IV.—The Irish eskers

• Published in: Philosophical Transactions of the Royal Society of London. Series B, Containing Papers of a Biological Character
• Format: Article in Journal/Newspaper
• Language: English
• Published: The Royal Society 1921
• Subjects: Ice Sheet
• Online Access: http://dx.doi.org/10.1098/rstb.1921.0004; https://royalsocietypublishing.org/doi/pdf/10.1098/rstb.1921.0004

The term esker is Irish and its definition should be settled by reference to the eskers of Ireland. They are hills of sand and gravel, which are typically ridges, but are sometimes mounds or groups of mounds. Excluding various suggestions only of historic interest, there are three chief theories of the formation of ridge-eskers. According to the first theory, due mainly to Hummel (1874, p. 3), eskers are accumulations of gravel along sub-glacial rivers. According to the second theory, due largely to Holst (1876-7, p. 97), eskers are deposited along rivers flowing either over, or perhaps partially within, glaciers. According to the third theory, which was advocated almost simultaneously by Hershey (1897, p. 241), Baron de Geer (1897, pp. 377-386), and Baron von Toll (1899, p. 22), eskers are the deltaic deposits of glacial rivers, and their ridged form is due to their continuous deposition at successive positions by the slow recession of the river mouth during the retreat of the ice sheet. An esker so formed should show an annual banding due to the seasonal variations in the volume of the glacial river. This banded structure is well developed in the Swedish osar (åsar), for many of which this receding delta theory may be regarded as established. Prof. Sollas adopted Hummel’s theory for the Irish eskers. He described them (1896, p. 803) as the casts of systems of sub-glacial rivers. He justly objected to the supraglacial theory, since the eskers were clearly accumulated in their present positions ( ibid ., p. 819) and had not been redeposited at a lower level on the melting of the ice. His map of the esker system of Central Ireland, compiled from the maps of the Geological Survey, shows some eskers arranged in convergent series like a river and its tributaries; and it was this arrangement which suggested the fluviatile origin of the osar of Sweden. The Irish eskers have therefore been regarded as exactly equivalent to the Swedish osar.