The Cretaceous Chalk in Southern England

By Brieanna Graham

During the Cretaceous Period chalk deposits built up over most of the planet (Wicander 2004). This was a result of almost the entire planet experiencing the same environmental conditions. The earth was going through a warming period following the Permian, and it reached a climax at the end of the Mesozoic, the Cretaceous (Wicander 2004). This environment was characterized by warm, shallow seas covering most of the continents and no ice caps (Wicander 2004). The image below shows the location of the landmasses in the Late Cretaceous and sea level at that time (Scotese 2003).

 

On a worldwide scale sea level had risen 600 meters and “many continental areas, which had been land since the Paleozoic, were flooded by shallow water” (Oxford Museum 8 Aug 2006). Because of this warm environment a thick layer of chalk formed in most of the world’s oceans and can now be seen in places such as France, Scotland, Greenland, Asia, and Texas (Rayner 1967). Along the Jurassic Coast of Southern England the chalk is “synonymous with the Upper Cretaceous beds” (Bennison 1969). It can be seen at various points along the Jurassic Coast and is the youngest rocks found in this World Heritage Site (Gallois 1995). A long and complex story is associated with the chalk.

            Chalk is “made up of whole or broken coccoliths…up to 10μm in diameter” (Melville 1982). Four hundred million coccoliths would be necessary to cover a single £1 coin (Gallois 1995). Below is a model of a coccolith enlarged 10,000 times in the Museum of Natural History.

More specifically, “coccolithophores are small, single-celled autotrophs covered with disks of calcium carbonate (coccoliths) fixed to the outside of their cell walls” (Garrison 2002). These tend to live near the surface of the ocean in areas where there is an abundance of sunlight (Garrison 2002). Coccolithophores can become so abundant that they will give the water a milky or chalky appearance (Garrison 2002). In warm water coccoliths bloom and in clear, undisturbed water their skeletons can sink to the ocean floor and be preserved to form chalk (Brundsen 2003). Coccolith shells form a matrix in which other calcium components are often trapped and subsequently embedded into the chalk (Melville 1982). The coccoliths in Southern England “are present in vast numbers and in all stages of disintegration down to individual component crystals” (Chatwin 1960). In this area many other calcium carbonate remains are found, including, “calcispheres (Oligostegina), formanifera, and fragmentary debris of many larger invertebrates (especially the bivalve Inoceramus)” (Melville 1982).

The chalk in Southern England is considered a calcareous ooze, which is an “ooze composed mostly of the hard remains of organisms containing calcium carbonate” (Garrison 2002). The accumulation of ooze on the sea floor depends on “a delicate balance between the abundance of organisms at the surface, the rate at which they dissolve once they reach the bottom, and the rate of accumulation of terrigenous sediment” (Garrison 2002). Oozes accumulate very slowly, at a rate of only about 1 to 6 centimeters per thousand years (Garrison 2002) but the deposition of the chalk in Dorset took place 100 million years ago until the end of the cretaceous 65 million years ago (Gallois 1995) so it had a long time to build up over 410 meters of chalk (Rayner 1967).

            The environment that the chalk formed in was significant to how and why it formed as it did in Southern England. The chalk layer “indicates the most considerable change in conditions in Purbeck times, namely, an invasion of the sea which spread over a large part of the south of England” (Melville 1982). At the beginning of the Mesozoic the supercontinent Pangaea began rifting apart and drifting to its present day locations (Wicander 2004). By the Cretaceous Period “America continued to drift away from Europe” and the “Atlantic Ocean and Mediterranean expanded” resulting in the entire area being drowned in a warm, shallow, extensive sea (Brunsden 2003). Most of Britain was under water and “evidence of shorelines [was] scanty even in the western outcrops” and near Scotland (Bennison 1969). Throughout the Cretaceous “shallower water conditions became established at times but there is no evidence that any area became emergent” (Bennison 1969). Bioturbation and nodular beds are often used as evidence to show that the water was shallower at times (Bennison 1969) as well as ripples, or current action (Chatwin 1960). “Marls indicate an increase in terrigenous material possibly consequent on the elevation of neighboring land areas” (Bennison 1969). The environment in which the chalk was deposited was not consistent; there is evidence of cyclic deposition “with episodes of low energy and steady accumulation alternating with ones of higher energy and perhaps shallower water, when deposition was interrupted” (Melville 1982).

            The climate was much warmer when the chalk formed than it is now. We know that the water was warm because of the composition of the rock. The billions of cocolithophores which made up the chalk only survive in warm, relatively pure water, so this sea must have had these conditions (Gallois 1995). Calcareous oozes can only form in water less than 4500 meters in depth, so the chalk was not formed in any sort of deep sea environment (Garrison 2002). This chalk had to have been deposited in water approximately 200 to 300 meters in depth because this is the only way the small, delicate cocolithophores would have been preserved (Melville 1982). The chalk is thought to have formed on the outer edge of a continental shelf, the biggest in any Jurassic sea found along the Southern Coast of England (Melville 1982). At this point in time England is thought to have been situated at “a more southerly location…than at present” which would have augmented the already higher temperatures (Lovell 1977).

The Chalk Sea lasted for approximately 30 million years (Ensom 1998) which was enough time for billions of coccolithophores shells to be deposited on the seafloor (Gallois 1995). One thing that makes the chalk so unique is the near “absence of terrigenous material” (Chatwin 1960). This is due to the fact that land was a great distance away from the depositional environment (Chatwin 1960). Also, the land had to have been fairly flat and would not have had a good mechanism with which to bring detrital matter out to the open sea (Rayner 1967). The chalk is especially unique because it has the greatest outcrop area of any formation in England (Rayner 1967).

            Within the chalk there are three subdivisions, the Lower Chalk, the Middle Chalk, and the Upper Chalk. The contacts between these layers are gradational, indicating a gradual change in environment (Wright 1981). The layers of chalk can be identified by their lithological characteristics, as well as the fossils they contain (Chatwin 1960). Over time, the water in the area became deeper and the chalk became purer (Gallois 1995). Only the Upper Chalk has many economic uses, “it writes readily on wood, and is fit for the cooper, for whiting, for lime, and for manure” (Wright 1981). The Middle Chalk is noted as only being fit to use on highways (Wright 1981). Overall however, the chalk is hard enough to be a significant building tool but only on the local level because it is so much softer than the surrounding limestone (Ensom 1998).

            The Lower Chalk is situated directly above the Glauconitic Marl and is mostly composed of Chalk Marl and Grey Chalk (Melville 1982). The lowest portion of the Lower Chalk, called the Varians Zone, begins with “the glauconitic ‘Chloritic’ marl, sandy and nodular, which grades up into grey marly Chalk” (Bennison 1969). The sponge Exanthesis labrosus is very common in the Varians Zone in the Isle of Wight (Melville 1982). The Lower Chalk as a whole is characterized by “ammonites of the genera Schloenbachia, Mantalliceras, and Hypoturrilies, bivalve shells including ‘Aequipecten’ beaveri, Entolium sp., Plicatula inflata and Inoceramus crippsi, and tubes of the worm ‘Serpula’ umbonata” (Mellville 1982). The Upper Part of the Lower Chalk is characterized by the belemnite Actinocamax plenus and ends at the Plenus Marls where it then gives way to the Middle Chalk (Melville 1982).

            The Middle Chalk varies in thickness by up to 60 meters, but in most places it is much less than that (Melville 1982). The base is very nodular and is “hard and white in places and elsewhere containing slightly yellow nodules and lumps in pale grey streaky marl” (Melville 1982). This basal bed is called the Melbourne Rock (Bennison 1969), but is not recognizable in South Dorset and the Isle of Wight (Melville 1982). Common fossils found in the Melbourne Rock include: brachiopods and the echinoids as well as the bivalve Mytilides mytiloides (Melville 1982). The rest of the Middle Chalk is mostly “white and homogenous, with marl partings” showing up occasionally (Melville 1982). The Middle Chalk is usually without flints except for the uppermost 30 feet (and in the Isle of Wight is completely without flints)” (Bennison 1969). One break in the homogeny is that in Swanage and the Isle of Wight, “the upper part of the Middle Chalk becomes lumpy and nodular” and is termed the ‘Spurious Chalk Rock’ (Melville 1982). The most common fossils found in the Middle Chalk are brachiopod shells, echinoids and bivalve shells (Melville 1982). In the upper part of the Middle Chalk the echinoid Holaster planus and Micraster sp. are common, as well as the bivalve Inoceramus lamarcki (Melville 1982). Throughout the Middle Chalk actual ammonites are rare, but there is evidence of their presence (Wright 1981). “Ammonite aptychi and impressions of ammonite shells [are found] on the bases of corals, oysters and other originally calcitic cemented forms” (Wright 1981).

            The Upper Chalk is the largest portion of the entire chalk beds (Melville 1982). It is up to 404 m in the Isle of Wight and 260 m in Dorset (Melville 1982). It is described as “milk-white, fine in its grain and smooth when cut” (Wright 1981). The base of the Upper Chalk is marked by the hardest layer of all, the Chalk Rock (Melville 1982). It has a nodular character and was deposited in unusually shallow water (Bennison 1969). It contains the fossils of sponges and the moulds of ammonites and gastropods (Melville 1982). Above the Chalk Rock the chalk becomes very smooth, white, and “massively bedded, except for nodular beds and hardgrounds in…the pilula Zone of the Isle of Wight and Dorset” (Melville 1982). There are many different types of fossils present in the Upper Chalk (Bennison 1969). These include remains of fishes, in particular teeth are quite common, as well as the remains of sharks and teeth of ray fishes (Melville 1982). This provides extra evidence to support the statement about how calm the water was in which the chalk was found. Sharks have a cartilaginous skeleton, and are usually not preserved well since they are so fragile (Wicander 2004). To find a shark skeleton means that the water was not disturbed much over a long period of time. Echinoids such as Micraster and Echinocorys are common and can be used to correlate different layers of chalk (Melville 1982). Ammonites are also used for correlation including the Hyphantoceras reussianum (Bennison 1969). Other fossils present include: gastropods, bivalves, oysters, brachiopods, and sponges which are commonly enclosed in flint (Melville 1982).

            An important characteristic of the chalk is the formation of chert. Chert is a dense, smooth rock made up of silica (Chesterman 2005). It forms when “certain algae and protozoans, which, having accumulated on the ocean bottom [are] compacted and partly recrystallized to form chert of opaline composition” (Chesterman 2005). Other cherts form when silica is precipitated out of sea water and condenses into rock (Chesterman 2005). The chert can be found within the chalk. As the chalk breaks apart nodules or layers of chert are exposed (Davies-Vollum 28 Jul 2006). Along the Jurassic Coast the chert is typically yellow colored though it can also be various other colors (Chesterman 2005). The chert is very hard and fractures conchoidaly, like glass (Chesterman 2005). A certain specific kind of chert was especially common along the Jurassic Coast, flint (Davies-Vollum 28 Jul 2006).

Flints are common along certain areas of the Jurassic Coast where the chalk outcrops. Below is a picture of flint embedded in the chalk at Lulworth Cove.

 

For a long time the boundaries between the Lower, Middle, and Upper Chalk were determined by the abundance of flints before they were adjusted to where they are now (Wright 1981). The Upper Chalk contains an abundance of flints, while the Lower Chalk completely lacks flints (Bennison 1969). Flints are “a fine grained form of quartz, formed during the early stages of hardening of the chalk sediment” (Gallois 1995). Flint generally forms near the surface of the earth in a low pressure, low temperature environment (Chesterman 2005). It contains no crystals and has a hardness of 7 on a scale of 1 to 10, with 10 being the hardest substance on earth (Chesterman 2005). Flint is commonly found on the surface of the chalk because as the softer chalk erodes away the hard flint pieces are left on top (Oxford Museum 8 Aug 2006). “Flint formed at several different periods during and after the deposition of the Chalk” (Melville 1982). Flint filled in burrow holes in the chalk, allowing these to be preserved in areas that the burrow holes normally would be indistinct (Melville 1982). “At a later stage these may have been joined together by further growth of flint to form more or less continuous courses of nodules” which had to have formed prior to the Tertiary deposits since the flint often shows up as pebbles in Tertiary gravels (Melville 1982). Within the flints fossils of “shells of sea urchins, bivalves and sponges” are common (Gallois 1995). Flints are a “major source of aggregate for the construction industry” (Ensom 1998) and ideal for the manufacture of stone tools due to their composition and durability (Gallois 1995). In the town of Beer flint has been used as a local building tool due to its durability and availability (Brunsden 2003). However, there is a great deal of controversy about whether or not it would be wise to remove these flints because they are actually very good at preventing erosion of the chalk cliffs (Ensom 1998). Up until the 1970’s the flint was mined until “growing fears concerning depletion of the beaches led to a ban being imposed” (Ensom 1998).

            The chalk outcrops in several places along the Jurassic Coast including Beer, Axmouth to Lyme Regis, White Nothe, Lulworth Cove, and Old Harry Rocks (Brunsden 2003).  Old Harry Rocks are located on the eastern edge of the Jurassic Coast and Beer is located on the western end of the Jurassic Coast (Brunsden 2003).

Old Harry Rocks is a massive Upper Chalk formation near Sawnage (Brunsden 2003). At Old Harry Rocks the chalk has been eroded extensively. Waves will find weaknesses in the rocks and will erode these areas until they form arches or caves. These then continue to erode until the arch collapses and there becomes a tall island of chalk called a stack. Finally, the chalk stack or needle (a stack that comes to a point) will be eroded to the point that they collapse completely (Brunsden 2003).  As recently as a few thousand years ago the area around Old Harry Rocks was connected to the Isle of Wight, but they are now separated by the English Channel (Brunsden 2003). As of 1770, a person could climb out to Old Harry Rocks which are now considered to be a stack of rock not connected to the rest of the chalk (West 2006). In 1896, the Wife of Old Harry, another chalk stack, collapsed (West 2006). However, it is important to note that while some of the chalk formations are collapsing, others are constantly being created as the waves continue to erode weaknesses in the rock (West 2006). Eventually the small arch in the rocks on the left will look like the rocks on the right.

                 

Clearly, these land forms are in a constant state of erosion and change. At the site of Old Harry Rocks the chalk is nearly horizontal, only dipping at 8° North (West 2006). Several small faults and joints can be seen in this area but they are not big enough to indicate any sort of major tectonic activity (West 2006).

            There is some major faulting within the chalk, though not seen at Old Harry Rocks. At Ballard Point the chalk is curved to a near-vertical orientation and this can be viewed by boat from the English Channel (West 2006). The chalk above the faulting is from the Upper Chalk and is 76 meters thick (West 2006). Image from West 2006.

 

The thrust fault was caused by compression and probably originated from tectonic movements related to the uplift of the Alps (West 2006). The current structure of the Jurassic Coast has not been altered much since this event (Melville 1982).

            Another major geologic feature along the Jurassic Coast where the chalk can be seen is Lulworth Cove. Here the Lower and parts of the Middle Chalk make up the resistant back wall to the bay (Burnsden 2003). Lulworth Cove was formed when a stream first breached the resistant Portland Limestone (Brunsden 2003). This allowed sea water to wash back into the stream and consequentially the weaker Wealdon Beds located behind the Portland Limestone began to erode (Brunsden 2003).  As the Wealdon Beds were eroded more and more the area began to take on the shape of a cove until it finally reached the more resistant chalk beds which greatly slowed the spread of the cove to the north (Brunsden 2003). The chalk found at Lulworth Cove has been greatly altered by tectonic activity. The chalk was deposited in the Lulworth Cove area 90 million years ago (Gallois 1995). However, 30 million years ago the entire area was compressed due to Alpine Earth Movements (Gallois 1995) and now parts of the chalk have been completely overturned (West 2006).

            The chalk can be seen along the Jurassic Coast at the site of the great unconformity which can be viewed at Branscombe and Golden Cap (Brunsden 2003). The unconformity represents a time gap between rock ages, implying there was deposition of rocks, and then erosion so a great portion of the geologic record is lost. Along the Jurassic Coast “The rocks were tilted east in the Mid-Cretaceous, and then eroded by seas and rivers. There was little erosion in the east of the [Jurassic Coast] Site but in the west, all the Jurassic and Lower Cretaceous rocks are missing and the Upper Cretaceous rocks lie directly on the eroded surface on the Triassic” (Brunsden 2003). In Seaton the white chalk can be viewed right beside red Triassic rocks (Davies-Vollum 3 Aug 2006).

The chalk is very hard in this spot, and much more splintery than at Old Harry Rocks. It is closer in composition to the Beer Stone which is located below the chalk. In this spot the Triassic rocks are very soft, red, and eroded. The fault has weakened the rocks joining the two beds and consequentially there is a large V in the land between the chalk and Triassic rocks (Davies-Vollum 3 Aug 2006).

            The landscape in Southern England has been largely dominated by the chalk. The Isle of Purbeck is considered to be an island because of the chalk. The chalk has created a hard band of hills that can only be crossed in two locations, by Corfe Castle and on Highway A351 (Davies-Vollum 28 Jul 2006). This makes the Isle of Purbeck an island because it is as isolated as an island. Corfe Castle was strategically places in one of the dips that was naturally cut by a river. This means that anyone wanting to go into the Isle of Purbeck had to pass by Corfe Castle so it was an excellent place for trade and consequentially dictation of the surrounding area (Davies-Vollum 29 Jul 2006). The chalk is also responsible for the topography of Lulworth Cove, because its hard structure does not allow the cove to expand farther to the north (Gallois 1995). In addition to the topography, the chalk is responsible for some of the vegetation of the region. Because it breaks down to form such poor soil only short turf can grow there, and hardly any trees (Melville 1982). The area where this is most obvious is at Lulworth Cove. Vegetation growing on top of the chalk is sparse and short. It is easy to see where the chalk meets the Greensand because the vegetation changes so dramatically from short grasses growing on the chalk to the more shrubby, lush vegetation on the Greensand.

The Cretaceous ended in a catastrophic manner 66 million years ago (Wicander 2004). The Chalk Sea retreated and the rocks were exposed to erosion and slight folding (Melville 1982). The cause of the enormous change in environment was the arrival of the asteroid which hit earth and resulted in the second largest extinction in earth’s history (Wicander 2004). The dinosaurs, giant marine reptiles, and ammonites were all gone. An entirely different global environment dawned; one dominated by mammals, flowering plants, and grasses, like we have today (Brunsden 2003).

 

 

References Cited:

Bennison, George M., Alan E. Wright. 1969. The Geological History of the British Isles.       New York St. Martin’s Press. New York.

Brunsden, Denys Ed. 2003. The Official Guide to the Jurassic Coast, Dorset and East         Devon Coast World Heritage Site- A Walk Through Time. Coastal Publishing.       Wareham.

Chatwin, C.P., 1960. British Regional Geology The Hampshire Basin and Adjoining Areas             3^rd Edition. Her Majesty’s Stationary Office. London.

Chesterman, Charles W. 2005. National Audobon Society Field Guide to North American   Rocks and Minerals. Alfred A. Knopf. New York.

Davies-Vollum, Sian. Lecture 28 Jul, 29 Jul, and 3 Aug 2006. On site.

Ensom, Paul. 1998. Discover Dorset Geology. Dovecote Press. Wimborne.

Gallois, Ramues. 1995. Holiday Guide: Lulworth Cove Area. British Geological Survey.        Exeter.

Garrison, Tom. 2002. Oceanography An Invitation to Marine Science 4^th Edition.    Wadsworth Group. Pacific Grove.

Lovell, J.P.B. 1977. The British Isles through Geological Time A Northwest Drift. George   Allen & Unwin Ltd. London.

Melville, R.V., Freshney, E.C. 1982. British Regional Geology The Hampshire Basin and     Adjoining Areas 4^th Edition. Her Majesty’s Stationery Office. London.

Oxford Museum. 8 Aug 2006. “Cretaceous Chalk” Second Floor, Display Case #13.

Rayner, Dorothy H. 1967. The Stratigraphy of the British Isles. Cambridge University           Press. London.

Scotese, Christopher R. 2003. Paleomap Project. <http://www.scotese.com/cretaceo.htm>.

West, Ian. 2005. “Lulworth Cove—Dorset” and “Harry Rocks and Ballard Point”             <http://www.soton.ac.uk/~imw/Lulworth.htm> and <http://www.soton.ac.uk/    ~imw/harry.htm> respectively.

Wicander, Reed, James S. Monroe. 2004. Historical Geology Evolution of Earth and Life   Through Time 4^th Edition. Brooks/Cole. Belmont.

Wright, C.W., W.J. Kennedy. 1981. The Ammonoidea of the Plenus Marls and the Middle Chalk. Palaeontographical Society. London.

 

 

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