Clew Bay Drumlin's

Clew Bay 2008 by The Banner

Drumlins either oval or elongated hills are believed to have been formed by the streamlined movement of glacial ice sheets across rock debris or otherwise known as till.  The name itself was derived from the Gaelic ‘druim’ a rounded hill or mound. It first appeared during 1833. [i]

The impact over several million years produced a series of glaciations that extended across much of western Europe.  Within Ireland the most recent of these glaciations saw ice perhaps on occasion more than a thousand metres thick over the land.  As a result of the erosive work of slow – moving ice plus depositional activity under the ice or at its margins it produced a huge impact throughout.  Smaller drumlins are in west Mayo where actually some are submerged in the Clew Bay area.  Drumlins are usually three hundred metres long or perhaps possibly over one hundred metres wide. [ii]

In spite of its current moderate climate Ireland displays impressive features of past glaciation from periods of geologic history known as Ice Ages. One example is the drumlin field of Clew Bay with Newport Bay on the Mayo island’s North Atlantic coast.  The name drumlin is derived from the Irish word droiroimnín (littlest ridge).  Drumlins may be explained as features created while the glacial front melted back. The glacier the pushed up over the debris that the ice had moved forward. An inverted spoon shape is the classic form, or a narrow ridge elongated in the direction of the ice flow. Several were steepest on the upstream side then trailed off towards the newly exposed ice – free area.  The drumlins showed wave erosion that created till banks at the sea’s edge as the resultant sea level rose with the force of the melting glaciers.  The Clew Bay Ireland Coordinates are: 53.833333, – 9.8. [iii]

Clew Bay has according to tradition three hundred & sixty – five islands: one for every day of the year.  The largest one Clare Island guards the entrance to the sheltered bay.  The islands in Clew Bay are partly drowned drumlins, which are elongated, steep – sided hills that were sometimes described as whale  -backed. They were formed when glaciers reshaped the landscape during the last ice age.  Several of the hills on land around the bay are similar drumlins.  Collanmore Island near Westport is the largest island within the inner bay.  This site includes a video of Clew Bay, its islands, its people plus the site’s formation from a BBC Coast Team. [iv]

The Clew Bay drumlin field proved how powerful ice was in the shaping of the landscape. Drumlins of ice debris left by ice – sheets formed when the sea rose then drowned the low land. [v]

Michael Cusack in his research for his book ‘The Story of Clew Bay, from Granuaile to John Lennon’ stated in an article in The Irish Times of 14th June 2016 that some twelve thousand years before Granuaile Clew Bay was covered in ice.  As the temperature rose the ice retreated with wave – like patterns that left sediment on the surface of the land.  This occurrence resulted in drumlins from West to East with their massive clay boulder cliffs.  He also discovered that Clew Bay was often mentioned as a stunning example of a drumlin swarm…there are one hundred & forty – one named drumlins plus countless unnamed tidal – islands.  Cusack includes the following quote from the great nineteenth century author William Thackeray who penned of Clew Bay that, “…the bay and the Reek, which sweeps down to the sea, and the hundred isles in it, were dressed up in gold and purple and crimson, with the whole cloudy west in a flame. Wonderful, wonderful!” [vi]

Drumlin from the Irish word Droimnin or‘little ridge’ was first recorded during 1833, it was according to the classical sense an elongated hill in the shape of an inverted spoor or a half – buried egg  discovered in glacial ice that had acted on unconsolidated till or ground moraine.  Drumlins may have comprised of  layers of claysilt, sand, gravel or boulders in various proportions.  These were glacial landforms composed primarily of glacial till.  They formed near the margins pf glacial systems or within areas of fast flow deep under ice – sheets.  These drumlins were typically one to two km, less than fifty metres in height also below three hundred to six hundred metres in width, they may have had possibly a length or width ratio of between 1:2 & 1:3.5 km.  There are two main theories of drumlin formation: constructional, in which the form of sediment is shaped ie. by subglacial deformation or remnant / erosional.  The latter proposes that drumlins were formed by erosion of material from an unconsolidated bed.  A hypothesis that catastrophic sub-glacial floods formed drumlins by deposition or erosion challenges conventional explanations for drumlins.  It includes deposition of glaciofluvial sediment in cavities scoured into a glacier bed by subglacial meltwater also remnant ridges left behind by erosion of soft sediment or hard rock by turbulent meltwater.  This hypothesis required that huge subglacial meltwater floods produced raised sea level by tens of centimetres in a few weeks. [vii]

Dominant Mountains surround Clew Bay in Mayo on both sides where small islands called Drumlins are located.  Ten thousand years ago these were created as Glaciers when ice melted then retreated with the resultant small sand or shale islands formation.  The reported three hundred & sixty – five islands contain some partially submerged sandbanks only. [viii]


These images may be viewed at this site along with a Map of Ice Age Europe by Stephen Hannon:

Drumlins in Clew Bay, Co. Mayo –Copyright of the Geological Survey of Ireland 2006.  Drumlins in Clew Bay – Courtesy of Google Earth. [ix]

The islands of Clew Bay are listed at this link:


[i] Drumlin Geology ( [assessed 31st May 2020]

[ii] The Work of Ice ( [assessed 31st May 2020]

[iii] Drumlins of Clew Bay ( [assessed 31st May 2020]

[iv] Clew Bay & the Islands ( [assessed 31st May 2020

[v] Mayo: County Geology ( [assessed 1st June 2020]

[vi] The Story of Clew Bay ( [assessed 1st June 2020]

[vii] Drumlin ( [assessed 1st June 2020]

[viii] A Weekend in Westport ( [assessed 1st June 2020]

[ix] The Work of Ice ( [assessed 31st May 2020]


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