Geology & Geomorphology

Share this:
The shape of the land is the result of the natural forces of erosion that have worn down rocks and then deposited accumulations of sediments in other places.  The study of landforms is a branch of Geography called Geomorphology.

County Sligo has a varied and attractive landscape partly resulting from the diverse types of rock that make up the geological foundation.  One of the most obvious effects of rock type can be seen in the mountain scenery where limestone erodes to give vertical cliffs with flat summits to the hills, as seen in the Dartry plateau and at Knocknarea; as compared with the metamorphic rocks of the Ox Mountains that have eroded into rounded hills and mountains.
Ox Mountains near Ballysadare showing more rounded and worn down profile with rounded rocky outcrops.  The very green pasture fields in foreground are underlain by limestone.  Photography: Mr. Don Cotton
Throughout Ireland the iconic landscape of Benbulbin and the Dartry Mountain plateau is so well known that in many people’s minds it symbolises County Sligo.  This magnificent rock edifice rises abruptly some 526m from sea level and is largely composed of Carboniferous limestone that is very nearly horizontally bedded, although it does sit on layers of clay-rich shale near the base. This limestone rock was carved by glaciers coming west down Glencar and around from the north-east cutting the face of Benwiskin on the way.  Since the glaciers cut the vertical faces out of the limestone rock, and melted away, the processes of sub-aerial erosion have taken their toll as the limestone has gradually fallen away to create the characteristic talus slope composed of limestone talus or scree.
Dartry Mountains showing limestone scenery of flat plateau summits & cliffed walls. From left to right:  Benwiskin, Benbulben and King’s Mountain. Photography: Mr. Don Cotton
Whilst writing about Glencar it is important to note that this is a magnificent example of a glacial U-shaped valley with a ribbon lake.  Glenade situated just a few kilometres further north in County Leitrim is less well known, but is also a U-shaped valley with a glacial ribbon lake.

Glaciers not only carved the landscape but they also carried vast amounts of sediment that were shaped and deposited beneath the ice as drumlins and eskers, or were washed out by streams and rivers as the glaciers melted and dumped as more sorted deposits.
There are several good examples of storm beaches along the Sligo coast.  As their name implies, these are formed when the sea has high swells out in the Atlantic Ocean that send high energy waves with a long reach, and often with long wave lengths, crashing on to the shore.  The result is a steep beach composed of rounded cobbles and shingle that most of the time is above the normal tidal range and therefore above the sandy beach or wave-cut platform.  Examples can be seen at Cloonagh (see photograph), Ballyconnell and at Pollacheeny.
Estuaries are defined as semi-enclosed, tidal river mouths.  Sligo has five estuaries of which the four largest are all river valleys that were drowned when sea levels rose after the melting of ice at the end of the last glaciations.  Sand was then deposited at their mouths by long shore drift which partially closed off their mouths. 
There are many examples of swallow holes where surface water ‘disappears’ underground to come up again through springs where it may create important ecological habitats called fens and flushes.  Lough Nasool (the Lake of the Eye) near Riverstown is named after a large swallow hole in its centre that gets blocked with natural debris and mud, then periodically becomes unblocked and the water of the lake can drain away overnight!
A turlough is a temporary lake that forms in limestone districts when the water table is high but drains away when the water table falls. Sligo and Leitrim are at the northern limit of turloughs in Ireland but never the less, there are several water bodies that qualify. One of the finest examples is Lough Gowra in the Bricklieve Mountains.
Lough Gill is Ireland’s 14th largest lake and its origin is similar to two Mayo lakes, namely Lough Corrib (2nd largest lake) and Lough Mask (6th largest lake).  All three of these lakes overlie limestone but have one shore dominated by acidic rocks.  It is believed that in the case of Lough Gill, the acid water running off from the Ox Mountains (Killerry Mountain and Slieve Daeane) have dissolved away the limestone to form a hollow that is now occupied by the large lake.
The walk at Raghly provides an opportunity to view the sea on both sides of tombolo.
Two glacial erratics laying on Carboniferous limestone at Streedagh Point
Peat outcrop on Strandhill beach
A glacial erratic – the “split rock” near Easkey

No walks reviewed yet

Why not be the first to review a walk?

link to flippbook

Please view the Sligo Walks brochure online.