340 S. V. Wood — Origin of the Loess. 



can penetrate the permanently frozen part below, and that the water 

 aiising from the dissolution of the snow, and from the summer rain 

 which penetrates this thawed portion, must convert it into sludge ; 

 and that having no vertical downward escape, such as that which in 

 climates where the soil is not permanently frozen supplies water 

 to springs, and wells, all escape of the water thus saturating the 

 thawed surface, which evaj)oration does not account for, must be 

 lateral. 



Whei'e the material acted on is porous sand or gravel, this lateral 

 escape may take place without any diplacement of the material 

 itself; but in imperfectly permeable material such as clay, and 

 especially in such perfectly but slowly permeable material as lime- 

 stone, the latter of which this perennial alternation of frost and thaw 

 must rupture and convert into loam or marl, this escape cannot, I 

 imagine, occur until the material has become so saturated as to be 

 mobile.^ In that condition its tendency must be to slide from higher 

 to lower positions ; and thus, while accumulating in the lower, to 

 tmcover and expose in a corresponding degree in the higher 

 positions, more and more of what would otherwise be the perma- 

 nently frozen portion to this action of alternate thaw and frost. 



To this action during the minor glaciation I have, in the concluding 

 part of the memoir referred to, attributed the origin of the " Warp " 

 of Trimmer, the " Trail " of Fisher, and the " Landwash or formation 

 of great submergence" of Prestwich, as well as that amorphous 

 Cave-earth which has in so many cases evidently found its way into 

 the caves through fissures in the rock from the surface above ; while 

 to the same but more prolonged action during the major glaciation, 

 I have referred the clay with flints, which covers those parts of the 

 chalk of England which were above the line to which the great 

 submergence that was coincident with such glaciation in this country 

 reached, and were not occupied by the land-ice. Up to that line the 

 waves as the land gradually sunk removed whatever of this forma- 

 tion had previously accumulated, and as the glaciation endured for 

 only the earlier portion of the emergence, whatever formed during 

 that emergence below this line was small in quantity, and insuf- 

 ficient to give rise to such a distinguishable and uniformly dis-. 

 tributed material as occurs above it, and which is susceptible of 

 delineation on map.^ Similarly this action during the minor glacia- 

 tion has not been sufficiently prolonged to cover the surface in 

 general with any considerable thickness of this material, or else, as 

 England was during that glaciation nearly all in the condition of 

 land, that part of the country of which the surface consisted of 

 material favourable to the action in question would be enveloped 

 by this material, instead of its being for the most part confined 



1 I propose in a subsequent communication to offer some remarks as to the peculiar 

 material which has resulted from this action upon the chalk with flints, and which 

 is not marl. 



2 It has in fact been mapped by the Geological Sui-vey in parts of Herts and 

 Bucks ; and though I am not aware that the Herts portion is published, I have 

 been kindly allowed to examine the MS. sheet containing it. 



