Buckingham sand pit

Geology notes

The sand pit at Buckingham preserves a rare opportunity to see Ice Age geology in England. Despite the fact that large areas of England have been affected by periglacial activity and a varied mix of glacial deposits covers many areas, very little is exposed for geologists to see directly.

The clays, sands and pebble layers within the pit are the only direct evidence we have that Buckinghamshire was once in the grip of a moving ice sheet that measured at least 2 km thick. This occurred during a period known as the Anglian around 500,000 years ago. Following this time Buckingham would have been in the path of the meltwaters of the receding ice, finally to be laid bare in the cold icy winds of the tundra environment. The last chills of the ice age ended 10,000 years ago. We are presently experiencing a warm interglacial period – but beware, the ice will be back!

Ice ages – what causes them?

What we term the ‘ice age’ is actually the last of several global coolings, that lead to massive ice growth and advance. It is within the Quaternary period starting 2.6 million years ago. This is sometimes called the ‘Great Ice Age’, but it is not as intensely cold as others preceding it. Some of these preceding ice ages are highly likely to be the cause of mass extinctions associated with these periods of time – extinguishing at least 70% of all living species of the time. The main ice ages are:

  The causes of ice ages is still one of hot debate (excuse the pun!). There is also confusion over the difference between the onset of a global cooling (ice age) and ice advances and retreats within that period.

The causes of the onset of a global cooling are large scale factors – the big Earth-scale factors such as major plate movements, atmospheric gas composition, and disruption of the ocean currents. When ocean currents are interrupted the large-scale distribution of warm waters from the equatorial regions to the poles breaks down. This may lead to colder higher latitudes and hence a build-up of ice sheets in these areas. Ice/snow has a high albedo and this results in more of the sun’s energy being reflected back into space, further enhancing the cooling effect (a positive feedback mechanism).

The causes of the advance and retreat of ice sheets within an ice age is the result of planetary influences that have been well described by Milankovich. The gravitational attraction of other planets on the Earth (notably the gas giants) subtly alters 3 factors of the Earth’s orbit around the Sun:

1. Obliquity – the Earth’s axis is tilted and the tilt varies over a cycle of 41,000 years. It varies between 21.8 and 24.4o and the greater the tilt, the more extreme the seasons.
2. Precession – the tilt of the Earth ‘wobbles’ about its axis with a cyclicity of 27,000 and 105,000 years. This effects seasonality, as it effects the time of year when the Earth is close to or away from the Sun.
3. Eccentricity – the path of the Earth’s orbit changes from more circular to elliptical which occurs in cycles of 110,000 years. In a highly elliptical orbit the Earth receives more radiation when near the Sun and much less when in the far ends of the elipse.

Whatever causes ice ages, the effects are clear to see – changes in world climate, dramatic sea-level changes, changes in habitat – all sufficient to produce a world-wide signature in terms of sedimentation and extinctions.

Buckingham Sand Pit

-         the advance of the ice

  Moving ice leaves behind a very distinctive sediment – till. Till is often called ‘boulder clay’ on old maps or books, as it describes the common appearance of this deposit, which is full of all sizes of clasts from boulders to clay. However, as it does not have to contain boulders it was thought to be a misleading name, hence it’s modern name it till.

  Till can be seen from the western viewing platform. The sediment here is very poorly sorted with large pebbles set in a sandy-silty-clay matrix. The pebbles appear to ‘float’ within the matrix showing that this has not been sorted by water. This is the typical appearance of till, although the clasts and the character of the matrix can vary enormously.

  This till is Anglian in age – laid down half a million years ago during the most extreme ice advance of the Quaternary. When this till was laid down it would have been under c. 2 kilometres of ice. Looking at the till of this ice sheet – which can be seen and traced from numerous old and current sections from Buckinghamshire across to the Norfolk coast – it has an interesting history. It seems as though it changed it character from being an earlier cold-based glacier to a warm-based. We see the deposit from the warm-based glacier here at Buckingham Sand Pit.

  Cold-based glaciers are frozen to the bedrock underlying them and they literally pluck up large areas of this rock or frozen sediment, which becomes entrained within the ice or crushed at the base. For this reason, the fragments contained within the ice range in size from ‘rock flour’ to massive boulders. They are also angular in shape.

  Warm-based glaciers develop a thin layer of water at the base. This lubricates the ice sheet enabling it to flow much more quickly than a cold-based glacier. Warm-based glaciers do not pluck and erode sediment as effectively as cold-based, but tend to crush the debris contained within it which is left behind as thick layers of till. Hence, till is direct evidence of the past presence of an ice sheet in that area. Till is found throughout north Buckinghamshire as far south as the Vale of Aylesbury. The Anglian ice may have reached further south, or indeed crossed the Chilterns, but no direct evidence has been found. Some geologists argue that ‘not till’ is evidence of ‘no ice’, some argue that the evidence may have been eroded.

The direction of the ice flow?

No directional evidence exists in Buckingham Sand Pit (unlike some of the Norfolk coastal localities), except for the clastic component. The following rock types can often be recognised from the till or from the sorted deposits at the eastern viewing platform:

The ice melts

When ice melts, it starts gradually but can turn into a torrential flow very rapidly. Any clasts beneath, or at the front of the glacier, get caught up in the running water. Water will then sort the clasts and deposit them in graded deposits with characteristic sedimentary structures. These deposits can be seen at the eastern viewing platform. The flow of the current can be inferred by two methods:

1. Pebble imbrication – where pebbles with long axes have been pushed over and hence aligned with the direction of water flow.
2. Cross-stratification - the 'lines' seen within sands which point in the direction of water (or wind) flow.

Pebble imbrication

              Cross-stratification

The interesting factor about these water currents is that they were flowing within an esker – a tunnel beneath the ice. The additional evidence for this is the long, narrow and sinuous sand body (much quarried away by the pre-1950s working of this site) and also the ridge-like topography just seen on the present land surface. This makes Buckingham Sand pit a rare opportunity to see an esker. There are no other examples in Bucks, but others include Blakeney Esker in Norfolk and the Tweed Valley esker in Scotland. The sediments seen in the deposit at this site show the often torrential flow (the boulders marking the base of channelised deposits) which gradually slows to deposit finer sands.  Eventually the eskers choke up their own channel – the step sides and sinuous ridges are preserved well as they are confined within the ice. As long as the ice melts slowly as it finally retreats, they are preserved.

The climate warms up

As the climate warms the ice continues to recede. Eventually the ice is distant enough for the ground to be occasionally thawed, but the layers beneath remain permanently frozen. This is the tundra environment. It is possible for specialised flora such as lichens, mosses and ‘arctic-type’ low-level plants to thrive. The evidence for tundra can be seen from the eastern viewing platform in the form of contorted sediments and tiny fragments in organic-rich layers. These are discontinuous, thin deposits just a few centimetres in thickness and c. 1 to 3 metres wide. They would appear to be the deposits remaining within isolated pools of water. Some of these layers contain charcoal and the interpretation and source of these fragments remains a mystery. Other organic material enhances the precipitation of iron oxides and are stained red.

A sketch reconstruction of the Buckingham area at the end of the Anglian glaciation, just under half a million years ago. Glacial lakes occur nearby in the Milton Keynes area. Till is on this site and it also covers large areas of north Bucks and Bedfordshire. Eskers are found in this site, as well as organic debris collecting in isolated pools of water. Periglacial features can be found over much of Bucks and the neighbouring counties.

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