172 G. H. Kinahan — Re-arranged Glacial Drift. 



to two or three feet in thickness. The re-arranged angular detritus 

 in the marl pits, north of the Forth Mountains, is rarely over eight 

 or ten feet deep, though this is probably not its maximum thickness, 

 as a marl deposited near a high cliif ought to have a greater depth of 

 this detritus over it, and in some sections on the coast the detritus 

 covering is much thicker ; therefore it is probable that inland its full 

 thickness has not been discovered, as the raising of the marl would 

 not pay if too great a surface had to be removed. 



It has now been demonstrated that all the so-called " upper glacial 

 drift " of S.E. Ireland is probably only Glacialoid drift, or " re- 

 arranged glacial drift," either by marine or meteoric action. It 

 must, however, be allowed, as pointed out by me in a former paper,^ 

 that during the " Esker sea period " some of the mountain groups 

 had still their glaciers, and possibly during this time some glaciers 

 may have existed in the valleys of the Wicklow and Carlow Hills, 

 which sent down their detritus on to the shelly drifts ; consequently 

 in a few subordinate places a true glacial drift may cap the shelly 

 drifts ; such deposits, if they exist, must necessarily be isolated, and 

 of subordinate importance, and do not affect the general question. 



The loose practice, adopted by many observers, of grouping 

 together Glacial and "re-arranged or Glacialoid drift," has led them 

 into the error of imagining that two distinct Glacial periods existed 

 over tracts of country, where in reality there is only evidence for 

 one ; the re-arranged or Glacialoid drift being a member of the 

 gravelly or fossiliferous drift, all being products produced contem- 

 poraneously by very similar causes. If, however, they still insist 

 on keeping the stratified re-arranged glacial drift as a member of the 

 Glacial group, they should also include therein the Esker and other 

 gravels and sands, and every kind of accumulation that has been 

 formed by the re-arrangement of the glacial drift. 



In a paper like this some reference should be made to the shelly 

 drifts of the Wicklow and Dublin Mountains, which were found at 

 an altitude of about 1200 feet, by the Eev. M. H. Close on the 

 Three- rock Mountain, This extraordinary height to which the 

 shelly drift ranges in this portion of the mountain group it is 

 hard to account for ; but I am led to believe it is due to the elevation 

 of an oblong tract with a general N.N.E. and S.S.W. direction, here 

 raised, much more than in the rest of the country, while associated 

 with the elevation was the formation of great Post-glacial faults.'* 

 These N.N.E. and S.S.W. curves of elevations and their associated 

 faults would seem to have had their greatest upthrow in the neigh- 



1 Notes on some of the Drift in Ireland, Dublin Quarteriy Journal of Sciences, 

 vol. vi. p. 249 et seq. 



2 The faults traversing this country have not been worked out ; but I would suggest 

 that a fault with an upthrow to the N.N.W. runs along the valley of the Bray river. 

 This, however, would seem to be shifted and displaced by transverse faults, such as the 

 faults in Glencullen, Sally Gap, Glenmacanass, Glenavonbeg, etc. ; while there are 

 marked axes of elevation, nearly parallel to the valley of the I3ray river, one crossing 

 the mountains obliquely in a" S.S.W. line from Laragh, while a second nearly 

 parallel, occurs a little further to the N.N.W. 



