362 



D. MACKINTOSH ON HIGH-LEVEL MARINE DRIFTS. 



ice. — The idea of the granite pebbles of the Frondeg dis- 

 trict having been brought by land-ice is opposed by the fact 

 that few Eskdale-granite pebbles (so far as I have noticed) are 

 to be found between Frondeg and the estuary of the Dee (the 

 direction in which the ice would most probably move) excepting 

 near the Dee estuary. There is likewise an equal absence of shell- 

 fragments in the drift-deposits where the latter are present ; but 

 it is a remarkable fact that a great part of the area intervening 

 between Frondeg and the Dee estuary is free from drift. Sup- 

 posing the erratics and shells to have been pushed forward under 

 the ice, they ought to be represented over the whole, or at least the 

 greater part, of the intervening area, while no one would suppose 

 that the numerous extra-rounded pebbles of Frondeg could have 

 come in the form of a supraglacial moraine. 



6. Probable Decrease in the Rate of Submergence. — It has already 

 been remarked that from about 500 feet to about 1000 feet on the 

 east side and flanks of the mountain-range under consideration the 

 stones in the drift are generally angular or subangular, while 

 above 1000 feet they are generally rounded, and in many places 

 extra-rounded. It is likewise true that of the standard Frondeg 

 erratics, namely Eskdale-granite, not one in fifty can be found in the 

 comparatively low-level drifts between the mountain-range and the 

 railway, while in the Frondeg district, above 1000 feet, Eskdale- 

 granite pebbles are very numerous. We have no reason for sup- 

 posing that the sea in this latitude was sufficiently warm to melt 

 the floating ice, so as to cause the precipitation of many erratic 

 stones. On the contrary, the great number of granite pebbles 

 which have found their way as far south as Shrewsbury (where they 

 are rather numerous in the lower Boulder-gravel, though not in the 

 upper clay), and the much greater number (within a limited 

 breadth of area) which reached as far south as Burton (S.W. of 

 Broseley)* would seem to indicate that where many of these 

 pebbles are found crowded in a small compass (as in the Frondeg 

 district) they were left by the stranding and consequent breaking- 

 up of floating ice. At this period the district was probably in the 

 condition of a littoral zone, which may have lasted for a time 

 sufficient to enable the waves to round the stones and to allow 

 the Mollusca to multiply in the littoral and sublittoral zones, and 

 thereby furnish many shells destined to be reduced to fragments by 

 the rolling and grinding of stones on a much-exposed sea-coast. 

 But a protracted sojourn of sea-waves in what is now the Frondeg 

 district is likewise indicated by the time required for the accumula- 

 lation of the immense number of erratic stones. That these 

 stones were rounded approximately in situ is evident from the fact 

 that at the high levels in the Eskdale district, from which the 

 stones must have been launched, the stones are all more or less 

 angular. 



* See Quart. Journ. Geol. Soc. vol. xxxv. (1879). p. 425. The Frondeg erratics 

 are not mentioned in this paper, as I had not discovered them when the paper 

 was written. 



