THE. SOUTH IRELAND PENEPLANE*
By A. FARRINGTON
The South Ireland Peneplane (600-800 feet) has been
recognized for many years as a remarkable feature of the
landscape, but except for a short account of it by Hallissy1
little serious attention had been given to it until Miller, who
gave it its name, published a more extended study in io,39.a
The existence of this relict surface, although deeply
dissected and completely gone over wide areas, cannot be
denied. Miller has ascribed its formation to marine abrasion;
but in the opinion of the present writer there are many
difficulties in this interpretation of the surface. One is led to
explain the surface as one of marine abrasion by the remarkable
bench on the south side of the Knockmealdown ridge in
County Waterford. This occurs near Mount Melleray at
between 600 and 700 feet O.D. and is backed by a steep rise to
1,300 feet, which, it is suggested, represents a denuded cliff.
Miller states that a similar feature may be found on the nearby
Nagles mountains and " less clearly against the south and east
flanks of the Boggeraghs and on the south and east flanks of
the Blackstairs Mountains " (Miller, p. 324).
Miller prepared, for the Knockmealdown ridge between
the valleys of the Suir and the Blackwater, a curve by plotting
areas against altitude and also gave a table of measurements.
In Table I a similar method has been used except that in order
that the different groups of figures might be more easily
compared only percentages have been given. Only altitudes
between 200 and 1,400 have been considered and the figures are
the distances between adjacent hundred foot contours
expressed as percentages of the total horizontal distance
between the two extreme contours.
* Since this paper went to press an interesting discussion of the
peneplane has appeared (Martin, S., "Problèmes morphologiques de
l'lrlande du Sud." Annales de Bretagne, Tome LIX, No. 1, Rennes,
1952). Martin is in agreement with the writer of the present article in
considering the surface to be one of sub-aerial erosion.
211
TABLE I
Knockmealdown Mts.
(1)
(2)
(3)
1,300-1,400
1,200-1,300
1,100-1,200
1,000-1,100
900-1,000
800-900
700-800
600-700
500-600
400-500
300-400
200-300
%
I-I
I.-9
i-i
1-9
5-1
"•5
13-4
16-6
17-9
13-0
10-5
Nagles Mts.
(4)
0/
4-1
4-2
o-8
o-8
/o
0-4
17
3-9
5-4
77
8-9
1-5
3'1
1-4
1-2
12-0
IO'O
2-1
127
127
19-6
25-1
9-2
9-9
47
8-4
5-3
15-4
16-2
8-3
19-0
18-0
14-0
6-i
7-2
4-0
6-o
(1) After Miller; 12 sections at two mile intervals.
(2) Seven sections at one mile intervals, avoiding north
and south valleys.
(3) Six sections, along interfluve ridges.
(4) Eight sections at one mile intervals.
The first column in the table is prepared from the figures
given by Miller (Miller, p. 325). Here it is seen that 82 % of the
summed intercepts lie between the 300 and 900 foot contours,
but nowhere between these limits does the intercept between
any two contours stand out very prominently although there is
a slight culmination between 500 and 700 feet. The method
used does not give a clear result for the following reasons:
(1) the distance of two miles between each section is somewhat
wide for the area covered: (2) the Knockmealdown ridge,
within the limits of the twelve sections, is split longitudinally
for half its length by the well-developed valley of the Araglin
river which must be later than the peneplane: (3) the ridge is
scored by deep north and south valleys parallel to the lines of
section. The first would tend to make the figures obtained
somewhat uncertain, the second and third would tend to make
the lower contour intercepts too prominent. If one or two of
the sections unluckily happened to hit north and south valleys
the result would not give a true picture of the predominant
flats on the main ridge.
To try to avoid these objections sections were drawn at
intervals of one mile and all those which crossed the Araglin
valley or fell within the influence of the north and south
212
valleys were excluded. The result is shown in Column 2 and
here it is seen that the benching is most likely to be found
between 600 and 800 feet. To confirm this intercepts were
measured on sections which run along the crests of the interfluves between the north and south valleys, as it is here, of
course, that the benches are best preserved. In this case the
predominance of the intercepts between 600 and 800 is most
marked.
TABLE II
1,300-1,400
1,200-1,300
1,100-1,200
1,000-1,100
900-1,000
(1)
(2)
(3)
(4)
(5)
(6)
0/
0/
0/
0/
0/
0/
0/
/o
/o
/o
/o
/o
2-6
5"I
2-5
I "6
i*6
1-2
/o
4-1
i-8
4'I
1-4
2-8
/o
5-0
2-5
2-5
2-5
7-5
4-0
7-4
32-8 . 27-9
2'0
4-0
207
(7)
800-900
9-6 8-i 34-2 5 7
5-0 5-1 i-6
700-800
37-6 29-3 8-2 31-3 8-2 13-8 107
600-700
4-0 7-4 8-9 35-7 36-9 37-3 17-3
500-600
4-8 7-4 6-9 5-1 15-5 n - 5 42-1
400-500
3-2 5-9 6-i
5-1 12-3 9-2 8-2
300-400
i-6 3-0 4-8 3-1 n*4 4-6
200-300
2-4 3 7
4-0 5-1 4-1 4-5
Profiles along the crests of the interfluves. The highest
points of the sections are: (1) Knockalough, (2) Labbacallee,
(3) Knockcorragh, (4) Knockaunarast, (5) Dyrick, (6) Coolgortaboy, (7) Coolgortaboy west to east.
The examination of these interfluve ridges reveals another
very interesting point. As one goes from west to east the
elevation of the greatest contour intercept rises and then falls.
Table II brings this out clearly in the first six columns. The
seventh column is got from a section drawn from the same
summit as that used in column 6, but it is drawn from west to
east down to the bottom of the Ballinamult pass which
separates the Knockmealdown range from the Monavullagh
group. The disposition of the benches on the interfluves shows
that we are dealing with a surface which slopes a matter of
300 feet in ten miles along the side of a mountain ridge to join
a flat which passes through the ridge by a wind-gap. This is
quite unlike what one would expect if the surface were one of
marine erosion and strongly suggests that the South Ireland
Peneplane in this area is a subaerial erosion surface related to
a master stream.
213
Working on this suggestion one is led more and more to
doubt the hypothesis of marine erosion simply because the
features of the present topography are more easily explained
otherwise. The Knockmealdown ridge is a complex anticline
with the typical east-west axis of the Armorican of southern
Ireland. The main anticlinal ridge is divided longitudinally bj'
a secondary syncline in which the valley of the Araglin river
has been eroded. Farther south other secondary synclines with
their axes parallel to the main axis are found (Miller, p. 342 and
Plate XXXI).
In this structure one may, perhaps, find the reason for the
marked assymetry of the ridge, all the higher summits being
towards the northern side, suggesting that the more folded
southern part had yielded more readily to erosion. It is not
improbable that there may have been some Carboniferous
strata folded down into the secondary synclines as is still to be
seen in the Araglin valley.
These small synclines have also affected the stream
courses, giving to several of them marked east-west sections
where the valleys are crossed by the synclines. Miller
discusses these valleys (Miller, p. 341) and adopts the explanation that they are " the surviving portions of temporary
subsequents, surviving in the position at which they stood at
the end of the first cycle of the erosion on the South Ireland
Peneplane, and that they marked the position at that stage of
the O.R.S.-Carboniferous junction." This is a suggestion of
much interest and evidence in favour of a cover of Carboniferous strata over the Old Red Sandstone could be adduced
from other parts of Southern Ireland but it does not support
the hypothesis that the South Ireland Peneplane was formed
by marine erosion.
If we consider that the South Ireland peneplane was a
surface of marine erosion then the streams which developed on
it, or drained across it from pre-existing head waters, would
have been consequent streams flowing south and such changes
of direction to east and west might have been due to capture.
In no case is there now any trace of such a capture.
In every case if one produces the east-west part of one of
these valleys one arrives onto the peneplane surface to find no
trace of the valley. In some cases the valley lies so far below
the peneplane surface that it seems to have no relation to it.
The valley pattern can much more easily be fitted into a
scheme of continuously developing sub-aerial drainage with
adjustment to structure than into a scheme which necessarily
•
214
requires a system of consequent streams flowing across an
emerged coastal plain.
At the time of the South Ireland Peneplane a considerable
river must have passed through Ballinamult gap, which is well
matured down to the 500 foot contour. There must, therefore,
have been higher land to the north of sufficient extent to give
an ample catchment at this period. The Carboniferous Limestone country to the north now lies for the most part at about
200 O.D.; much of it is lower. No trace of the South Ireland
Peneplane now remains except upon uplands of more resistant
rock. The Galtee mountains reveal little. There are inconclusive benches to be found at 800 and 1,000 feet on some of the
spurs. Slievenamon shows a few small benches and one
considerable flat at 1,000 feet. Miller (2, p. 327) mentions the
summit levels of the Slieveardagh hills at 900 to 1,000 feet and
also the Castlecomer tableland which has remarkable flats at
1,000 feet. On this tableland only one or two points touch
1,100 feet yet there are very extensive areas over 1,000 feet.
Such summit flats, however, must be suspect as evidence of
peneplanation for they occur with such suspicious frequency
on denuded uplands at so many different levels that there may
be some other reason for their existence.
If we correlate all these flats and benches as being
remnants of a single surface, a highly speculative proceeding
at this stage, it is clear that if it were a plane of marine erosion
it must have been considerably tilted as parts of it are now
200 feet above the " cliff " as Miller points out (2, p. 327).
(N.B.—The fall is 5 feet to the mile, not 50 as appears on
p. 327). But this introduces another difficulty in that where
the South Ireland Peneplane is preserved to the south of the
Knockmealdown ridge it lies at a somewhat higher level on the
Watergrasshill ridge than does the occurrence already described on the flank of the Knockmealdown ridge. The peneplane, therefore, slopes in both directions towards what is now
the valley of the Blackwater, a circumstance which again
suggests normal drainage development under sub-aerial
conditions. In his paper Miller makes the point (2, p. 339)
" that at one stage no less than 10 considerable consequent
streams must have flowed southward across this [Watergrasshill] ridge. The summit of this ridge, as has already been
described, is bevelled by the South Ireland Peneplane, and
I regard it as a very strong evidence of the marine origin of the
erosion surface that the early drainage should have been
characterized by so many parallel consequent streams separated
by such short distances." The point is illustrated by a sketch
215
map (Miller, fig. 10, p. 348). This would, indeed, be a strong
argument in favour of marine erosion if the suggested stream
courses would stand critical examination. The succession of
cols which are taken as defining the courses of these consequent
streams may, perhaps, represent the remnants of such stream
courses but they cannot, without very detailed examination of
the problem, be advanced as proofs. Some of the courses
suggested invite much criticism being defined by a small series
of widely-spaced cols at levels some far below and others far
above the surface upon which the streams were supposed to
have been consequent.
The only argument remaining in favour of the marine
erosion hypothesis is the existence of the " cliff." This does
not give strong support. For example, column 4 in Table I
shows no striking change of slope on the Nagles Mountains and
the same might be demonstrated for any other group of
mountains in the area. Furthermore, on the Knockmealdown
ridge itself there are much sharper breaks of slope at levels
above that of the peneplane; for instance at 900 feet on Dyrick,
at 1,200 feet on Knocknafallia. For many miles the whole of
the north slope of the ridge is steeper than the supposed
" cliff " and it has never been suggested that it is the product
of marine erosion.
It is the opinion of the author of the present note that if
the surface of the South Ireland Peneplane really represents
a stage in a cycle of erosion, and even this cannot be said to be
proved, it is much more likely that it was one of sub-aerial
erosion than marine planation. In this case it would appear to
have been a surface developed on the Carboniferous Limestone,
or on strata overlying the limestone, with a base-level of about
500 feet (Ballinamult gap). The limestone surface has since
disappeared and the only remnants left are a few bevels on
spurs and ridges of Old Red Sandstone and other resistant
rocks.
In conclusion another suggestion may be introduced.
Much of the topography, particularly on the lower slopes of the
sandstone ridges, suggests exhumation rather than contemporary erosion and one cannot ignore the possibility that
features which we are now attempting to explain by normal
processes of erosion may be pre-existing features which have
not been greatly modified since they were uncovered. This
caution is particularly necessary in dealing with a landscape
formed from two rocks so strongly contrasted in resistance to
erosion as the Carboniferous Limestone and the Old Red
216
Sandstone of the south of Ireland. The limestone may* be
resistant in comparison to shale, given conditions when
solution does not act, but under no circumstances is it resistant
in comparison with the Old Red Sandstone, which in all cases
in the south of Ireland stands up as lofty ridges above the
comparatively flat limestone land. The remarkably steep
features which constantly occur where the Old Red Sandstone
plunges beneath the Carboniferous shale or limestone owe more
than has been recognized to the structure.
1. Hallissy, T., Mem. Geol. Surv. of Ireland; Barytes in Ireland,
pp. 61-69, 1923.
2. Miller, A. A., "River Development in Southern Ireland", Proc.
Roy. Irish Acad., vol. 45, pp. 321-354, 1939.
217