SoLLAS — A Map to sJiotv the Distribution of Eskers in Ireland. 791 



the inland ice. If we think of an ice-sheet melting away, we must picture the 

 water, continuously produced on its surface, finding its way down to the bottom 

 through crevasses, producing there tunnels, the size, form, and distribution of 

 which will depend on the movement of the water ; and this, in its iurn, depends 

 on the slope of the upper surface of the ice, its crevasses, and the slope of the 

 underlying ground. Let a valley exist on the upper surface of the ice, then we 

 might expect to find the tunnel below the ice taking the same direction as this 

 valley ; further, let a correspondence exist between the direction of the valley on 

 the ice, and of a valley on the underlying ground. Then, from the sides of the 

 ice valley, and at right angles to it, the water, streaming down, will proceed, 

 maintaining its directions through the ice ; the streams of water will meet some- 

 where in the mountain valley below the ice, and produce a powerful effect on its 

 lower surface. Something similar might occur, independently of the relief of the 

 ground, if the crevasses increased in depth towards the middle of the ice-flow, and 

 diminished towards its sides. 



Such streams of water must naturally occur and precipitate themselves in 

 numberless places along the whole length of the ice valley ; streams of water, also, 

 coming from opposite sides, will meet along a line, more or less crooked, under the 

 ice, and thus will arise a long extended vault, or row of vaults, in the bottom of 

 the ice, running in tlie same direction as the ice valley. 



Given the possibility of such action of the water on the bottom of the inland 

 ice, and the explanation of the highland eskers becomes comparatively simple. 

 The ground-moraine is exposed to the action of the flowing water ; it becomes 

 converted by degrees into a water-worn material, which is transported by the 

 moving water to the glacier tunnel, and is forcibly pressed in, until the tunnel is 

 filled with it ; the lateral streams are compelled, then, to form a new tunnel by 

 the side of the old ; the new one is, in turn, filled up, and another excavated, and 

 so on. The ice thus acts both by determining the movement of the water, and by 

 affording a mould within which water- worn material is collected. The greater 

 the thickness of the ice, and the greater the fall of water, the larger the sub-glacial 

 tunnels, and the greater the dimensions of the eskers. On quite level tracts of 

 boulder- clay the eskers attain no great height, but still present a plainly recognis- 

 able form, and are often bordered by peat-filled depressions at the sides. 



These lateral depressions — esker moats — are very characteristic features ; 

 while in some places inconspicuous, in others they attain a considerable size, and 

 not seldom form a large basin from which several eskers of well-defined form 

 proceed. When not due to the form of the glacial tunnels, they may result from 

 the excavating action of water, which, pouring down the sides of the ice, scours 

 out the moat, and afterwards builds up the excavated material into the esker. 

 Thus the moat might be the first stage in the formation of the esker. 



