Wright and Muff — Pre-glacial Raised Beach. 271 



The upper head rests on the terrace of drifts where the pre- 

 glacial cliff rises behind it. In some sections the upper head 

 differs from the lower head only in that it contains a few pebbles 

 derived from the boulder-clay ; in others it becomes very loamy, 

 and the rounded pebbles rather abundant. In the latter case it is 

 not easy to separate from the weathered top of the red boulder- 

 clay. The fact that it forms small flat cones at the mouths of 

 valleys has already been noticed. 



The recurrence above the boulder-clay of a head similar to the 

 lower head is of interest, as showing that the conditions, under 

 which the lower head was formed, returned for a period after the 

 ice had melted away. The preponderance of the lower over the 

 upper head is no doubt due to the greater steepness in pre-glacial 

 times of the dominating cliff or slope from which the head was 

 derived. It is, as a consequence, not to be taken as any indication 

 of a longer lapse of time between the elevation of the beach and 

 the period of glaciation than between that period and the present 

 day. 



5. General Conclusions. 



In his classic paper, " On the Mode of Origin of some of the 

 Biver- valleys of the South of Ireland," 1 Jukes proved that the 

 river-system is superposed on an old plain, which he believed 

 to be one of marine erosion. This plain slopes from north-north- 

 west to south-south-east, and is truncated by the pre-glacial 

 coast-line. Expressing Jukes' results in the terms of a later 

 nomenclature, the rivers occupying the deep and narrow gorge- 

 like valleys, which trench the plain from north-north-west to south- 

 south-east, are consequent on the plain, whilst the more open 

 west and east valleys which conform to the strike of the rocks are 

 eroded by subsequent streams. Cork Harbour, and the estuaries 

 of the Blackwater, Suir, and other rivers, are parts of this river- 

 system now submerged beneath the sea. The occurrence of the 

 pre-glacial beach fringing their shores is therefore a point of con- 

 siderable interest, since it proves that their submergence took 

 place in pre-glacial times. 



"Whether the old plain be one of marine erosion or the result 



1 Quart. Jour. Geol. Soc, vol. xviii, p. 378, 1862. 



