Hughenden Rocks: a walk through time
This is a 2.5 mile walk that explores the geology of Hughenden and how the landscape has been formed. The walk will take you from the shallow tropical seas of the Cretaceous to the cold depths of the Pleistocene Ice Age. It finishes at the era of the Victorian railways when geological materials could be brought to Hughenden from far afield.
How to follow the walk
This walk is not waymarked. It is best followed using a hand-held device, such as a smart phone or tablet computer. (NB. Not all the graphics will be available on printed copies.)
Map
Start:
Start at the visitor welcome kiosk. Grid Ref SU860955
1
From the visitor welcome kiosk, retrace your steps towards the main car park. At the T-junction of paths by the Dew Pond turn left. There is a purple way-marker on a post. You will soon reach another purple way-marker on another short post. Here turn right to follow a more or less level path. At a viewpoint on your left, opposite a bench on the right of the path, pause to take in the view across Echo Valley towards Manor Farm and the D'Isreali Monument: this is a good vantage point from which to consider the general geology at Hughenden.
The Geology of Hughenden
The geology of the Hughenden Estate is dominated by two rock types. Underlying the whole landscape are great thicknesses of a sedimentary rock called chalk, up to 200m thick. Above the chalk and covering the summits of many of the Chiltern Hills is a layer of Clay-with-Flints. The upper chalk and the clay both contain lots of hard flint nodules. The clay within the Clay-with-Flints is thought come from younger Palaeogene clay that covered the chalk. The flint nodules from the chalk became concentrated at the top of the chalk and then in the clay by the dissolution and weathering of the chalk plus mixing with the clay at some time since the clay’s deposition. The timing of this dissolution and mixing is uncertain but least some would have occurred during the Pleistocene ice ages as a result of freeze-thaw action.
2
Continue along the main path through Disraeli’s ‘German Forest’ until, on your right under some yew trees, you see some old badger sets where the ground has been disturbed to expose fragments of chalk and flint. When you are ready, continue on the level path until you reach a junction of 5 paths. At the junction, take the first left that heads downhill. (This path can be very rutted in places as it is used by vehicles for forestry work.) Ignoring any crossing paths, follow this path until it reaches the base of the slope and meets a bridleway running along the bottom of the valley. Note the absence of any form of river or stream in the valley bottom. Here turn left, heading along the bottom of the dry valley towards a gate. You are now in Echo Valley. Go through the gate and continue across the field until you reach another gate.
Badger Miners
There are few places where chalk is exposed at the surface at Hughenden, but the badgers have come to the rescue by digging some of the rock to the surface from beneath the thin ‘rendzina’ soil! At any of the abandoned badger setts, pick up a fragment of white, soft chalk and examine it. You can also pick up fragments of flint for comparison. In your hands you have fragments from an ancient tropical seabed. Around 95 million years ago, in a geological period called the Cretaceous, the planet was warm and sea level had risen so much of Hughenden was beneath the waves. The sea was no more than 300 metres deep and crystal clear. With little sand or clay available, a calcareous ooze formed on the sea bed from millions of the microscopic remains of plankton, especially the disc-shaped calcite plates or coccoliths that make up the spherical coccolithophores. Every handful of chalk contains millions of these tiny fossils and very occasionally there are larger fossils of ammonites or sea urchins. More will be said about the flint further into the walk. Please return any rock you have picked up to where you found it.
3
Go through the gate and turn left and follow the wide track sign-posted towards Hughenden Manor. The track runs parallel to the route of an ancient stream (in the field on your left) that dried up towards the end of the Ice Age. As you reach the woodland, just after a pair of field gates on either side of the track, you reach the point at which this ancient stream bed would have crossed the track. In the winter the track and the surroundings are often waterlogged, reminding us of this long-lost stream. Continue into the woodland, following the track uphill and ignoring any crossing paths to a four-way junction by a wooden signpost.
Asymmetrical Echo Valley
Echo Valley is a typical Chiltern ‘dry valley’ that was carved from the chalk during the Ice Age. The valley would have formed during the glacials (long cold periods), from about half a million years ago. During these times the ground would have been frozen like modern-day Arctic permafrost. The pores within the chalk, which normally allow water to drain down through the rock, were blocked with ice, so the water from spring snowmelt and summer rain would have run as a torrent over the surface, carving the valley. A feature of many Chiltern Valleys is an asymmetrical profile. Looking down the valley towards Hughenden, the slope on your left, leading up to Hanging Wood, is significantly steeper than the slope to your right leading up to Manor Farm. This is not controlled by the geology, but by differences in the rate of weathering and erosion. During the ice age, the southwest facing slope was more exposed to wetting, freezing and thawing, weakening the rock. The valley’s fast flowing stream would then have eroded this material away down-valley, changing the depth and profile of the valley.
4
Here take the first right, following a footpath initially downhill and then uphill, with a steep bank on your left. You may catch glimpses of the west side of Hughenden Manor, the west bank garden and the corner of the Parterre garden on your left. When the path starts to descend again into dense yew woods look out for a branching path to a gate on your left that leads into Hughenden Park. Go through the gate and head left towards Hughenden Manor across an area of open grassland with an area of woodland and some of Hughenden Manor’s beehives on your left. Close to the beehives is a good place to view the south façade of Hughenden Manor.
Bricks made from local clay
The oldest bricks of Hughenden Manor (called Chesham Blues) date to around 1737. These were originally hidden beneath white stucco, but Disraeli had this removed. The bricks were almost certainly made from Eocene clay from the Reading Beds, which were deposited locally on top of the chalk about 56 million years ago. Unlike the chalk, these sediments are non-marine, indicating that sea levels had drastically fallen, due to the relative rise in land level. At this time, the Chilterns and the Weald were being uplifted and the London and Hampshire Basins were warped downward. This was caused by huge tectonic forces that folded and contorted the rocks of Europe in a mountain building period called the Alpine Orogeny. The Reading Beds are mostly composed of iron-rich mottled clays and estuarine sands that formed under a very hot and humid climate, and they have been the source of the clays used in the brick and tile industry in the Chilterns since Roman times and possibly before.
5
On approaching the fence to the gardens of Hughenden Manor, bear right, skirting the edge of the gardens. Pass an ornamental gate, decorated with Disraeli’s castle motif, and you will soon reach a kissing gate. Go through the gate and continue alongside the edge of Hughenden Manor’s garden until you reach a brick ha-ha wall on your left. From here you have an excellent view of the park and Hughenden Valley, with a variety of large, mature trees and you should be able to see the course of a small stream in the valley below you.
6
From your vantage point, make your way downhill and to your right to pick up a steep path that heads downslope parallel to the fence that separates the north and south parts of the Hughenden Park. Head towards a bench by the right-angled bend in the stream at the bottom of the valley, where it has been managed with weirs to form ponds. Take great care in wet weather as the slope can be slippery. (In very wet or icy conditions it is advisable to zig-zag down the slope, heading initially towards the church and then towards the stream on a less steep gradient.)
Chalk Streams and Dry Valleys
Most valleys in the Chilterns are dry, as rainwater drains down into the porous chalk until it reaches the water table, deep underground. Chalk is an aquifer, so it is able to soak up and hold a lot of rainwater. Under gravity, the water flows through the rock until it re-emerges in the valley bottoms in the form of springs that feed the chalk streams, like the locally named River Hitchen, which runs through Hughenden Park. Since groundwater levels in the chalk vary according to rainfall and season, the flow of chalk streams is naturally intermittent. In the summer the streams may dry up completely for months at a time. During the winter, with its higher rainfall, the aquifer will be topped up and the head of the stream moves up the valley as the water table rises. This section is called a winterbourne because it usually flows only after the winter rains. Chalk streams are globally rare, confined to North West Europe and to the UK and France in particular. In fact, of the 200 or so chalk streams in the world, over 160 are found in England.
7
Turn left, following the bank of the chalk stream, passing to the right of an information board about the stream. Eventually you come to a small bridge where the access road to Hughenden Manor crosses the water. The ground beneath you is recently deposited alluvium deposited by the stream over the last 10 000 years. Take a look back along the stream and the Hughenden Valley and imagine how much has changed in the last 40 000 years.
Sarsen Stones
As well as the concrete weirs, you will see a number of large boulders that have been deliberately placed in the stream for decorative purposes. Such large boulders are not natural features of a chalk streams, but they have not been brought far; indeed they may well have been removed from neighbouring fields in the Hughenden Estate to allow farm machinery to be used. These are sarsen stones and they represent the remnants of an ancient river deposit that formed many metres above you on top of the chalk. As the chalk has eroded away, the sarsen stones have been lowered from their original position and can be found lying in fields and woodlands. They are made of a very tough material called silcrete. This formed in a semi-arid climate around 20 million years ago, when Britain was under a much warmer climate than today. Look out for more sarsen stones later on the route.
8
Head uphill, parallel with the access road to Hughenden Manor, then through a metal gate at the end of the car park to reach St Michael and All Angels Church. A church has existed on this site since the 12th century, but the building was extensively restored and extended between 1874 and 1890 using traditional building materials. Get close to one of the walls of the church and look in particular at the finely knapped flints in the church’s walls, and also at the sandstone used on the corners and window frames.
Hughenden in the Last Glacial Period
What did Hughenden Valley look like 40 000 years ago in the Late Glacial Period? The vegetation would have consisted of lichens, mosses, small numbers of low-growing grasses and flowering plants. Stunted birch trees and shrubs may have grown in a few sheltered locations. We know from fossil evidence that mammoths, woolly rhinoceroses and wolves inhabited parts of the Chilterns. The animals would have wandered through the landscape in a constant search for food. Nomadic Stone Age hunter-gatherers probably survived the early part of the last glacial period in small numbers in sparsely wooded areas, occasionally leaving behind their primitive flint tools. There is evidence of Neanderthal humans living in Britain from around 60,000 BC. By 40,000 BC modern humans (Homo sapiens) were spreading across Europe, soon reaching Britain. It is not known exactly when Neanderthals died out but there may have been a period when Neanderthals and modern humans both lived in Britain. Contrast this with the rural Chiltern landscape around you today.
9
Go to the east outside wall of the Church where you will see the grave of Benjamin and Mary Anne Disraeli.
The Church Walls
Unlike chalk, flint nodules found in the upper chalk are very strong indeed making it a useful local building material. Flint is a form of silica, the origins of which were the silica-rich organisms such as diatoms (microscopic algae), other plankton and marine sponges. When these creatures died, their silica skeletons dissolved in the water near the sea floor. The deep, quiet waters of the chalk sea became locally saturated with silica, which was re-precipitated, giving rise to flints, which replaced some of the chalk, which dissolved away. Initially this was a sticky jelly-like substance but, over a very short period, the gel solidified into the very hard flint we find today. Often fossilized remains of sea-urchins and sponges can be found in the cores of flint nodules, suggesting that they may have triggered the crystallisation of the flint nodules. Flint has been used in the Chilterns since the early Stone Age for making tools such as axe-heads, cutting tools and arrow heads. Later, flint was used in the flintlock mechanism in firearms. As a building material, flint is limited because of its irregular shapes and sizes. Notice that sandstone, rather than flint was used for the corners of the church building and for window surrounds, since it can more easily be worked into regularly shaped forms. Sometimes the flint nodules are ‘knapped’ (shaped) into more regular blocks, but this is a highly skilled and costly process, so it is reserved for finer buildings like St Michael and All Angels Church.
10
From St Michael and All Angels Church, head uphill on the left side of the church (away from the carpark) until you reach a gate adjacent to the cattle grid for the road. Continue through the gate and uphill until you reach the Stableyard building opposite the main entrance gate to Hughenden Manor, looking out for more sarsen stones alongside the path in the left side of the drive.
The East Wall and Disraeli’s Grave
Sandstone has been brought to Hughenden for the corners of the church building, and for window surrounds, which is well illustrated by the east wall. However, you will see that the softer sandstone is flaking away in places around the area above Disraeli’s grave. This is evidence of frost weathering where rainwater has penetrated the pores of the sandstone and in the winter months the water has frozen. Towards the buttress to the left of the grave, the weathering of one of the corner blocks has exposed some cross-bedding, which tells geologists much about the direction of flow of the water or wind that deposited the sand. The headstone of the grave has been constructed from a slab of polished crystalline pink granite: an igneous rock. The large crystals tell us that this rock formed from slowly cooling magma, deep in the earth’s crust, and certainly not at Hughenden.
11
There is a restaurant, a shop and toilets in the Stableyard and opposite is the entrance to the Manor House. Entry to the house is free to National Trust members. Non-members can buy tickets at the welcome kiosk, which is on the route back to the car park. When you are ready to continue, follow the narrow road that curves uphill to the left of the Stableyard until you reach the visitor welcome kiosk where you started.
Victorians, Railways and Rocks
The Stableyard was built from red brick by E B Lamb in the 1860s and the yard is paved with sandstone cobble stones. It is a less ornate building than the Manor, making it much easier to see the roofing material, which is made from another ‘exotic’ rock called slate: a metamorphic rock. The original slate was almost certainly brought over 200 miles to Hughenden by rail from quarries in North Wales. Slate started as fine silt, clay and even volcanic ash on an ancient ocean floor, which was compressed into soft shale. Then, under huge tectonic pressures, the shale became compressed and heated into a fine grained foliated metamorphic rock, in which the original minerals have been altered. A unique feature of slate is the foliation that does not correspond to the original sedimentary layering, but instead it forms in planes perpendicular to the direction of metamorphic compression. A very strong foliation is called ‘slaty cleavage’, and it’s this feature that enabled skilled craftsmen to expertly ‘cut’ or split the slate by striking it parallel to the foliation with a specialized tool, forming smooth flat sheets of stone which have long been used for roofing, floor tiles, and for many other purposes. Formed up to 490 million years ago in the Caledonian Orogeny, these slates are by far the oldest rocks at Hughenden, even though most of the slates you see are likely to be replacements.
End:
Start at the visitor welcome kiosk. Grid Ref SU860955
