OUGS Field Trip 10/5/15: Kelvin Huff

Geological background

The village of Cerne Abbas lies in the Cerne valley in the Chalk downland of central . The steep-sided valley has been cut through the Chalk into the Upper Greensand by the action of water. Initially the freeze-thaw action, under periglacial conditions in the Pleistocene period, would have broken up the Chalk, aiding later erosion. Springs issue from the Cann Sand which overlies the Gault Clay and the Chalk. Around and Parva, and Upcerne, the Cann Sand is overlain by a narrow outcrop of Sandstone of the Upper Greensand. Above the Greensand the steep hillsides show a succession through the Chalk, in the Cenomanian and Turonian stages. The village has been built on the Cann Sand with the remains of the Abbey on the Zig-Zag Chalk (Cenomanian, Grey Chalk subgroup, (formerly Lower Chalk). A narrow outcrop of the Holywell Nodular Chalk (Turonian) forms the base of the steep hill and the Giant is cut into the New Pit Chalk (Turonian). Above these, as the slope lessens, is the Lewes Chalk (Turonian). The top of the hill is covered with Clay-with-Flints, mostly angular flint gravel in a red-brown clay matrix. On the highest part of the hill Clay-with-Flints covers the Seaford Chalk (Coniacian) which is low in the Upper Chalk sequence.

Country Walk 10.30-1.00 approx.

1. Lime Kiln and Yelcombe Bottom Dry valleys have been carved through the Chalk into the Greensand below by fluvial action during the Ice Ages. In the winter the surface was frozen but spring thaws of snow and soil water created considerable run-off forming deep dry valleys. Yelcombe Bottom may still have a winterbourne after high seasonal rainfall. 2. Chalk Quarry (SY 668017) A huge quarry has been cut into the side of the hill on the left. The quarry is mainly in the New Pit Chalk and is the probable source of the Chalk building stone in the village but no exposures can be seen nowadays. 3. SY 667028 Cross a field showing Clay with flints. Then descend into Minterne Parva following a field on the Zig Zag chalk. Minterne Parva Farm is on the Upper Greensand (Shaftesbury Sandstone). Go left at the farm, and note dry stone wall rubble of Upper Greensand. Dower House, Minterne Parva: The house has been built of Upper Greensand rubble with Ham Hill Stone dressings. Opposite, the stump of a parish cross is of Ham Hill Stone. 4. SY 663033 The Cann Sand may be visible in badger holes at the sides of the sunken road. 5. Upcerne The first two cottages are late 17th and early 18th century respectively and are built of flint and chalk block, with strengthening courses of stone including some Ham Hill Stone rubble. The church and manor house are not accessible but they can be seen from the fourth gate (going uphill). The front is of Ham Hill Stone and the north wing was built in the late 17th century of brick. 6. SY 656027 You can see the Upper Greensand (Shaftesbury Sandstone) in the field on the right of the road and as the footpath turns left into the next field. Lumps of the glauconitic, calcite cemented, fossiliferous sandstone are visible in the upper half of the field. 7. The lane back to the main road crosses a dissected bench of Upper Greensand. between valley sides where the base of the Middle Chalk forms another bench. The Giant is now visible, cut into the New Pit Chalk (Turonian).

(Lunch at Kettle bridge picnic site)

Village Walk 1.30-2.30 approx.

1. Kettle Bridge and the flood defences. A barrier has been constructed by the Environment Agency to hold back flood water. Gabions (stone- filled wire bags) have been used for part of the barrier and also for the river channel downstream of the barrier to Kettle Bridge to minimise erosion of the banks. Note the weathered Upper Greensand in the bridge. 2. Beauvoir Court or North Barn. The large chalk blocks (of up to 4 tons) are Holywood Nodular Chalk in appearance with scattered flints. These blocks are so large that they must have come from the huge quarry seen at the beginning of the countryside walk. This quarry has provided Chalk for building, mostly from the Cenomanian, or lowest Chalk. 3. Follow the Mill Stream down past Mill Cottage. There is a workshop on the left hand side with a variety of stone types including a couple of pieces of Purbeck Marble which was probably derived from one of the Abbey buildings after the dissolution of monasteries by Henry Vlll. Other stone in this building includes Upper Greensand which is also showing significant signs of weathering with spalling of the surface. 4. The former police station with Ham Stone, flints, Portland or Purbeck limestone seen in the front elevation. Bricks are seen on the side elevation. 5. The front wall of the New Inn is a patchwork of brick and stone. At the right hand end, a mixture of material, including Ham Hill Stone (orange/brown), Chalk (white), Purbeck Stone (small, light grey) and flint goes up to the roof. Below the windows the stone is mainly small blocks of Purbeck with the mix as before between them. The window surrounds are of Ham Hill Stone. At the left hand end the striped appearance is created by using flint banded with Chalk. The Chalk has a greyish look and contains scattered crystals of glauconite (dark green). On the eastern corner quoin, above head height, a block of Chalk has weathered so that a Calycoceras ammonite and Inoceramus bivalves can be seen. The roofing slates are of HHS and Purbeck Limestone. 6. The Old Bell, where the lower wall is of thinly bedded Purbeck limestones. The bedding and weathering is characteristic of stone from the Lower Purbeck Ridgeway quarries. In the Old Bell, from the lower windows upwards, flint is banded with the greyish Zig Zag Chalk. Glauconite crystals are visible with some phosphatic nodules and at least one specimen of Turrilites acutus. This identifies the rock as Cenomanian – the Zig Zag Chalk. The lower windows are built of Ham Hill Stone. 7. The Royal Oak Inn was built in the early 16th century while the Abbey was still standing. Its walls are of flint and rubble. 8. St. Mary’s Church The tower and entrance porch are of Ham Hill Stone with the plinth of the nave being of Ham Hill Stone and Portland Limestone ashlar. The 19th century South wall of the nave is banded with small blocks of Purbeck Limestone then ashlar blocks of Ham Hill Stone and Portland Limestone banded with flint. Opposite the church the Pitchmarket, built in about 1500, has walls of flint banded with both Purbeck and Ham Hill Stone and timber frame above. The cobbles in front of this row of houses are flint. 9. Abbey Farm at the end of the street incorporates part of the Abbey buildings. The south wing with the buttresses, facing Abbey Street, is partly the 15th century Abbey gatehouse. The walls are of Purbeck Limestone and flint with some Ham Hill Stone. Much of this was rebuilt after a fire. 10. Graveyard and St. Augustine’s Well. The graveyard has a wide variety of gravestones of different ages. There are Ham Stone sarcophagus-type tombs. There is a 15th century preaching cross of Ham Hill Stone. In the grounds is St. Augustine's Well. Again this water source is from the Upper Greensand. By the spring there is a slab of ripple marked limestone. Although rather eroded it looks very much like Purbeck Limestone seen in the Ridgeway quarries and on Portland. 11. Cerne Abbey (£2 fee to enter the Abbey) The Abbey is of Saxon origin but was replaced by Norman structures after the Norman Conquest. In the Abbey grounds, the 15th century Hospice or guest house has walls of flint banded with Purbeck Limestone and some Ham Hill Stone windows and quoins. The oriel window at the side is mostly constructed of Portland Limestone. Inside, the walls are lined with Chalk block. The Abbot’s Porch, built 1497-1509, has walls of Ham Hill Stone and Portland Limestone. The Purbeck Limestone is recognisable by comparison with the limestones used from Weymouth northwards. It weathers in a totally different way from the Middle Purbeck Limestones quarried in the Isle of Purbeck. The Portland ashlar seen in the church, the Hospice and the Abbot‘s Porch is similar in character to the Portland ashlar used for the Abbey at Abbotsbury, also a Benedictine foundation. At Abbotsbury this has been identified as coming from Portesham quarry, the westernmost quarry on the Ridgeway.

Acknowledgements:

1) DGAG Coast and Country Guide written by Jo Thomas

2) A geological stroll around Cerne Abbas by Alan Holiday (DGAG newsletter)

3) ‘The Geology of Cerne and its impact on village life’ published by the Cerne Historical Society (.pdf)

4) Various books on the geology of Dorset.

Return to car park and move on to St. Catherine’s Flint Products for 2.45/3.00

Geological map showing spring-lines

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2

1. The marl band at the level of the Holywell Nodular Chalk causes springs around the base of Giant Hill. 2. Springs at the Upper Greensand/Zig Zag Chalk boundary (St. Augustine’s Well)

The lime green in the centre of the valley is the Upper Greensand, comprising the Cann Sand (plain) overlain by the Shaftesbury Sandstone (with yellow dots). The grey-green underlying the gently sloping lower valley sides is the lowest Chalk layer, the relatively soft greyish-white Zig Zag Chalk. The dark green layer marks the change to the steeper slope below the Giant. This is the Holywell Nodular Chalk, a very hard, nodular layer, more resistant to erosion, thus the change in slope. The light green layer is the New Pit Chalk. This is a moderately hard, blocky, white chalk generally free of flints and is the layer in which the Giant has been carved. The mid green layer is the Lewes Chalk, a hard, blocky, white chalk with flints, which underlies the flatter top slope of Giant Hill. The brown layer on top of Giant Hill is Clay-with-flints and represents the weathering residue from a considerable overlying deposit of Tertiary age. This covers the Seaford Chalk at the hill top. The pink is Quaternary Head and the yellow is alluvium.

Geological Column with rocks seen in context