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Surveying
the Stones
During
1998 a survey was carried out to record all wavestones known to us at
that time. A number of others have been reported to us subsequently, and
these were all duly visited. To date, wavestones have been recorded in
Counties Offaly, Tipperary, Galway, Clare, Cork, Roscommon and Westmeath.
Lough Leane in Kerry was visited because of the reported similarities
in the morphology to wavestone erosion in the limestone around the lake
edge. However, the erosion around Lough Leane was much rougher and more
jagged than the smooth erosion characteristic of the wavestones and thus
not really comparable.
Naming
the Stones
A
number of the more striking stones have their own local names, some of
which appear on the Ordnance Survey maps. One of the stones in the townland
of Creevagh, south of Clonmacnois in County Offaly, is named on the six-inch
map as the Piper’s Stone. Crinkill 1 stone outside Birr is known as the
Nun’s stone because of its similarity to a nun’s wimple. Sraheen is known
as the Banshee stone. In the townland of Clonkeen (in a field close to
the road from Clonbullogue to Clonad House, in County Offaly) is a stone
known as Finn Mac Cool’s stone. According to legend this is a stone which
the mythical Irish giant tried to throw from the top of Croghan Hill at
another giant standing on the Hill of Allen in the Kildare hills. The
stone however fell short of its target in Clonkeen, where it was spotted
by a dog as it rolled along the ground; the dog is said to have stopped
it with its paw. A credulous eye can still make out the mark of a dog’s
paw on one side of the stone and the giant’s hand on the other.
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Stones
are named here according to the townland in which they are found,
with the addition of a number where more than one occurs. For example,
Moyvannan 3 is the third stone examined in Moyvannan townland.
The
location of the stones recorded during the survey was determined
using GPS, and plotted on six-inch maps. Global Positioning enabled
accurate location of the stones on a map.
Where
the more accurate GPS was used it allowed the level of the ‘notch
height’ (i.e. the limit of ancient wave action) to be determined
approximately. It is important to discover if all the notch heights
are at the same level. If they are, it shows that the change in
lake level was due to a drop in the water table. If not, it may
have been due to other causes, such as uplift of the land surface,
which might affect different areas to different degrees.
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Global
Positioning Systems
A
Global Postioning System (GPS) is a world-wide radio-navigation
system formed from a constellation of 24 satellites
and their ground stations. GPS uses these satellites
as reference points to calculate positions on the earth's
surface. With sophisticated forms of GPS, measurements
(positions on a grid and height above sea level) can
be made to one centimetre accuracy. A GPS receiver works
by picking up radio signals from as many of the satellites
as it can contact at any one time, and from the wavelengths
of the signals can calculate exactly where it is on
the earth's surface.
GPS
receivers have been miniaturised to just a few integrated
circuits and so are becoming very economical to use.
However, there are a few complications, one of which
is atmospheric interference and the need for information
from a fixed GPS at ground level to take this into account
in the reading. Without this differential, accuracy
is less precise. Hand-held GPS receivers have this disadvantage
while the larger systems do not.
Many
of the stones were recorded with the more cumbersome
but very accurate GPS system. Other stones, however,
had to be positioned using only a hand-held receiver,
so the notch heights of these stones are not yet known.
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[
Illustration: Andrew Hendrickson]
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