ASPECTS OF IRISH GEOLOGY EXPLAINED — SIMPLY
Paddy Gaffikin on what we need to know about the most recognisable rock in Ireland:
pocket near Killarney, it only occurs now in the north east. The most likely reason for this is that the basalt, which was extruded over the chalk around 60 million years ago (Early Palaeogene), in the north east of Ireland, prevented the erosion of the chalk.
CHALK
Dull would he be of soul who could pass by A sight so touching in its majesty.
Why is the N. Ireland chalk so hard?
W. Wordsworth (1770-1850).
Description
With the single exception of a very small exposure at Ballydeanlea, near Killarney, Co. Kerry, the chalk in Ireland is exclusive to the north-east of the island – the bulk of it being in Co. Antrim. Now often called the Ulster White Limestone, it is very easily recognisable because, when fresh, it is pure white in colour, although parts of it are darkened due to weathering and soiling. The reckoning of the Antrim Coast Road to be one of the most scenic coastal routes in Europe, is due in no small part to the majestic chalk cliffs on the inland side of the road. The white colour contrasts noticeably with the overlying black basalt and the dark bluish clay below. It is composed of up to nearly 100% pure calcium carbonate (CaCO3). In many places, flint nodules occur in the chalk. The Stone Age people, who first arrived on our shores around 9,000 years ago, used this flint for the manufacture of axe-heads, arrow-heads and scrapers – artefacts which played a significant role in their survival.
2cm
A sample of Co. Antrim Chalk showing a fresh broken surface.
Why is the chalk not found over most of Ireland?
It could be speculated that the Cretaceous sea, in which the chalk formed, covered all or most of Ireland. Except for the small Tiny cocoliths, too small to be seen without the aid of a microscope, accumulated to form chalk. This one, viewed by an electron microscope, is about 8 µm across.
When and how did the chalk form?
Fossils embedded in the chalk indicate it formed some 80 million years ago (Late Cretaceous). To put this in perspective, this was around the time when dinosaurs such as the giant Tyrannosaurus rex were at their numerical peak in N. America and probably China. It formed in a warm shallow sea from accumulations of fragments of the calcareous ‘shells’ (called coccoliths) of microscopic algae called coccolithophorids. In other words, the Ulster White Limestone is a mass of microfossils! They are so small, however, you could not see them even with an optical microscope – you would need a complex instrument called an electron microscope, which can give magnifications of many thousands.
The chalk in N. Ireland is harder and denser than say most of the contemporaneous English chalk – e.g. that of the White Cliffs of Dover. One explanation for this may be that the weight of the overlying basalt compressed the chalk thus causing it to harden. (compression of rock can generate pressure and heat.) But there is a problem with this theory because the Yorkshire chalk, which was not subjected to such metamorphism, is nearly the same hardness as the Ulster White Limestone. So, further research would be needed in order to solve this puzzle.
Fossils found in the chalk
BELEMNITES (Time range: Late Carboniferous to Early Palaeogene – but were most numerous during the Jurassic and Cretaceous Periods.) These would be about the commonest macrofossils in the Antrim chalk. In life, these now-extinct carnivorous marine invertebrates, would have resembled the present-day squids and cuttle fish. A belemnite fossil consists of a calcareous bullet-shaped shell, which would be quite a bit shorter than the whole length of the living animal. While living, the hard (internal) shell was totally surrounded by soft tissue, which, except in very rare cases, is not preserved. SEA-URCHINS (Time range: Ordovician – Present-day.) Also called echinoids, sea-urchins in the past were exclusively marine and this is also the case with ones living today. Their fossils can have a globular, heart shape or flat shape. While living, their surface was covered with spines but these usually break-off after death. The spines offer protection from predators and would also assist with movement. The globe-shaped ones moved across the sea bed while the heart-
SCIENCE SPIN Issue 66 Page 23