Burren and Cliffs of Moher UNESCO Global Geopark - Geology

Erratic Rocks
A notable feature of the exposed rock of the
Burren are the large boulders that sit on top of
the limestone. Some of the best examples are seen
along the coast road between Ballyrean and Black
Head or Rock Forest near Mullaghmore. These are
called ‘glacial erratics’ and they were transported to
their present position by large ice sheets that moved
across Ireland during the last period of glaciation
which was at its maximum about 22,000 years ago.
Boulders such as this are known from across
northern Europe and North America and during
the 17th Century they were originally thought to be
evidence of the Great Flood. It wasn’t until the 19th
Century that the idea that major glaciations in the
past had shaped the Earth’s surface became accepted
thanks to publications by eminent geologists such
as Charles Lyell’s Principles of Geology in 1830.
The name comes from the Greek word ‘errare’
meaning to wander. The term is applied to rocks
that have been carried some distance from their
original outcrop. We have some excellent examples
of large granite boulders that have come from
across Galway Bay. There are no outrops of Galway
Granite in the Burren so the boulders must have
been transported here. The vast majority of our
erratic boulders are limestone and local to the
Burren, so the transport distances aren’t huge.
One of the useful features of erratics is that
they can tell us the direction that the ice flowed,
so we know that the Galway granite boulders
were carried south to the Burren by ice sheets
flowing from Connemara. In southern County
Clare at the Bridges of Ross we have erratics that
were carried there from County Kerry whereas
in eastern Clare we find erratics carried from
the Slieve Aughty mountains. This tells us that
local ice sheet flow spread out from regional
upland areas and that while the overall flow
of the main ice sheets in Ireland was from the
northeast, the local pattern can be more complex.
While currently these boulders stand proud and
exposed it is possible that they were once part
of glacial Boulder Clay deposits (a mixture of
boulders and ground up rock clay) which have lost
all the finer material due to erosion. The amount of
Boulder Clay that has been eroded, the erosional
processes involved or when that erosion happened
has yet to be established. Some boulders are split in
half or thirds, this is most likely due to the action
of freeze-thaw processes in the periglacial period
after the rock was deposited when severe seasonal
freezing and thawing would have been common.
More recently, these erratics can be used to
establish when the last ice sheets melted. Dr.
Gordon Bromley, a glaciologist from NUI Galway
is currently studying our Buren erratics. By
calculating when these boulders were last exposed
to cosmic radiation, it is possible to estimate when
they were exposed after the last ice sheets melted.
For an alternative and thoroughly entertaining
story about how the Burren got covered in boulders
I can recommend Eddie Lenihan’s book ‘Irish Tales
of Mystery and Magic’ published by Mercier Press.
Article written by Dr. Eamon Doyle, Geologist for
Burren and Cliffs of Moher UNESCO Global Geopark, Clare County Council.
www.burrengeopark.ie