SUCCESSIOlSr IN THE TRENT BASIN. 441 



main watercourses. Like the Boulder-clays, the gravels and sands 

 are of two varieties, each variety possessing distinct lithological 

 characteristics. 



Each of the physical changes which caused the Pleistocene 

 deposits to vary over wide areas, from Boulder-clays to gravels, 

 sands, or brick-earths, will be considered as marking a definite stage, 

 the commencement or close of which was not perhaps always 

 exactly coincident over the whole district. No hard-and-fast line 

 really separates the various stages from each other, for the Pleisto- 

 cene deposits I have had to deal with seem to indicate an almost 

 continuous series of changes from early glacial times down to the 

 present day, the exception being an apparent break at the close of 

 the Older Pleistocene epoch. The development of each deposit also 

 varies largely in different localities. 



Few branches of dynamical geology have, of late years, given 

 rise to more divergent views than the question of the origin of 

 Boulder- clays. It is not my intention at present to enter fully into 

 the subject, but a short statement of the opinions formed by their 

 study in the field wiU help to render my meaning clearer when I 

 come to deal with them in detail. 



I do not incline to either an entirely subaqueous or an altogether 

 subglacial hypothesis. 



The Boulder- clays may be divided into four distinct types : — 



1st. Aqueous Boulder-clay formed near the shore or the terminal 

 front of a glacier where it entered water tolerably free from currents. 



2nd. Moraine piofonde formed beneath a thick ice-sheet by the 

 breaking up of the preglacial rocks over which it moved. 



3rd. Moraine profonde formed beneath an ice-sheet by the 

 ploughing up and confusing of aqueous Boulder-clays, sands, &c. 



4th, Deposits collected at the terminal fronts of glaciers not enter- 

 ing the sea. 



Any particular section of Boulder-clay may contain one or more 

 of the above typical forms, or any intermediate variety, for they 

 graduate by insensible stages one into the other. 



The aqueous Boulder-clay is generally a fine, stiff, sandy, homo- 

 geneous deposit, of various colours, charged with striated, grooved, 

 and polished erratic boulders, many of which have travelled great 

 distances. It often rests upon or underlies beds of sand, with which 

 it is clearly interstratified. Where sand-beds, which frequently 

 show no signs of disturbance, and aqueous Boulder-clays inoscu- 

 late, the sands frequently lose their false-bedded character, show- 

 ing that the Boulder-clays were deposited in still water ; everything 

 points to their having been formed by the sediment which the sub- 

 glacial streams brought down and deposited in comparatively stiU 

 water. The absence of stratification is due to the constancy of the 

 supply of sediment and to there being no strong currents. 



The included morainic masses of clay which it sometimes contains 

 are derived from rocks which the ice was denuding at the time, 

 while the striated boulders were released from the thawing under 

 surface of the ice. 



