278 Correspondence. 



bave originated in dislocation of the strata, throngli the action of 

 subterranean forces; but here there is no trace of such dislocation. 

 The valley has been hollowed out of a uniform level bed of chalk. 

 Other valleys have been excavated by torrents from the surrounding 

 hills, but here there is no evidence of any river-current capable of 

 producing the supposed effect. We remark further, that valleys 

 scooped out by river torrents become narrower as they get deeper, 

 and though they may afterwards be filled with gravel or sand, the 

 original channel cut out of the rock has always the character we have 

 ascribed to it. Other valleys are formed by glaciers ; but in the 

 neighbourhood of the Somme there is no trace of glacier action, as 

 far, at least, as the accounts given by Sir C Lyell and Mr. Prestwich 

 show. 



If, as we said, we rightly understand the accounts given of the 

 valley, there seems to be but one cause to which its formation can be 

 assigned. That is the action of floating ice, carried backwards and 

 forwards by a tidal current. 



If we suppose the Somme, at first, to have flowed into the sea, 

 through some little narrow creek, the ice formed on its surface, at a 

 time when a boreal climate prevailed, must have rapidly worn away 

 the chalk which formed its banks. Wlien the mouth of the river 

 gradually enlarged into a long narrow estuary, that estuary would be 

 filled in a great measure with fresh water, which would be frozen 

 over in winter. The flux and reflux of the tide would be like that 

 which we find in the Solway Firth at the present time. It would 

 produce very powerful currents, and give to the ice on its surface an 

 impetus which a substance so soft as chalk could not resist. If the 

 sides of the depression had been formed of any of the harder rocks^ 

 they would not only have been better able to withstand the shock of 

 the floating ice, but the fragments broken off from them would have 

 formed beds of gravel which would have lessened the force of the 

 ice. The abraded chalk would be diffused through the water and 

 carried out into the ocean ; the embedded flints only would remain. 



Since various considerations have led to the conlusion that a boreal 

 climate prevailed at the time when the valley was formed, it seems 

 no improbable conjecture to suppose that masses of floating ice, with 

 sand and gravel adhering to the bottom and sides, were the means by 

 which the excavation was originally formed. 



Yours truly, 



James Brobie. 



ON THE ORIGIN OF VALLEYS. 



To the Editor of tJie GEOiiOeiCAL Magazine. 



SiK, — My friend Mr. Scrope, in his article " On the Origin of 

 Yalleys," published in your last number (p. 193), has rightly re- 

 presented me as a convert to his opinions on that subject ; but by 

 remarking that I had at last acknowledged the correctness of his 

 views, he might lead your readers to infer that I had obstinately 

 maintained an opposite theory, until a very recent periods 



