Summer Field Walks in Suffolk Coastal
A report on five field walks led by Bob Markham, with thanks to Terri Tarpey for botanical input.
Newbourne Springs Thursday 15th August
The purpose of this walk was to investigate the unusual water regime, which gives this Suffolk Wildlife Trust reserve its name. We met in the car park (274432) and went first to look at the crag pit across the road. We discovered a face of Red Crag, which deposit is permeable to rain water, and nearby a pool of water retained by an impermeable deposit beneath the crag. Borehole evidence shows this to be London Clay. We returned to the reserve and walked along the east side of the valley floor, which was quite marshy and muddy. We noted a spring entering the stream from the left and also lesser water parsnip growing in the stream, which indicates alkaline / neutral water. As we climbed the valley side, there was an abrupt change to dry, sandy ground with steep slopes down to the stream. We crossed the next bridge and inspected the peat on the valley floor.
Then we retraced our steps, scrambling down at one point (with permission from the warden) to investigate a spring entering on the left bank of the stream. Despite the beautiful, hot, sunny weather there was a steady flow of water issuing from this, bringing lots of fossil Red Crag shells with it. This explained the pH of the water.
Spring sapping had clearly created the steep valley side here. We then walked back along the west side of the stream and found an exposure of Red Crag in the valley side behind Crag Cottage. Obviously, the stream has eroded through the Red Crag to the perched water table on the London Clay below, and springs issuing from this have steepened the valley sides and provided a steady flow of water.
We made an approximate measure of the discharge of the stream at the two bridges we crossed (272435 and 268437). We measured the width of the water surface, the maximum depth and calculated the cross-sectional area of the stream, assuming a rectangular channel. We made three velocity recordings, using dog biscuits as ‘Pooh sticks’ and timing them over a measured distance. In both locations the discharge was close to 0.05 m3sec-1, which compares with 0.187m 3sec-1 measured on a previous visit in November, (see ‘Geological Notes’ White Admiral, 40 Summer 1998),
Dunwich Friday August 16th
Medieval Dunwich is one of the best-documented examples of coastal erosion in
Britain and we came to look for field evidence of this once great settlement.
We met in the car park (479707) and walked first along the beach to the south
to find out why Dunwich is so vulnerable to erosion by the waves. The cliff
here is Norwich Crag Sands channelled by Westleton Pebble Gravels,
unconsolidated material that is easily removed. Although the weather was calm
and sunny, and the waves quite small, the wave frequency was high. (We made
five counts and came up with a mean figure of 19 waves per minute.) Each new
wave was breaking into the backwash of the one before and we observed net
movement of pebbles down the beach.
Having watched erosion in action, we walked inland through the village to St James Church. Here we found the remains of the Leper Chapel with its original Norman arches, which was part of a hospital built in the 12th century to house lepers at a safe distance outside the town. Also in the churchyard is a buttress from All Saints’ church rebuilt here in 1923 after the original church fell over the cliff in 1919. From here we walked to the ruins of the Greyfriars Priory and followed the footpath on the south side. This intersects Middlegate Street, one of the main routes to Medieval Dunwich, but cut off now by the sea. We walked along Middlegate Street to the cliff top and then turned north into the grounds of the Priory, noting the use of Coralline Crag as a building stone in the walls. Almost immediately on leaving the Priory to walk along the cliff top path we came across the last two remaining gravestones from All Saints’ Church. Both these and the Coralline Crag wall are within a few metres of the cliff top which means that they will not survive for long.
Bridge Wood Monday August 19th
The purpose of this walk was to investigate the wooded
slopes of the Orwell Estuary in the Orwell Country Park owned by Ipswich
Borough Council. We met in the car park at 188408 and walked south along a
sandy ridge with steep slopes on either side. We descended the slope on our
left and discovered a series of springs issuing from beneath an exposure of
sand on the eastern edge of the Park.
Impermeable London Clay beneath has created a perched water table in the sands above and
spring sapping has eroded the steep slope. We walked down to the beach and
turned north along the shore of the estuary, walking inland again up the next
valley on the west side of our original path along the ridge.
The footpath crossed uneven, hummocky ground, the remains of great landslides which
occurred in the London Clay here at some time in the past. Several large, backtilted
oak trees at the foot of the slope bore evidence of rotational slip when the
landslides occurred, the inference being that these started life at the top of
the cliff and were moved and back-tilted by the landslide. Sycamores growing on
the steep slopes behind were all quite straight and much smaller, so presumably
they began life after the landslide occurred. We measured the circumferences of
some of these and produced a mean figure of 0.58 m measured at 1.5 m above the
ground. Assuming an increase of trunk circumference of 1.25 cm a year (see “A
Slip in Time” - White Admiral 37 Summer 1997) these trees are about 50 years
old, which gives a minimum date for the landslide. However, all of these
sycamores were multi-stemmed and had probably been coppiced in the past and so
were older than this dating implies. We then descended to the estuary again and
looked at exposures of London Clay along the shore before returning along the
west side of the Park to the car park.
Shingle Street Tuesday August 20th
The area of active shingle deposition on the coast just
south of North Weir Point, the most southerly extremity of Orford Ness, forms a
sizeable ‘ness-shaped’ mass at the present time. The party met at the telephone
box near the Martello Tower (367426) with the purpose of making a preliminary
survey of this landform. We started at the Beacon Cottage, setting up a survey
line from there to the Coastguard cottages and then running five transects down
the beach at right angles to this. We discovered that the ‘ness’ stretches almost from Beacon Cottage to the Martello Tower, a distance of about 600m and at its widest, it is about 240m from the fence at
the back of the beach to the high water line. It encloses two lagoons, which
are brackish and tidal, the larger being about 64m wide. The shingle forms of a
complex series of ridges (about 15m at the widest point) of sub-rounded flint
pebbles 2- 80mm ‘in size, which show some degree of grading in places.
The beach north of Beacon Cottage is quite narrow, comprising two main shingle ridges, which turn abruptly away from the coast south of the cottage and can be traced round the outer edge of the ness, enclosing the larger lagoon. A third large ridge branches off from these two just south of Beacon Cottage where the ness begins to widen and runs along the landward side of the lagoon. The shingle ridges display excellent vegetation succession, with isolated individuals of plants such as sea pea, horned poppy and sea kale present on this third ridge and increasing percentage cover and number of species on subsequent ridges away from the sea. An interesting discussion ensued on the relative ages of the various ridges.
Orford Wednesday August 21st
We met at the car park near Orford Quay (425497) and walked to the Quay where we
turned north and walked along the river wall. After about 500m we descended
onto the salt marsh that fringes the River Ore. The mud here has been deposited
in the low energy environment of the estuary, sheltered by the Orford Ness
shingle spit, which was clearly visible across the river from our viewpoint.
The roots of the salt marsh plants sea purslane and sea lavender here bind the
top of the mud to form a solid mat. We jumped up and down and made our
surroundings wobble, a reminder that we were standing on a few cm of root mat
on top of several metres of jelly-like mud! We pushed (easily) a 2m-bamboo
stick into the marsh and pulled it out again, it showed black deoxygenated mud
below the first few cm.
We climbed back up the river wall and turned inland following the footpath to the north across the flat reclaimed marsh. The footpath showed grey/black clay dried and cracked by the summer sun and we found marine shells such as oysters and whelks which may be evidence that this was a marine mud, although these are edible species which may have been dropped here by people. At about 500m inland the footpath started to climb the gentle slope up the valley side, the soil changed, becoming much more sandy and we began to find pieces of Coralline Crag and some small fossil bryozoans along the field margin. Just before we joined the road we could see a small Coralline Crag outcrop across the field and behind this a heavily overgrown crag pit. We joined the road and walked back through Orford to our cars, noting the use of Coralline Crag as a building stone in the walls on the east side of the road.
We then drove 3km north to Crag Farm (430523), where we saw (with permission from Mr & Mrs Black) a fine example of Coralline Crag used in situ as a building stone in the stack yards in the farmyard. We left our cars here and walked a short distance to a Coralline Crag pit, which showed the upper decalcified ‘rocked’. In this part of the Coralline Crag the aragonite fossil shells have been dissolved by ground water leaving the fossils as casts in the rock. The lime has been redeposited in the pore spaces of the rock thus cementing it and rendering it hard enough to use as a building stone.
C. Markham
Don't forget you can download the whole of each White Admiral as a .zip file.
(Typically under 700 kb) ~ See the top menu.