The Minnaun Sandstone is exposed in two principal areas:
1. A continuous section of the Formation is exposed from Fohernadeevaun (just southwest of Port) in the west to Benaderreen (opposite the Céide fields). The rocks are accessible in but few places, as in the vicinity of Port (grid ref 10260 34211), and at the peninsula of Minnaun (grid ref 10365 34196). Even there access is difficult, sometimes hazardous, and unsuitable for field parties.
2. from Bunatrahir Bay, north of Ballycastle, towards Downpatrick Head.
Details of the thickness, and lateral variation, of the Formation were obtained by measuring a series of sections remotely - that is, by suspending a weighted, marked, rope from the cliff edge, and then, with the rope hanging close to cliff face, examining the section from a distance by telescope. Each of the "stick sections" in the figure above represents one such observation point (except for those between Bunnahowna & Stella Maris which can be examined directly).
The basal Carboniferous unconformity at Fohernadeevaun, looking south from grid ref 10223 34196 on the Port peninsula.
The lower part of the Formation is dominated by thick bedded to massive pale sandstone units, often with erosional bases and usually with flat tops. There are subordinate medium to thin bedded sandstone and siltstone units. Apart from a thin pebbly sandstone unit above the basal unconformity, pebbly or conglomeratic horizons are absent, with the exception of "cornstone conglomerates". These are composed of micritic calcareous concretions that have been eroded from occasional caliche horizons in mudstones and siltstones.
channel sandstone units, looking northeast from grid reference 10321 34190
A sequence dominated by fine red sandstones, siltstones and mudrocks (silty mudstones) occurs in the middle part of the Minnaun Sandstone, and is well displayed in cliffs on the west side of the Minnaun peninsula (see picture below). The upper part of this unit is accessible (with some difficulty) just to the southeast of the peninsula. Rippled and occasionally mudcracked surfaces occur, and there are a number of distinctive very fine grained structureless limestones and dolomites.
red beds in the middle part of the Minnaun Sandstone, telephotograph looking east from grid reference 10358 34187
The sequence above the red bed interval is exposed in the high cliffs between Minnaun and Benaderreen, opposite the Ceide Fields Centre. There is a reversion to thick pale sandstone units, interbedded with grey, reddish and greenish bedded sandstones and siltstones.
The most westward section where the contact with the overlying Downpatrick Formation can be seen is at the stream section at Bunnahowna (grid ref 10644 34093) where the appearance of trace fossils indicates the onset of marine conditions.Back to North Mayo Index