Sandville : Portal Tomb

CountyDerry
Grid RefC 388 049
GPSC 038834 04913 (7m)
Longitude7° 23' 41.1" W
Latitude54° 53' 25.3" N
ITM east480366
ITM north584435
Nearest TownArtigarvan (3.9 Km)
OS Sheet7
UTM zone29U
UTM x449041
UTM y5761192
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Visit Notes

Sunday, 29th October 2006

The OS map has this site marked as 'Standing Stones'. The inventory has them down as the possible remains of a portal tomb .

The two stones are said to stradled a field boundary, but I could only see one stone here. The other may now be hidden in the gorse that is enveloping the visible stone. This stone certainly does look as though it could be a portal stone. It is over 2m tall and slab-like. The only thing that speaks against it being from a portal tomb is that the entrance would have faced north or south, which would be quite unusual.

Portal tombs are what most people wrongly refer to as dolmens. They are, to me at least, the most strikingly designed of the megalithic tombs. They are called portal tombs because they have two large upright stones, usually very well matched, in front of the chamber that seem to form a doorway.

Resting upon the portal stones and the chamber a large capstone rests (sometimes there are two capstones - see Knockeen (County Waterford)), usually at an angle of around 22 degrees from the horizontal. Although these were originally incorporated into one end of a long cairn there are none left in this state today, although traces of the cairn can sometimes be seen upon the ground. The portal stones can be up to 3.5m tall, which combined with a thick capstone can produce an imposing monument over 5m tall. Capstones can reach in excess of 70 tonnes, with that of Browne's Hill (County Carlow) being estimated at over 120 tonnes.


Often betwen the portal stones there is a door slab, blocking the width of the entrance, but not always the full height. Door slabs are either half height, three quarter height or full height, describing the amount of the portal that they obstruct. All portal tombs would have had door slab, but this has often been removed to facilitate entry into the chamber.

Quite rarely the portal stones are the same height as the chamber and the characteristic slope of the capstone is created by the profile of the capstone (see Glendruid (County Dublin)).

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About Coordinates Displayed

This is an explanation of (and a bit of a disclaimer for) the coordinates I provide.

Where a GPS figure is given this is the master for all other coordinates. According to my Garmin these are quite accurate.

Where there is no GPS figure the 6 figure grid reference is master for the others. This may not be very accurate as it could have come from the OS maps and could have been read by eye. Consequently, all other cordinates are going to have inaccuracies.

The calculation of Longitude and Latitude uses an algorithm that is not 100% accurate. The long/lat figures are used as a basis for calculating the UTM & ITM coordinates. Consequently, UTM & ITM coordinates are slightly out.

UTM is a global coordinate system - Universal Transverse Mercator - that is at the core of the GPS system.

ITM is the new coordinate system - Irish Transverse Mercator - that is more accurate and more GPS friendly than the Irish Grid Reference system. This will be used on the next generation of Irish OS maps.

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