Ken Brooks (UK)
This is the first of three articles on the geology and fossils in the cliffs and foreshore to the east of Hastings. This one is intended as a field trip. The geology here is all Lower Cretaceous and is some of the best in Britain if you are interested in this period of time. Follow the Hastings seafront eastwards to the ‘Old Town’ and the famous ‘net shops’ in Rock-A-Nore Road. Below the high, sandstone cliffs of the East Hill, you will find the Fishermen’s Museum, the Blue Reef Aquarium (a sea-life centre), a large car park and public toilets.
This field trip begins at the last stone groyne and continues along the beach towards Ecclesbourne Glen, nearly one kilometre (half a mile) to the east. The massive sandstone cliffs of the Upper Ashdown Formation are overlain by the shales and sandstones of the Wadhurst Clay. A distinct junction between one horizontal bed of rock and another often marks a period of erosion. This may have been followed by a change in the environmental conditions where a different grade or type of sediment was deposited.
In this area, the lower part of the cliff is hidden under a scree slope of broken rocks, but there is one small exposure in situ at beach level. Here, there are flattened branches of carbonised wood lying horizontally within a silty mudstone. These were probably washed into a river or lake, then later covered and compressed by sedimentary layers. The leaves of small ferns, such as Weichselia, may also be found at this location. These delicate plant remains are preserved as fossils only because they contain carbon – which is derived from the same carbon that was fixed by photosynthesis over 100 million years ago.
Fragments of the tree fern Tempskya occasionally fall from a layer near the top of the cliff. After the plant had died and been covered by sediments, minerals in its ‘trunk’ were gradually replaced by silica (quartz). In fact, with a hand lens, tiny crystals of quartz may be seen lining the spaces between a mass of brown-stained fibres. The outer part of a section is sometimes covered by fusain, a friable layer which represents the carbonised remains of external tissues.
The fine-grained sandstones and siltstones occasionally contain specimens of the gastropod, Viviparus. This is also the name of its present day descendant, which lives in ponds and streams. This similarity suggests that the fossil Viviparus probably also lived in a freshwater environment. Isolated specimens of this gastropod may sometimes appear among shells of the small bivalve, Neomiodon.
This bivalve is the most common of all fossils in the Hastings area. The bedding planes of sandstones are often covered with its shells, and layers of them can be seen in cross-section on the sides of blocks. There are also larger bivalves which may be found locally, such as Filosina and Unio, but these are less common. Small tube-like and oval structures are often found in rocks near the shells of Neomiodon, and these have been identified as its burrows and faecal pellets. Where the sediment has been disturbed or burrowed by any animal, it is known as ‘bioturbation’.
Some siltstones have preserved the remains of conchostracans, small crustaceans which are often described as ‘clam shrimps’. These were enclosed in shell-like carapaces, usually measuring about 7mm in length and, because of this, their fossils are sometimes mistaken for bivalve shells. Even smaller are the oval-shaped ostracods, Cypridea, which are also crustaceans, but as they measure only about one millimetre in length, a hand lens is required to examine them closely. Although their tiny seed-like carapaces can be found in large numbers on the bedding planes of thin shales, it is easy to overlook them on a field trip. Some species of Cypridea have ornamented carapaces with raised ‘pimples’, while others have smooth surfaces.
The siltstones also show evidence of the insects that lived around the streams and lakes. For example, the larval tubes of caddis flies Trichoptera may be found. The larvae of these insects used locally available materials to build the tubes which would serve as temporary homes while they lived in the water.
Fish and reptile remains are usually found as isolated fragments, probably because they were broken up and dispersed by streams or rivers. These fossils include the glossy, brown scales and small rounded teeth of the fish, Scheenstia (formerly referred to as Lepidotes), and the pointed teeth and fin spines of the shark, Hybodus.
Individual scales from Scheenstia are common, but to find an undamaged specimen of this fish is rare. Part of a body with well-preserved scale impressions is still visible in a hard sandstone block near Rock-A-Nore, and a few years ago a complete Scheenstia was discovered at Covehurst Bay (near Fairlight).
There are also the scutes (hornlike plates) from turtle shells and the bones and teeth of crocodiles, such as Goniopholis and Bernissartia. Turtle shell scutes can be smooth or covered with raised ‘pimples’, while the dermal plates from crocodiles’ skin have circular dents in them. These can sometimes be found in iron-stained ‘pebble beds’ or on the bedding planes of a friable sandstone. This rock’s crumbly surface is the result of disruption by salt crystallisation from sea water, and freshly exposed layers of a particular bed frequently contain fossil specimens.
Some of the rarer fossils from this area include the phalanges (finger bones) of the pterosaur Ornithochierus, one of the largest of all the flying reptiles. Such specimens may be identified as pterosaur remains, as the once-hollow bones which allowed them to fly are usually crushed flat. Perhaps the most impressive finds are the teeth and bones of dinosaurs. These can occasionally be discovered in the shale and clay where the scree slope meets the beach.
Trace fossils, such as footprints and burrows, sometimes provide valuable information about an animal’s presence and behaviour, particularly where no other evidence has been found. However, the nature of the species itself can be difficult to deduce, as different organisms may have had a similar mode of life. The large tridactyl (three-toed) footprints of dinosaurs, such as Iguanodon, which may be seen on the bedding planes of certain siltstones are relatively common.
Some of the prints can measure up to 60cm from heel to toe. These dinosaurs might have been walking by a river or lakeside and left their footprints in the mud. This quickly dried out in the hot sun, and water then deposited silt or sand in the hardened impressions, later forming infill casts. Gradually, over millions of years, these sediments turned to rock. Today, either the footprint impressions or the casts may be revealed when the weaker rock layers are removed by erosion.
It is fascinating to consider that, while all fossils are natural reproductions of shells, bones and teeth, a dinosaur footprint is the actual impression left in the mud millions of years ago by a living animal. Slabs of a bluish-grey rock with strange bulbous shapes may be seen on the beach in this area.
These are fallen blocks of Tilgate Stone, which come from various strata within the Wadhurst Clay. In the past, this rock was known by local quarrymen as ‘Bluestone’ or ‘Hastings granite’. It is a hard, fine-grained sandstone, which is strongly cemented by calcium carbonate (calcite) into tabular and nodular masses. The Tilgate Stone’s unusual dome-shaped structures may have been formed by the attraction of calcium carbonate from groundwater around a tiny fragment of shell or a crystal of calcite. The concentric hardening of the rock gradually produced the rounded structures (concretions) that we see today, now that the poorly cemented outer layers have been removed by erosion.
About half a kilometre (quarter of a mile) to the east of Rock-A-Nore, the horizontal strata of the cliff are displaced by the Foul Ness Fault. However, this is obscured by a section of cliff that has slipped down to form a small plateau. Below this is a scree slope of broken rocks and shale where segments of the horsetail Equisetites may be found in mud slides on the beach. These specimens usually measure between 3cm and 8cm in length. They can be recognised by their ‘striped’ appearance and tiny holes where leaves once grew around the stems. Other interesting discoveries that have been made in this area include fossil bones and an Iguanodon tooth.
From here to Ecclesbourne Glen most of the fallen rock and scree material lies directly below vertical cliffs and, for safety reasons, close examination is not recommended. However, there are still features of interest in scattered rocks along the foreshore, and even the shingle itself is worth closer study. It is important to realise that the flint pebbles covering the beach do not originate from this area. The sea has washed them along the coast from Beachy Head, where the high Chalk cliffs contain many nodules and thin beds of flint.
Between 100 and 65 million years ago, Britain was covered by a shallow sea where sponges and other marine animals extracted silica from the water to strengthen their fragile structures. Later, over a period lasting many millions of years, their skeletons dissolved and the silica eventually recrystallised as concretions of flint.
It should be pointed out that not all geologists agree with this explanation and that the chemical processes involved in the formation of flint may be far more complex. For example, the silica could have been precipitated directly from sea water. Heart-shaped echinoids (sea urchins) are among the fossils that may be found in the shingle, but more common are the hollow flints that once enclosed delicate sponges. These often contain very small and beautiful crystals of quartz (chalcedony), which are best seen with a hand lens.
Ecclesbourne Glen is an ancient river valley. It was carved out by vast quantities of melt-water, which flooded across southern England at the end of the last ice age, around 10,000 years ago. Since then, the sea has gradually cut back into the cliff to form a hanging valley with a small waterfall about 6m above the beach. This location marks a suitable turning point for walking back to Rock-A-Nore.
The next article is a short summary of the geology of the area.
|Rock-A-Nore (Hastings), Sussex (OS map 199. BGS sheet 320/321. Grid ref. TQ 831 095)|
|The other articles in this series are as follows:|
|Hastings (Part 1): Field trip from Rock-A-Nore|
|Hastings (Part 2): Geology and fossils|
|Hastings (Part 3): When Dinosaurs Roamed|
Early Cretaceous Environments of the Weald, Guide No 55, by Alistair Ruffell, Andrew Ross and Kevin Taylor, The Geologists’ Association, London (1996), 81 pages (softback), ISBN: 0900717882.
English Wealden fossils, Palaeontological Association Field Guide to Fossils No 14, edited by David J Batten, The Palaeontological Association, London (2011), 769 pages (softback), ISBN: 9781444367119.
Geology and Fossils of the Hastings Area, by Ken Brooks (2nd edition), Ken & Diana Brooks (2014), 76 pages (softback), ISBN: 9780957453050.