Soil Erosion at Garradreen, Coolstuff, around 500 AD


DR. EDWARD CULLETON

The coming of Christianity to Ireland also heralded a period of more intensive agriculture. This seems to have been related to the introduction of a heavier type plough which enabled more land to be brought under tillage. Lime-rich soils were much sought after and were often recognised by the abundance of elm and ash growing on them. But it has been found that around 400 - 500 AD the amount of pollen from these trees dropped and the pollen associated with agriculture such as grasses, plantain and bracken increased.

The pollen of every plant is different and all are long lasting. Hence, scientists are able to count the numbers of different types of pollen in any given deposit. From this they can calculate the dominant types of vegetation growing in an area at a given time: By this method they were able to establish that a major episode of deforestation occurred around the time Christianity reached Ireland.

But even more dramatic evidence of deforestation and subsequent soil erosion was found in the townland of Garradreen, near Coolstuff, in 1973. When examining a deep, freshly dug drain in the field opposite the old Culleton homestead, the author (another Culleton, but no relation) noticed dark coloured organic matter and charcoal under almost a metre of earth at the lower end of the field. A sample of the charcoal was sent for radio-carbon dating. This revealed that the charcoal, the result of burning wood, was around 1450 +/- 85 years old, or about 500 AD

This sequence of deposits is likely to be due to the combined effects of forest clearance and subsequent cultivation. Under forest vegetation erosion is negligible, because both canopy and ground litter protect the soil from the full impact of rain drops. Deforestation, ploughing and probably harrowing leave the soil vulnerable to erosion in topographically suitable situations. This possibility is increased by the absence of fine grass rootlets and root hairs which bind aggregates together when living, and, on decay, provide organic matter within the mineral soil for aggregate stabilisation. Prolonged rainfall would then result in translocation of fine material downslope. The organic matter, being on the surface, would move first, to be buried later under inorganic sediment, as at Garradreen.

The location of a small, univallate (single-bank) rath some 600 metres from the site may be related to forest clearance in the area. These raths were the homesteads of strong farmers who practised mixed farming. Cereal growing was an extensive and important activity.

The evidence for the kind of plough used is confused, but some coulters and shares suggest a heavy type. The Anglo-Saxons are known to have used a heavy plough, drawn by multiple ox-teams, to cultivate the heavy soils in East-Anglia and it is conceivable that some of these new methods were brought into Ireland by exchange of ideas between monasteries in Ireland and Britain. Cultivation of heavy- textured soils in the area would only seem possible using a fairly heavy implement.

Probably a strong farmer, attracted by the elm and ash growing on the lime-rich soil, established a rath at Garradreen and proceeded to clear the landscape for cattle grazing and tillage. Peculiarly, the rath at Garradreen is built on poorly-drained soil, but the climate of the period of its building may have been drier than at present. Ploughing and harrowing, followed by heavy rains, would then result in removal of fine material. At Garradreen the funnel-shaped nature of the site facilitated concentration of these erosion products, i.e. organic material, charcoal and silty loam, into one small area at the base of the slope.

This discovery at Garradreen has proved of great significance in landscape studies and the technical paper on it, written by the author and Professor Frank Mitchell, of Trinity College, is now quoted in every major publication on the early Christian period in Ireland.

Translocated silty clay with charcoal and peaty material at the base, overlying soft shale at Garradreen.

FOOTNOTES

  1. Organic matter is formed from the decay of leaves, roots and other vegetable material at the soil surface.
  2. ' Soil Erosion following Deforestation in the Early Christian Period in South Wexford.' in the Journal of the Royal Society of Antiquaries of Ireland, Volume 106, 1975 pages 120-123