Neolithic and Bronze Age Landscapes of North Mayo: Report 2011

The southern two thirds of the Mullet peninsula are part of the Erris complex, mainly gneisses with a
small area of more recent rocks to the extreme south. The Erris complex rocks are amongst the
oldest in Ireland, with some of those on the Mullet dating back to 1,900 million years ago (mya).
Other rocks in this complex range from 1,900 through to 900 – 650 mya. These rocks, which have
been significantly transformed by later metamorphism and deformation formed part of an ancient
North American continent which was later sundered from comparable rocks in Greenland and NE
America by the formation of the Atlantic ocean approximately 200 mya.
From the north of the Mullet peninsula eastwards, through Broad Haven Bay and to Glenlassra the
underlying rock is Dalradian in age, from the Grampian and Appin groups. Most of the Dalradian
rocks were deposited in a shallow sea following c. 750 mya, and they contain evidence for tropical
and glacial climates at different times. Following c 590 mya the Dalradian rocks were uplifted into
substantial mountains and eroded as the continental plates converged and moved apart. The
Grampian group includes the quartzite and psammitic schists that form the spectacular cliffs of this
part of the North Mayo Coast, whilst the northern slopes of the arc of mountains from Ben More to
Slieve Fyagh immediately to the south of Belderrig and extending west are mainly part of the Appin
group of quartzites and psammitic schists (Dalradian), with Carboniferous sandstones forming the
southern slopes and the summit of Slieve Fyagh. The complex of archaeological features in Belderrig
is associated with the Broadhaven group of quartzites and psammites. Dalradian pelitic schists of
the Inver Schist formation are the core of the area studied by Noel Dunne, including the hills of
Knocknalower and Dooncarton. The Dalradian rocks also include intrusive Caledonian metadolerites
associated with the opening and closing of the Atlantic (c400 mya). These are significant in the
Belderrig area, where they are visible through the psammites and quartzites and are associated with
massive quartz veins. Metadolerites to the south and south east of Belderrig form a significant part
of the Ben More range.
East of Glenlassra the region is dominated by Carboniferous sandstones and limestones deposited
from c 360 – 325 mya. Sandstones run from Glenlassra to the west of Killala Bay, which is underlain
by Carboniferous limestones (the Upper and Lower Ballina Limestone Formation). Some of the
Carboniferous sandstones were deposited on a coastal plain by rivers, but the Downpatrick
formation provides evidence of the encroaching Lower Carboniferous sea. The Ballina Limestones
indicate the deepening of this sea and fully marine conditions. North of Inishcrone, on the Eastern
side of Killala Bay these limestones include fossil corals. The Carboniferous limestones include cherts
and silicified limestones. These generally soft sedimentary rocks underlie the coastal lowlands and
low hills of the Ballycastle – Killala area, including Maumakeogh and form the dramatic Céide cliffs
outside the Céide Fields Centre and, most strikingly, at Downpatrick Head. This base geology
underlies the main Céide Fields complex and the complexes of field walls and associated structures
discovered by Gretta Byrne’s survey work between Ballycastle and Killala Bay.
Deglaciation and sea level change
The geological background provides the skeleton, but the surface of the North Mayo landscape has
been extensively reworked by the actions of ice and other associated processes during millennia of
glaciations and deglaciation. The last Ice Age culminated in Ireland being completely covered in an
extensive Ice Sheet at about 28-22,000 years ago. As this ice sheet retreated it left a series of
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