Correspondence. 277 



by the sea is accumulating, and consequently where the land is ad- 

 vancing. Their height above ordinary high-water-mark depends on 

 the height to which the billows rise at spring tides and in storms. 



While the advancing billow throws up gravel and sand above high- 

 water-mark, the retreating billow carries back similar material into ' 

 the deep water. We might, therefore, expect to find level terraces 

 formed imder the water, as well as above its level. This is 

 generally believed to be the case, and in that opinion the writer 

 of these remarks formerly concurred ; but farther examination shows 

 that the currents that sweep along the shores of the ocean prevent 

 such level formations, and throw up the debris carried down into 

 the deep in irregular ridges and banks. The level terraces, or 

 " ancient sea margins," from thirty to fifty feet above ordinary high- 

 water-mark, though some speak of them as " ancient sea bottoms," 

 seem all to have been formed by the advancing, and not by the re- 

 treating billow. 



The terraces on the Somme seem to have been formed in like 

 manner by the action of the sea. They are ancient " sea-margias," 

 throv/n up above high-water-mark by the advancing billows. 



They cannot be ascribed to the action of river currents. 



Mr. Prestwich's idea that the Somme at some former time carried 

 to the sea a vastly larger body of water than it now does, is dis- 

 proved by the fact that all the pebbles found in the valley appear to 

 have been brought from the rocks which are found in the basin 

 which it drains at the present tim-e. 



Even, however, if the volum.e of the river had been as large as 

 that of the Ganges or Mississippi, the force of the stream would not 

 have been sufficient to have brought down graveL The Somme at 

 Amiens, fifty miles from the sea, is only fifty feet above sea level, 

 ^and there is no reason for supposing that it was different at a former 

 time. No river current, with so gentte a declivity, as one foot in a 

 mile, could bring down stones and gravel such as are found in these 

 terraces, or could scoop out the hollow between them, as Sir Charles 

 Lyell supposes. 



In order to put the question more fully to the proof, let those, who 

 look on these terraces as having been formed at the bottom of the 

 river or frith, jsoint out an instance of any such level having ever 

 been formed under water, in any place exposed to the action either 

 of river or of tidal currents. 



We remark further, that if these terraces are the result of water 

 flowing down to the sea, any large stone that may be found in them 

 will have a bank or "tail" of sand behind it, and these tails will all 

 point downwards to the sea. Stones also of a flattened shape will 

 be found having an inclination to the sea. If the gravel has been 

 thrown up by the action of the billows no such uniformity of incli- 

 nation will be found. 



Query Thieb. — How was the volley originally formed 1 



K we rightl3r understand the descriptions that have been given of 

 it, the valley of the Somme may be regarded as a long, shallow, flat- 

 bottomed trough. Its formation seems to be unique. Some valleys 



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