We were going to look at rocks of the third period of the Mesozoic Era, the Cretaceous rocks. The Lower and Middle Cenomanian of the Upper Cretaceous period include the Chalk (Greek creta ‘chalk’) Marls, and we were also able to study the underlying Albian Gault and Upper Greensand which form the top of the Lower Cretaceous. Along Foyle track we took note of the telegraph pole which is the marker for a zone of Lewes nodular Chalk (stained iron-coloured) which can be seen at the side of the track, as well as volcanogenic content in the marl, which assists in dating.

The Marl marker beds are very extensive – Anglo-Paris basin, N Sea and beyond. Rory Mortimer told us that a ‘marker bed’ is an easily recognisable bed which occurs over a wide geographic area and arises from the same event, so we can judge other events in relation to the marker bed in all places where it can be seen.

The base of the Lewes Chalk has nodular flints – hard nodules around sponges.

‘Chalk’ as an umbrella name includes different chalks with unique fracturing patterns which result in different geomorphology. The engineering fraternity only recently started to take this fact on board and to modify their structural expectations appropriately.

We were going to see on our field trip a variety of types of cliff collapse features – e.g. cliff collapses on fractures, rotational landslips, flowslides, rock avalanches and vertical joint sets leading to local collapse.

From our initial high point on the track path we could see to the left some cliff collapses on fractures back towards Eastbourne and to the right towards Beachy Head we could see the landslip terrain with rotational landslip on the Gault Clay. This year’s weather had produced lush vegetation which tended to obscure geologists’ interests, but our view of the Beachy Head Anticline at this point was the landslip area of the southern eroded limb of the anticline.

Out to sea we observed a lighthouse which is built on Wadhurst Clay, this being the firm sea bed at that point.

The Upper Greensand reefs were apparent when we stepped down onto the beach. A point to note is that the Upper Greensand base can be seen here only and nowhere else in Sussex. The Greensand is a dark green, the dark colouring caused by concentrated glauconite, a member of the iron-rich illite group. Why so concentrated here? A pulse of sedimentary deposition? Or perhaps a pulse of volcanics? What is happening on a broad scale? There is international discussion to try and assess the reason. There were numerous phosphatized pebbles to be picked up.

The Upper Greensand has fantastic trace fossils. Our leader advised us to make sure that, during study of the rocks, any re-worked fossils are differentiated from indigenous fossils. Caution must be exercised on dating rock with a burrow-filled zone - there could be 200mm depth of burrowing giving rise to a burrow-filled gradation layer which ‘fuzzes’ the age estimate.

Nodular chalk was found – this consists of small intraclasts of chalk within a main bulk of chalk. Were the intraclasts formed by bio-erosion? Not really likely. More likely to have been re-worked on the sea-bed. We observed – looking from the bottom of the rock face upwards – dark Greensand, burrow bed, pale limestone. The base unit of Chalk here is Glauconitic Marl which is distinguished from the glauconitic Greensand below by the numerous phosphatic nodules within it.

At Cow Gap and Head Ledge we looked out for Albian Gault Clay and Upper Greensand, and the Cenomanian Glauconitic Marl, the basal Member of the West Melbury Marly Chalk Formation. This contains indigenous body fossils, reworked, phosphatized body fossils, trace fossils marker beds (litho-and bio-). A number of fossils were found, one superb echinoid, Holaster subglobosus, by a chemist (rather than a geologist!) in the party. Another, Micraster, was found by Barbara, who has a reputation for being eagle-eyed. There are many solid trace fossils of infill dropped into burrows; these Paramoudra have a small ‘tube’ running up the centre which was filled originally by the animals’ body. .

What was the sea level doing at this point in time?

As we looked gradually from bottom to top of the sequence we could see shallow marine deposits, then calcareous, then less and less clay. This indicates that the water must have been getting deeper at this place – so there must have been flat land or land retreating i.e. the sea level rising, as can be seen at Malmbury.

Looking at the Head Ledge and Falling Sands area we were able to work up the Cenomanian succession to the Plenus Marls and Melbourn Rock members of the Holywell Nodular Chalk Formation.

The Plenus Marls mark change across all Europe; towards the SW margin of the ancient basin there is more flint. In the Chilterns (which were the basin shelves) a great 0₂ content has been measured in the Chalk, showing that there had been very high productivity.

A good spot to stop and ponder another ongoing worldwide debate. Why do the changes occur in sealife? Is it the result of rhythmic changes / regular events? A shift in the Milankovich Cycle? (i.e.eccentricities in the earth’s orbit leading to periodic changes in the amount of radiation received on Earth and hence change in climate). Study has been made of changes that can be recorded – e.g. nature of the sediment, shape of animal shells, chemicals, amount of silica. Several different cycle patterns have been suggested: ‘UK’ = 19-20,000 -year cycle, Germans = 41,000-year cycle, and every 5^th cycle – a bigger impact. These alternating layers of grey and white chalks can be seen in the cliffs. Sea-bed animals will follow suit, but gradually rather than instantaneously. A layer named Sub-bed 4 contains belemnites – these lived at a colder temperature than any of brachiopods on the seabed and so provide researchers with a guide to climate at that time. There were also bigger events going on alongside the smaller ones – erosional events, oceanic shifts, and tectonic activity.

We collected some easily available specimens of Inoceramus crippsi (a bivalve) – but crippsi under overhanging cliffs had to be ignored! At this point the wet weather gear came into its own whilst we waited for a rain cloud to pass!

West of the Plenus Marls we had a view of the huge cliff collapses that have occurred during the winter 2000-2001. These rock falls have covered most of the higher part of the Chalk. They include the Turonian and Coniacian (Holywell Nodular Chalk, New Pit Chalk, Lewes Nodular Chalk and Seaford Chalk Forms). Here we observed a flowslide collapse of the cliff which showed the characteristic varied angle conduits and the frequency of them in the Chalk. Looking at the rock bands from bottom to top we saw Zigzag, Holywell, Nodular beds. The Lower and Middle Cenomanian at Beachy Head are conspicuously thinned compared with those at Southerham Grey Pit further westwards near Lewes.

International discussion and opinion are contributing to a number of European research programmes aimed at increased knowledge of geology to help planners and engineers with cliff problems. Brighton University, and Rory in particular, is very involved with these projects which have attracted considerable sums from the EU.

All the while that we were enjoying investigating the cliff face and beach we were keeping a sharp eye on the tide! Now it was time to move smartly back to re-visit the Holywell Kiosk via Cow Gap for that anticipated cup of tea! On the way we had opportunity to observe the tabular flints high up in the Chalk which are stratigraphically consistent i.e. they appeared across a large area during the same time period (giving the ‘table top’ concept) and thus a good marker bed.

Most of the field trip group departed from the Holywell Kiosk to travel by car to the top of Beachy Head so that we could see the cliffs and foreshore below. We had a good view of the geology and cliff failures(!). With much publicity the Belle Tout Lighthouse had been moved back from the cliff edge at great cost – the question that springs to mind is - why no further back? As far as possible from the ‘edge’ might seem prudent. The Seaford Chalk Formation on which we were standing has vertical joint sets and we could see an example on the beach of ‘columnar Chalk collapse’ – the separated Chalk column had slipped in a ‘feet first’ manner toward the sea.

From our viewpoint on the cliff top we were invited by Rory Mortimer to look inland at the topography of the Chalk – the dry valleys and ridges some of which mark faults in the Chalk and occur in both the North and South Downs. The different type of Chalk and the faulting occur also under the Channel and in France and so there is collaboration between English and French engineers to research as much structural detail as possible. NOT ALL CHALK BEHAVES IN THE SAME WAY! – engineers need to take heed.

Then back into the cars again and off to Birling Gap where we could see the beds of the Seaford Chalk Formation down at beach level. The Seaford Chalk was seen high up the cliff at Beachy Head and by folding now appeared in a dip at a lower level in a syncline, known as the Birling Gap Syncline.

We descended wooden steps (reinstated and reinforced since the most recent cliff collapse) to the beach. We proceeded with a noisy crunching of pebbles underfoot to look at the Seaford Chalk Formation on a level with us. There was a narrow rusty-coloured hard band running horizontally in the Chalk and here we had the opportunity to observe the tabular flints at eye-level.

I was unfortunately not able to remain longer to join our leader and group in the conveniently placed pub, but thanks go to Rory Mortimer as leader and Di Clements as arranger for a very informative and enjoyable day’s coastal exploration of Cretaceous rocks.

Di started by talking about where rocks come from, the characteristics of the types of rocks and methods of classification. During the course of the afternoon two themes developed - firstly people were encouraged to try their hand at identification, whether they were geologists or not; and secondly “What would you like your marker to be made from, if you wanted it to last?” This is, of course, a reference to weathering and how the difference rock types react to the wind and rain, heat and cold.

As last time, we started by crossing the public road, aptly named Cemetery Pales, which divides the Cemetery into the original “Non-Conformist” and “Anglican” sections. These designations are adhered to no longer and there are active Muslim and Orthodox grounds in the former Anglican section. This cultural diversity of the Brookwood Cemetery is for me one of its main attractions and, without wanting to labour the point, has resonance and lessons for the difficult times in which we are living.

In the Non-Conformist section, Di was able to show us a variety of different rock types and was also able to point out the difference between the quarryman’s classification of rocks and the geologist’s. For example, a quarryman will describe almost any rock that can easily polished, especially limestones, as “marble”, whilst to a geologist “marble” is strictly metamorphic limestone. We saw many examples of true marbles, probably from Carrara, Italy.

On crossing back to the former Anglican section, we revisited a tomb that had puzzled us the year before. This was large box like structure, about 3m x 3m and 1.5m high and is formed entirely of slabs of a grey-green, black and white flecked rock, which have a distinctly “greasy” feel. Di identified it as being feldspathoid in nature, but was not able to be specific as to its origins. Research done at the stone mason’s firm that supplied it has proved fruitless as the records were destroyed when the firm was taken over a couple of years ago. Incidentally, a Worshipful Master of the Oddfellows has built the tomb in anticipation - there was no occupant to disturb.

After this, we made our way to visit some favourite graves with geological interest. The first of these is a grave in the shape of a barrow, an oval turf banked enclosure with a serpentinite monolith for a naturalist. Second, we found the memorial to Sir Charles Lyell, often referred to as the Father of British Geology. Our last stop was the grave of Harry Seeley FRS FGS FLS, whose marker sports a familiar crossed hammer and pick logo – now where have I seen that before?

You can see pictures of this walk on the Branch website and you can learn more about Brookwood and the Cemetery Society at their website www.tbcs.org.uk

And what are the best materials for grave markers and memorials? Clearly the hard rocks, igneous and metamorphic, are the best if you want them to last, but we did see examples of wooden markers over a hundred years old that still looked in good condition and a terracotta grave of similar age that appeared as if it had been erected yesterday!