120 Dr. Charles Richetts — On Subsidence and Accumulation. 



The ancients considered the formation of valleys to have been 

 dependent on atmospheric denudation ; that they have been excavated 

 by running water, and that floods have washed down hills into the 

 sea. This opinion was adA^ocated by Hutton and Playfair. By others 

 their origin has been variously attributed to the effects of a great flood, 

 notably the Noachian deluge ; — to marine action ; — that they are situat- 

 ed and dependent on lines of curvature, dislocation, and fracture ; but 

 none who have respectively advocated these opinions give an expla- 

 nation of the manner by which the materials, which once filled up 

 valleys, have been removed. Within the last few years the opinions 

 of Hutton and Playfair have been revived, more particular attention 

 having been directed to the subject in 1853 by Colonel George 

 Greenwood, in his work entitled " Eain and Eivers." These views 

 have since been advocated by the late Prof. Jukes, and by Profs. 

 Eamsay and Geikie, whilst numerous communications and essays 

 have appeared in the pages af the Geological Magazine from the 

 members of the staff of the Geological Survey and other observers. 

 These consider that valleys have in all cases been excavated by sub- 

 aerial agencies, including ice and glaciers, the debris being carried 

 forwards towards the sea by rain and rivers. This is the only theory 

 by which a satisfactory explanation can be given to account for the 

 redistribution of the materials which have been abraded from the 

 sides of valleys. 



It is almost universally conceded that perennial snow has existed, 

 and that glaciers have extended over districts much farther south 

 than at the present time ; to this ice-action much of the existing 

 contour of the land is to be attributed. In the neighbourhood of 

 Liverpool the Triassic rocks on each side of the Mersey are exten- 

 sively smoothed, grooved, and striated by this glacial action, these 

 markings in a comparatively soft sandstone having been preserved 

 by the deposition of the materials (sand and the Boulder-clay) re- 

 sulting from the grinding motion of the glaciers, which, in a similar 

 manner to what is now occurring in Greenland, issued as sub-glacial 

 rivers thickly loaded with mud and flowing into the sea, discoloured 

 the waters for miles and was eventually deposited on the bottom as a 

 thick coating of clay.^ A very considerable subsidence of the land 

 is known to have occurred during this Glacial period, both here and 

 in other parts of Britain ; this depression may be attributed to the 

 combined weight of ice and the Boulder-clay pressing down the sur- 

 face to below the sea-level ; the land being again raised to a con- 

 siderable extent when, upon the return of a more genial climate, it 

 was relieved of its load of ice and snow. A similar subsidence is 

 now in progress in Greenland, for the ruins of houses and factories 

 may be seen in places now entirely submerged at high- water, and 

 simultaneously there exists an increased severity of the climate and 

 an increase of the accumulation of snow. 



The formation of Deltas and alluvial plains has been considered, 

 even from the time of Herodotus, to be due to deposits brought down 



1 Dr. Eobert Brown, Physics of Arctic Ice. Quart. Journ. Geol, Soc, vol. xxvi. 

 p. 682. 



