Cabinet of Curiosities

At The Cutting Edge

Antrim chalk

Chalk is a sedimentary rock which formed on an ancient seabed around 75 million years ago in the Cretaceous Period. It is mainly composed of the accumulated fossilised remains of micro-organisms known as coccolithophores. These are a type of phytoplankton which secrete a calcium-rich protective shell made up of around 30 separate connected plates known as coccoliths, each of which is a mere 3 one-thousands of a millimetre in diameter. Pretty small for sure! Incalculable numbers of these minuscule shells rained down onto the seabed and were subsequently compressed, compacted and cemented together over millions of years to form the distinctive white coloured rock, the name for which derives from the Latin word ‘creta’ meaning simply ‘chalk’.

Flint nodule from Lyme Regis, Dorset
Knapped flint

The famous White Cliffs of Dover in England are composed of it. This rock once covered most of the island of Ireland but, through the combined processes of weathering and erosion over millions of years, has been stripped away. Mainly only surviving in Co. Antrim, where it is covered by a protective layer of basalt, it is visible as a thick white band along the coast. Flint occurs as irregularly shaped nodules in the chalk and, fracturing into flakes when struck with a hammerstone, was an ideal material from which prehistoric people fashioned everyday tools and weapons. It is a type of quartz

Flint nodules from beach shingle, Wexford
Ice Age oysters

The only sizable exposed in-situ source for flint in Ireland is in Co. Antrim. However, during the last Ice Age an extensive ice sheet filled the Irish Sea basin and, as it moved southward, bulldozed the sediments and underlying rock layers on the seabed. This material was later deposited by the melting ice as boulder clay on land, since which time large quantities of flint nodules have been eroded out and are now incorporated in the shingle on beaches along our eastern seaboard. The same boulder clay contains marine shells, including oyster (Ostrea edulis), which are certainly tens of thousands of years old. The art of flaking flint is known as ‘knapping’ and the resultant flakes were retouched by craftsmen who produced a range of items including knife blades, scrapers and projectile points. The consummate skill of one of our Neolithic forebears is readily apparent from this arrowhead which dates to sometime around 5,000 years ago.

Neolithic flint arrowhead

The earliest evidence to date for the use of flint in Ireland comes in the guise of cut marks on the front surface of a brown bear’s kneecap which has been dated to 10,500 BC. Inflicted by a flint blade when the carcass of the bear was being butchered, the marks were originally noted in 1903 by the team who excavated the ancient deposits in a cave near Ennis, Co. Clare. It was found along with thousands of other animal bone fragments. This makes the butchered kneecap the earliest definitive evidence for the presence of humans in Ireland, the only other explanation being that it belonged to a self-butchering bear!

Chert nodules in limestone

Chert is almost chemically identical to flint and was also used as raw material for making tools and weapons in our prehistoric past. Mesolithic hunter-gathers established a temporary camp on the shores of what was a shallow inland lake 8,500 years ago. The site is today known as Lough Boora, in Co. Offaly. Here they made tiny flakes or microliths from locally available chert. Only a few centimetres long, a number of these would have been mounted together as part of a composite implement, perhaps a spear or such like, used for fishing, hunting and fowling. Chert is found in Carboniferous limestone, Ireland’s commonest rock, in the form of thin bands and nodules.

Lipari obsidian

Obsidian is a black volcanic glass which, just like flint, fractures when struck. Its flakes were worked into razor-sharp tools and weapons. An important source is centred on Lipari in the Aeolian Islands north of Sicily. Recent archaeological research has established that the volcanic glass from this island was traded widely in prehistoric times.  



Cabinet of Curiosities

A Burning Issue

The compressed remains of ancient plants

When you pick up a piece of coal you hold in your hand the compressed fossilised remnants of a primeval forest. Around 300 million years ago, a time when most of what we now call Ireland lay about 10 degrees south of the equator, giant tree-ferns and clubmosses flourished in hot, humid conditions which saw the development of extensive areas of swamp. Over time, as the plants died, their remains accumulated in the prevailing waterlogged conditions and formed thick layers of semi-rotted dead vegetation or peat. This period was characterised by fluctuations in sea level which rose and fell on numerous occasions. Each rise flooded the swamps and covered the peat layers in deposits of silt and sand. When the water level once again fell, new swamps clothed in vegetation became established and so the cycle repeated itself many times over millions of years. During the passage of time, the alternating layers of silt, sand and peat were compressed and compacted into layers of shale, sandstone and coal respectively, comparable in a sense to a gigantic, multi-layered rock sandwich. This occured in the latter stages of the geological period called the Carboniferous, meaning ‘carbon-bearing’, a reference to the extensive coal deposits which were laid down at that time..

Stigmaria fossil

One of the main groups of plants growing in the swamp forests were the lycopods. These were giant clubmosses, some of which attained tree-size proportions. They include a species known as lepidodendron, which could reach heights of up to 38 metres. Its ‘root’ system is known in the fossil record as stigmaria and has a readily recognisable ‘dimpled’ pattern. Another common fossil clubmoss is sigillaria. It takes around 15 cm of peat to form 1 cm of coal. The large sample I am holding was gifted to me in 1977 by a friend whose father owned a fuel depot in Harold’s Cross in Dublin. It is 20 cm in thickness and therefore represents the compressed remains of a 3-metre deposit of ancient peat and weighs 10 kg. The upper surface displays the linear pattern of the trunk of a Sigillaria. It is estimated that as much as 70% of the coal in some deposits may consist of the compacted remains of lycopods. Smaller plants also grew in those tropical forests including many species of fern and Pennsylvania in the USA is famous for them. I regularly check through my coal before consigning it to the flames and over the years have been amply rewarded with some nice fossils.

300 million year old Sigillaria fossil
Fossil ferns from Pennsylvania
Clay-ironstone nodules

Clay-ironstone nodules are natural concretions which occur in shale and are commonly found during coal mining. They are composed of a blend of fine clay particles and an iron mineral known as siderite. Also known as ‘iron balls’, they were processed from the mid-sixteenth century onwards as a source of iron when coal extraction began in the Castlecomer area of Kilkenny. Indeed, some of the railings and gates in the locality are made of ‘Castlecomer iron’. The nineteenth-century ironworks in Arigna, Co. Roscommon, was supplied with siderite-rich nodules collected by men and women in the layers of shale exposed along the courses of the local rivers.

What would I give ….!!
Newgrange Visitor Centre

Clay-ironstone nodules come in a variety of shapes and sizes and over the years I have amassed a significant collection. When the Bru na Bóinne Visitor Centre in Co. Meath was being built some years ago, the machines removed vast quantities of the local shale and some large nodules surfaced. Some of these were retained and later incorporated in the water feature located just outside the main entrance to the centre. The largest specimen I have ever come across on my travels is on display outside the Natural History Museum in Bordeaux in France.