J. W.Davis — Erratic Boulders of the Calder Valley. 319 



submerged to the extent of some four or five hundred feet, and 

 the glacial clays subject to the denuding and abrading action of 

 water. The boulders released from the clay, and rolled hither and 

 thither by the waves, were gradually reduced to a more rounded 

 form, and by the same process the scratches were obliterated. There 

 can be little doubt that to this action is due much of the sand and 

 gravel existing in the Valley of the Ouse, and also in the Aire Valley 

 in the neighbourhood of Leeds. The climate appears to have been 

 still cold, and icebergs, broken off from the receding ice-sheet, or 

 masses of ground-ice bearing the boulders frozen from the bottom 

 into their mass, drifted in every direction, and, melting, dropped their 

 burden of boulders in new localities. 



Under such circumstances as these the Valley of the Calder would 

 be an estuaiy from the sea, of considerable width at its month, and 

 gradually closing inland to a comparatively narrow channel. The 

 united action of the floating icebergs and the tides would be amply 

 sufficient to account for the presence of the boulders which have 

 already been described. They are most abundant in the lower and 

 wider parts of the valley, and the boulders of distant origin pre- 

 ponderate in number, in every section, in the lowest beds, those 

 higher in the series being for the most part, and near the surface 

 entirely, composed of sand and boulders of local origin. The ice- 

 bergs would in the first place supply the erratics in the lower beds, 

 and as the ice-sheet still receded, and a warmer climate prevailed, the 

 rolling action of the tides reduced the boulders higher in the series 

 from the rocks of Carboniferous age which surrounded the valley and 

 were constantly being broken away by the action of the water wear- 

 ing away the shales supporting them. 



The fact that in all the sections which have been noted in the 

 lower part of the valley, the erratic boulders are always most 

 numerous, and that they are much larger in size, frequently a foot 

 or more in diameter, at the base of the section, diminishing in size 

 and frequency higher up until they are lost completely, and layers 

 with only stones of local derivation occur. In the higher parts of 

 the valley, as at Elland and North Dean, where the beds are half 

 the thickness of those at Wakefield or Dewsbury, no large boulders 

 have been found, and they are rarely seen to exceed two or three 

 inches in diameter. There is also a great decrease in the pro- 

 portionate number of foreign and local stones even in the lowest 

 beds. This arrangement of the heavier boulders at the base of 

 the sections, and especially their localization in the lower parts of the 

 valley, points most clearly to their eastward origin and the sorting 

 action of the sea. Had they come westward by river action, the 

 largest boulders would have been left in that part of the valley 

 nearest its source, or, if carried to their present positions, would have 

 been disposed indiscriminately with smaller ones throughout the 

 whole section. 



That the latter theory is probably the correct one, receives ad- 

 ditional support from the occurrence of erratic boulders in the 

 valleys which branch from that of the Calder, as, for example, the 



