The picture on the left shows Northern Drift deposits in Hanborough. The large stones are quartize cobbles from the Midlands.
This example of ice wedges was in the gravel pit where Millwood Vale is now. They occur, or are formed when the gravels broke up into large polygonal structures owing to shrinkage in intense cold periods.
On warming, debris fell into the 'wedges'.
Geology and Topography
How many of us wonder how the familiar landscape of our parish came to be as it is? What great natural forces in the remote past shaped these features?
Why, for instance, does our modest Evenlode stream flow gently at the bottom of a wide steep-sided valley; how is it that some of our garden soils are light and gravely, and others obstinate, sticky clay? To answer thse questions we need to take a closer look at the geology of the region.
The ground beneath our feet in Hanborough is composed of two very different kinds of material, known to geologists as "Solid" and "Drift". The solid geology is "bedrock", belonging to the Jurassic period (c. 180-130 million years ago). During this period Oxfordshire lay beneath the sea and thus accumulated marine sediments that were later uplifted to form land.
In Hanborough these deposits comprise either yellow limestones, which can be
seen on the slopes of the Evenlode Valley and in the Hanborough railway cutting, or Oxford Clay, a fine blue- grey mudstone covering the plateau away from the valley. Both formations contain the fossils of marine organisms such as sea shells, ammonites, or fish teeth. Corals are common in the limestones. Most of the stone built houses in the village were quarried from the local limestone, the most prominent working being Brown's quarry at the bottom of Swan Hill. In Pinsley Wood, a large depression marks the spot where stone was cut for the building of the Parish Church. Some of the older houses in the village still have roofing of the now worked-out Stonesfield Slate, a very hard limestone capable of being split into thin layers. As the name implies, this rock came mainly from the Stonesfield area. There were once miles of limestone walls marking field boundaries in the parish, but few remain today.
Much of the Jurassic
geology is overlain by the "Drift" material, which has been deposited within the last million years, during the Ice Age. The topography of today is largely the result of forces at work during the Ice Age, which not only laid down the Drift but also cut into the Solid geology.
The Ice Age, or Quaternary as geologists prefer to call it, actually comprised numerous fluctuations between long bitterly cold periods and weather somewhat warmer than today. During the coldest periods,vegetation ceased to grow and the landscape became a barren surface of silts, sands, gravels and stones. Although the ice-caps themselves never reached further south than the headwaters of the Evenlode, the Hanborough area became part of an enormous outwash zone where the formidable power of rushing meltwater loaded with glacial debris would have created a scene of utter chaos. The remnants of this debris surviving today are but a small fraction of the volume
of material that must have been washed down by an ancient Evenlode swollen to fill the whole valley and over spilling it.
The oldest of the Drift deposits, termed the Northern Drift, may be seen in the fields west of the village. Their characteristics are large brown water worn pebbles and cobbles made of quartzite that have been carried by the action of ice and water from the Midlands. Occasionally, stones are seen whose origin is as far away as Scotland. This material may be more than a million years old, and its widespread distribution in the Evenlode catchment indicates that at this time the Evenlode was the main river in the Upper Thames Basin, while the present Thames was but a tributary! The fact that it lies on top of the hill, up to 35 metres above the valley floor, and yet is thought to be a river deposit, shows that uplift of the landscape has taken place since this gravel was laid down, and the present Evenlode has cut down below the original valley floor of the Northern Drift.
The next deposit to have survived is the so
called "Hanborough Terrace", a thick bed of pea gravels underlying most of the village of Long Hanborough. It is composed of more local stones such as the Cotswold limestones. Geologists are not agreed on the complex question of the date of this terrace, but it is probably somewhere between 400,000 and 300,000 years old.
These gravels were extensively quarried in the past, and the old pitface may still be seen on the west side of Church Road between Long Hanborough and Church Hanborough, part of which has been designated a "Site of Special Scientific Interest", or SSSI in local government terms. The deposits have yielded molluscan fossils indicative of a cold environment and vertebrate remains that suggest warmer conditions. Examples of ice wedge casts occur, formed when the gravels broke up into large polygonal structures owing to shrinkage in intense cold periods. On warming, debris fell in to the 'wedges'.
Many
of the domestic gravel drives and pathways in Hanborough are composed of Hanborough gravel. So some readers may have a sample of the SSSI gravels in their driveways!
At a later date, perhaps around 200,000 years ago, another gravel terrace was deposited by the Evenlode at a lower elevation on the valley sides. This is the "Summertown-Radley Terrace" which lies mainly in the south of the parish towards Eynsham.
Finally, the valley floor itself, which is mostly flat, is composed of the most recent geological deposit of all, a soft river silt called alluvium. This is the product of successive local river flooding over the past 10,000 years, after the end of the Ice Age.
The complex geology described above has to be understood in order to appreciate the present day topography. Hanborough parish is bounded on the north and east sides by the river Evenlode, a tributary of the Thames. At the northeast corner of the parish the Evenlode is joined by the river Glyme, which has been dammed artificially in Blenheim Park to form the lake. South of this confluence, the river passes through a broad valley with gently sloping sides, until it flows into the Thames near Cassington. West of the junction with the Glyme, however, the Evenlode valley has cut a much steeper sided valley with a narrower floor, known to geomorphologists as the "Evenlode
Gorge". Through this gorge the river swings dramatically from side to side in a series of meanders stretching back to Stonesfield.
The formation of these meanders may be related to the sudden drop in discharge at the end of the Ice Age, but the exact process is not understood.
The remainder of the parish comprises a plateau sloping gently southwards from its highest point, some 107 metres above sea level, near North Leigh Common.
Continue to part 2
Copyright Terry Hardaker 2007
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