SOME OBSERVATIONS ON THE BEACHES OF THE
WEST COAST OF COUNTY DONEGAL
CUCHLAINE A. M. KING
University of Nottingham
Introduction
The coast of Donegal between St. John's Point and Bloody Foreland
presents a wide variety of landforms ranging from rugged cliffs to wide,
sandy beaches and sheltered estuaries. The coastline is extremely intricate owing to the varied geology. The effects of glaciation and changes
in sea-level have also played an important part in determining its present
character. The coast consists of alternating bays and headlands with,
long estuaries running several miles inland.
Depositional coastal features.
A wide variety of depositional features are to be expected on such an
indented coast, with deep inlets and many offshore islands in those areas
where sediment is available and where it can accumulate in relatively
sheltered conditions. For ease of description the accumulation forms have
been subdivided into the following categories : beaches, spits, tombolos,
dunes and tidal flats.
The beach material of the area varies from fine sand (0.125—0.25mm.)
as on the wide beach near Maghera in Loughros Beg Bay, to coarse sand
(up to 2 mm.) and boulders of the beaches in the little coves on the northern side of Cruit Island in the Rosses. In this neighbourhood adjacent
coves have beaches of strongly constrasting material ; some are sandy and
others consist only of boulders. Along the cliffed coast the only beaches
are small bay-head beaches such as those at Malin Beg and near Muckross
Head. Even smaller ' pocket ' beaches are found in the sheltered coves
amongst the higher cliffs as for example in the small coves along Slieve
League.
The largest beaches are found at the entrance to the main inlets into
many of which only small streams drain at present. The large amount
of sediment choking the estuaries appears to be re-sorted material brought
down the valleys by the glaciers. The inorganic nature of this material,
which is largely quartz, shows that it could well have been derived from
the local rocks. The broad strand at Maghera almost blocks an estuary
four miles long, into which only very small streams drain. There is
40
Bloody For«l«nd
WEST
DONEGAL
COASTAL FEATURES
*c/v*f
coastal
rocks
20.it
Heights abov« Chart Datum of raised
coastal {«attiras
—tft
Prodi« and umpi«
¡ü-;.j
Beach sand deposit*
posîtîoni
"! \ ''ft'
"""«*•
Fintrigh
Bay
kros Hd.
O
2
4
Fig. 1. West Donegal location map, showing some of the raised coastal features
measured and the position of the profiles and samples. The coastal geology
and the distribution of some sandy features are indicated.
Based on the Ordnance Survey by permission of the Government.
41
evidence that ice brought much material to this area. Many drumlins
occur at the inner end of the estuary and an exposure in one of them
revealed unsorted till, containing much sandy material, capped by a
layer of stratified sand. These glacial deposits probably extended further
seaward formerly, but some of the material has already been reworked
by the waves and wind to build up the coastal landforms.
Nearly all the beaches have smooth profiles characteristic of those
exposed to long swells. However, low ridges and runnels tend to form on
the lower foreshore in Loughros Beg Bay. These very low ridges are
situated at the levels of low neap tide and low spring tide, where the
GLEN BAY CUSPS
. sam pi« 3
|
OO
X 10
300
400
500FT
6OÖ
Fig. 2. Beach profiles, shewing position of samples. A profile across the beach
cusps in Glen Bay is shown.
42
swash can act for the longest period during each tidal cycle to build up the
ridges when long swells act constructively. Their presence suggests that
there is surplus sand on this beach, the gradient of which would be too
low for equilibrium in the absence of the ridges.
The beaches surveyed (Fig. 2.) illustrate very clearly the relationship
between sand size and the gradient of the upper part of the beach, the
slope of which is adjusted to the swash of the waves at high tide. A
regression correlation analysis of the data for five beaches (Fig. 3) showsthat there is a very close association, the co-efficient of correlation being;
0.995, showing that the sand size ' explains ' 99 per cent of the variation
in the gradient. The coarser beaches have the steeper slope as a result of
the greater permeability with increasing sand size ; this reduces the volume
and power of the backwash relative to the swash and helps to cause the
steeper gradient.
Table I and figure 4, showing sediment characteristics, also illustrate
other relationships of interest. The skewness indicates the extent of the
asymmetry of the cumulative frequency curve and the presence of a.
' tail ' of coarse or fine sediment is revealed. A negative skewness, when
using phi units instead of millimetres, is characteristic of beach sediments,
and is a factor by which they can often be differentiated from other types
of sediment. The negative value implies a distribution skewed towards
Fig. 3. Graph to show the correlation of sand size and beach gradient. The line
shows the mean of the two regression lines as they are too close to be shown
separately.
Fig. 4. Analysis of beach sand plotted on arithmetic probability paper in phi units.
43
the coarser sizes, which predominate in the tail of the sample. The coarser
particles in the samples analysed were largely of organic origin, consisting
of shell fragments.
Another interesting relationship is the correlation between sand size
and degree of sorting, the coarser sediments being less well sorted. This
is statistically significant at the 98 per cent confidence level, indicating
that this result would be expected to occur by chance less than once in
fifty times. The coarse material is probably not so well adjusted to its
present position as the finer sands, such as the sand of Dooey and Ballymahus beaches. The latter consists largely of shell sand, with many sponge
TABLE I.
Sample
M mm.
d
SAND SAMPLE ANALYSIS
S
0
M $
d
Mí
a <f>
orf
1
0.32
1.31
1.64
1.53
0.55
— 0.20
2
0.25
1.23
2.00
1.85
0.42
— 0.36
3
0.36
1.31
1.47
1.43
0.57
— 0,07
4
0.33
1.27
1.59
1.58
0.47
— 0.02
5
0.12
1.13
3.00
2.85
0.29
— 0.52
6
0.30
1.30
1.74
1.54
0.54
— 0.37
7
0.74
1.78
0.43
0.43
1.20
— 0.00
0.20
1.29
2.31
2.18
0.66
— 0.20
•Sample 1 Trahane beach-cusp crest
2 Trahane beach-cusp trough
3 Glen Bay, west—beach cusp crest
4 Glen Bay, west—beach cusp trough
5 Dooey beach raid-tide sample
6 Cruit Island beach mid-tide sample, upper sand
7 Cruit Island beach mid-tide sample, lower sand
8 Ballymanus beach mid-tide sample.
M^ mm.
Median diameter in millimetres.
'S Q
Trask sorting co-efficient.
"M j 4>
Median diameter $ u n i t s ( <f> = - log „ diameter in m m . )
•M ^
Mean, diameter <f> u n i t s = J ( <j>\6~t~^84 ^
o(j>
$ deviation measure - estimate of sorting = J ( <£g^—$,„ )
.a $
Skewness = M <f> — M , ^
44
spicules and other siliceous remains, such as radiolaria. The Dooey sample
contains much shell material and calcareous Globigerina foraminifera,
suggesting an offshore source, unlike the quartz sand beaches in the
estuaries. The samples from the coarser sand beaches, such as samples
6 and 7 from Cruit Island, consist largely of quartz grains, probably
derived by glacial scouring of the surrounding granite.
The presence of cusps on a number of beaches is of interest. These
features usually develop best in shingle, but in Donegal they were found
on several of the coarser sandy beaches,which had less well sorted sediment. Cusps were found on Trahane strand, in west Glen Bay, in one of
the coves on Cruit Island and on the northern end of the strand in Inishfree Bay. On Trahane strand there were 36 cusps in a distance of 1745
feet, but they disappeared towards the western side. Their mean width
was 48.5 feet, with a standard deviation of 5 feet. The standard deviation
<j(a= \£(X—X) , where X is the mean value and X is each separate
n
value and n is the number in the sample, indicates that two-thirds of the
values would be expected to lie between 43.5 and 53.5 feet. The mean
height of the crest above the trough was 0.65 feet. The sand of the crest
was coarser than that in the trough (see Table I and Fig. 4). The finer
sand in the trough helps to maintain the cusp because the backwash
can penetrate less easily into finer material. More water runs down the
slope over the trough than over the crest, causing further erosion and
increasing the depth of the trough. The swash sinks in more readily on
the coarser crest, reducing the backwash there.
Owing to the configuration of Trahane Bay waves could only reach the
foreshore at right-angles, and those observed must have been considerably refracted. They were low and had a period of 9.5 seconds. The
overall gradient of the beach was moderately steep, being 1 in 22. Remnants of larger cusps were visible higher up the beach, presumably formed
by storm waves. This suggests that wave size plays a part in determining
cusp size and that waves approaching at right- angles to the beach are
capable of forming cusps and indeed may be necessary to their formation.
In Glen Bay the sand is coarser, the beach steeper and the height
between the trough and crest of the cusps is greater and they were more
widely spaced. This bay is narrow ; in its 412 feet there were four cusps,
their lengths, diminishing eastwards, were 116 feet, 105 feet, 100 feet and
91 feet, and their heights were 2.25 feet, 3.18 feet, 1.67 feet and 2.04
feet (Fig. 2). A change of wave period seemed to have little effect on
them. This supports the view of Longuet-Higgins (1961) that wave period
does not correlate with cusp size.
One of the coarse sand-beaches at the north-western end of Cruit
45
Island had cusps at one end. The beach was very steep and dropped off
into deep water, resulting in vigorous wave action. The waves broke
irregularly, causing lateral movement of the swash. No cusps were
formed on the adjacent finer beaches, which had flatter gradients, even
though the wave action was more vigorous on some of them owing to
greater exposure. Cusps were observed on the strand in Inishfree Bay,
where they only occurred at the northern end. Wave action must often
be vigorous here, owing to greater exposure. These cusps were fairly
regular, being spaced at 83, 89, 86, 86 and 83 feet apart from the northern
«dge of the bay. Towards the centre of the bay the cusps became rather
wider apart, being spaced at 105, 108,102, 94 and 83 feet, but they died
out towards the southern end. The waves are usually largest in the centre
•of the bay, owing to the Tocky outcrops at the northern end and shelter
provided by a long rock reef at the southern end. It is significant that
on this beach strong springs emerge through the sand at^about the half
tide level. The water probably drains through the dunes from Mullagh•derg Lough that lies behind the beach. These springs caused marked
irregularities on the lower foreshore, with holes up to 4 or 5 feet in depth
forming in places. The holes caused the waves to break irregularly and the
swash flowed sideways, helping to initiate the cusps. All the cusps examined occurred at the high tide level. There appeared to be a general relationship between the size of the cusps and the size of the waves and
coarseness of the beach material, the three factors tending to increase
-together. On Trahane strand the sand size in the cusp trough was 0.25
mm. and the spacing was 48.5 feet, on the Cruit Island beach the corresponding figures were 0.31 mm. and 82 feet, and at Glen Bay they were
0.33 mm. and 104 feet. The more exposed beaches had the larger waves,
•coarser sand and more widely spaced cusps. This seems to confirm the
relationship found by Longuet-Higgins between cusp size and swash
length. On most of the beaches examined cusps formed when waves
approached the shore at right-angles. Once the cusps were initiated the
accumulation of finer sand in the troughs and the coarser on the crests
tended to perpetuate their form. In some areas at least irregularities on
the beach appeared important in initiating the sideways flow that is so
•conspicuous on a beach with well developed cusps.
The indented coast and plentiful supply of beach material would be
•expected to give rise to many spit forms. True examples are, however,
"not as common as might be expected. Nevertheless there are a number
of these features, such as the small one in Fintragh Bay and that built
across the entrance to Glen Bay, while the large sandy mass almost blocking the entrance of Loughros Beg Bay may also be classified as a spit.
The broad zone of accretion extending southwards into Gweebarra Bay
46
from Dooey Point is similar in character. The sandy accumulation on the
north-west side of Ballyness Bay in the north is also a spit in its present
phase of development. Nearly all these deposits are built up by long,
flat constructive swells, which form a gradient in equilibrium with the
size of the beach material and the wave length. They become orientated
to fit the direction of wave approach and are not primarily formed by
longshore drift, which must be restricted on such an indented coast.
Tombolos are more numerous that spits ; this is perhaps not surprising
considering the many islands in the area. Tombolos can be seen in all
stages of development. An early stage is visible in the north where the
north-western end of the spit in Ballyness Bay is growing towards the
island of Inishbofin. At low tide the exposed part reaches more than halfway towards the island, whilst another spit is extending towards it from
the island. Tombolo development at the island of Inishkeel, on the southern side of Gweebarra Bay, has reached a stage further. The island is
linked to the mainland at low spring tide by a developing tombolo, but
it does not emerge close to the island at low neap tide. These developing
tombolos are forming in the shelter of islands round both sides of which
waves are refracted.
Two of the main forms of tombolo are seen in Gweebarra Bay. One
is the type of tombolo that faces the direction from which the constructive waves approach, such as the sand and shingle feature linking
Termon to the mainland, and the sandy, dune-covered tombolo linking
Carnboy, near Burnbeg, to the mainland. North of Carnboy a less mature
tombolo of the second type, not yet built above high tide, links on the
island of Inishinny. This type forms in the shelter of the island. A third
type is estuarine in character, such as the ground linking Cruit Island to
the mainland, which dries out at low tide ; this also, however, is not fully
formed as the tide still rises across it.
The beaches, spits and tombolos illustrate the importance of factors
that determine the orientation of features built up by wave action. Most
of these features are composed of sand, which is built up by the long,
constructive swells coming in from the Atlantic. These, because of their
great length, are much influenced by the submarine relief and are greatly
refracted before they reach the coast. In nearly all instances the outline
of the beach, spit or tombolo is parallel to the submarine contours. The
configuration of these contours thus plays an important part in determining the orientation of the coastal forms.
Although the dominant, long swells approach this coast from only
one direction at any one time from the open ocean, by the time they
reach the coast refraction will have greatly altered then- orientation.
The vector resultant of the winds over force 4 at Shannon is almost due
47
south-west and it seems likely that the major swells approach from this
direction out in the open Atlantic. There are relatively few beaches
facing in this direction, however, although several of the beaches along
the north shore of Donegal Bay face between west and south. Muckros
beach, for example, faces south-west and it had by far the most violent
surf at a time when many of the neighbouring beaches, reached only by
the waves after considerable refraction, had much lower waves. Rip
currents were also active on this exposed beach ; these could be located
by the breaks in the surf, and must have been responsible for the irregularities in the beach at right angles to the water line. Good examples of the
parallelism of the offshore contours and the beach outline occur on the
seaward side of the tombolo linking Carnboy to the mainland and the
neighbouring beach which faces more to the north-north-west. The
beaches facing west and north on the developing tombolo growing towards
Inishbofin also illustrate this point.
The wide, sandy strands, the fact that both the prevailing and dominant winds are onshore and the considerable tidal range all help to account
for the large dune belts formed behind many of the wider beaches and
tombolos. These dunes are naturally more abundant in those areas where
glaciers have provided ample sand supplies. In most areas the dunes
are well covered by marram grass, while close to the tidal limit,sea rocket
is common in some areas. High climbing dunes of fine, firm sand were
noted hear Maghera where the beach is exceptionally wide. This very
wide flat stretch of fine sand is just above the high tide level and is being
colonised by isolated clumps of marram grass. This illustrates one of the
early stages in the process by which the wide belt of dunes behind this
strand has formed. The change in the river's course in the estuary may
have helped in the formation of this flat area. The river now drains out
on the north side of the bay, although the half-inch to one mile O.S.
map shows it draining out on the south side.
Although the dunes are very extensive, they rarely exceed ioo feet in
height. In the exceptionally high dune area in Loughros More Bay a
height of 84 feet above the lower dune limit or n o feet above chart datum
was measured. These dunes and others, such as those near Dooey Point,
have suffered much recent erosion, causing large blow-outs. The sand
from these has formed into younger dunes on the inland side of the main
dune belt. In other areas, such as Inishfree Bay, the dunes form a relatively thin cover over granite, which outcrops at intervals amongst thé
sand. Dunes have increased the height of spits and tombolos in many
instances and they have helped to widen these features, as the beach
builds up to allow dune formation. Good examples of dune-covered features raised in this way include the Maghera spit, the tombolo linking Termon
48
to the mainland and the Carnboy tombolo in Inishfree Bay.
Sandy tidal flats, or watts, are very extensive, as can be seen from
figure 1, but true salt marsh is very restricted and none of it is of the
muddy type. The absence of muddy sediment in the small streams
accounts for the lack of mud in the estuary. Glacial erosion must have
eroded the valleys below the present sea level, so that where they had
not already been filled with fluvio-glacial deposits, they became flooded
by the rising sea level of the Flandrian transgression. In some instances
the estuarine flats have become protected from the open sea by the growth
of beaches and dunes at their mouths. In other cases the natural opening
is too narrow to allow effective wave action, as for example at Trawenagh
Bay at the mouth of which two headlands approach to within less than
half a mile of each other.
Gweebarra Bay illustrates well all the landforms that have been
mentioned. A series of wave-built or wave modified features extend out
into the bay from the northern and southern shores. In Gweebarra
Bay four tombolos occur on the southern shore and one spit extends
from the northern shore. The most westerly tombolo has already been
mentioned as an example of a tombolo in an early stage of development.
The innermost tombolo has a very narrow, low neck of shingle, which
only extends a few feet above high water level. Solid rock outcrops near
the neck of the tombolo although drumlins conceal most of the solid
rock in the vicinity. This tombolo links a long, narrow island, lying at
right-angles to it to the shore, so that it and the island together are
T-shaped. The second and third features are of similar shape, but their
necks are wider and are probably formed only of sand. High dunes have
formed to raise them and widen them. The rocks that the three eastern
tombolos link to the shore outcrop in long reefs parallel to the elongation
of the bay. A spit extends southwards from Dooey Point on the northern
side of the bay. In front of this spit a wide, smooth sand-beach stretches.
It is backed by high dunes, widening southwards. Within the sandyfloored estuary the river follows a winding channel, while the ebb and
flood tidal streams have formed their own respective interdigitating
channels. The tide rises for several miles up the valley and several rocky
outcrops become islands at high tide. They are not linked to the shore by
tombolos because the lack of wave action prevents the formation of
tombolos in the inner recesses of the bay.
Conclusions
The indented coastline of Donegal allows beach material to differ in
neighbouring bays by restricting longshore transport. In the sandy inlets
49
a wide variety of landforms of coastal deposition have developed including
beaches, tombolos, spits, dunes and tidal flats. Because of the intricate
outline of the coast the long swells, approaching from the Atlantic, become
greatly refracted before they reach the coast and approach from a wide
range of directions in very shallow water. This accounts for the orientation of many of the wave-built features. The beaches illustrate interesting
relations between sediment parameters, gradient and cusp formation.
REFERENCE
Longuet-Higgins, M.D. and Parkin, D. W. Sea waves and beach cusps.
Geog. Journ. 128 (2), 1962, 194-201.
50