and causing folding, faulting and uplift as far away as southern England.
When placed under stress rocks may behave in a plastic fashion, and produce folds, or in a brittle fashion, and produce faulting. Both features are seen at Dryhill, which is part of the larger Wealden anticline (up-fold), but here the folding is far more obvious than the faulting.
Folds are caused by plastic deformation of rocks under compression, where the major direction of force is at right angles to the fold axis. When the forces are not equal the folds “lean away” from the direction of strongest force (i.e. are asymmetrical, with steeper dips away from the major force). See Figure 2 where the major force was from the right as the card was used to deform the sediment.
At Dryhill, the axis (line of the fold, as indicated by the creases in Figure 3) is almost directly east to west, indicating that the direction of major force was from north to south. Since the folds here are almost symmetrical (ie beds dip at similar angles on both sides of the fold) it implies that the forces were equal from north and south. From folds in other areas, particularly the Alpine folds, however, it is possible to deduce that the compressional forces were dominantly from the south, and are associated with the closing of the Tethys Ocean, from the south, by Plate Tectonic forces.
There are two basic types of folds: upfolds and down folds. Upfolds are called anticlines, because the dips (or slopes) of the beds are inclined away from the axis of the fold. Downfolds are called synclines because the dips (or slopes) of the beds are inclined towards the axis of the fold. This gives a general rule that, in simply folded rocks, beds dip towards a syncline and away from an anticline. This can be simply modelled using a sheet of paper.
Although it is easier to see folding in a section at right angles to the fold axis, it is important to recognise that folds are three dimensional, and can be found running through the rocks along the line of the axis, as is the case at Dryhill.
An additional complication at this site is that the quarrying for the “ragstone” seems to have favoured the straighter, dipping parts of the folds (see Figures 3 & 4), and excavated along them, leaving the curved beds in the axis of the folds behind. In these parts of the quarry the edges of the folded beds can seem to be horizontal, when in fact they are not. (See Figure 4)
When examined closely, some of the folds at Dryhill show faulting along the line of the fold, although, due to the slipping of sand from the sandstone beds, this can be hard to see. The result is that the beds on one side of a fold can sometimes be different from the beds on the other. Figure 5 shows a pebble exhibiting these relationships in a syncline.
Event c) Uplift, Weathering and Erosion
The rocks at Dryhill are now about 90 metres above sea level, and must therefore have been uplifted at least that far since the end of the Cretaceous period, since they were deposited below sea level.
Evidence for chemical weathering is provided by the thin weathering “crust” (about 1mm thick) on broken specimens of the limestone. This is caused by the weathering out of the calcite by chemical weathering. In the sandstones, loosely cemented by calcite in the first place, the chemical weathering of the calcite cement has produced large quantities of loose sand “screes” at the foot of the slopes. The glauconite, originally green, has been chemically weathered to the brown, almost black, flecks of iron hydroxides found most easily in the sandstone.
The outcrops themselves also show signs of biological weathering with the roots of trees penetrating the joints and bedding planes of these rocks. As part of the management of this site Kent County Council clear this vegetation from time to time. In addition, small children sliding down the sloping sandstone exposures also wear away the sand grains from the beds.
Finally these “ragstones” have been quarried away for aggregate and building stone, because the limestone has been found to be useful. Its main features being resistant to weathering (even though it is a limestone) and being physically resistant (i.e. “hard”).
The “Ragstone” or sandy limestone has been quarried away for aggregate and building stone since at least Roman times. The walls of Rochester fort were built of ragstone. It has also been used for seawalls and building stone as well as road aggregate due to its physical and chemical resistance.
The site has now been protected as a rare exposure of these beds, exhibiting evidence for the environment of deposition of these rocks, and their later folding and faulting. It is also a leisure facility for people in the area.
The beds at Dryhill show evidence of a previous rock cycle of weathering, erosion, transport and deposition, followed by uplift and present day weathering and erosion. In the future the weathered material from this area might be expected to reach the River Darent and be transported north to the Thames and then eastwards to the North Sea, where they will become deposited to form new sedimentary layers, containing present day fossils.
Earth Science Principles
In this area it is possible to demonstrate the following Earth Science principles.