244 Pi'of. Sedgwick on the Origin [April, 



ground, often forming the channel of the waters, and, where the 

 level of the country admits of it, sometimes surmounted by an 

 accumulation of newer alluvial materials. By the ordinary 

 action of the waters, the two distinct classes of deposits some- 

 times become mixed and confounded ; but 1 have never seen an 

 example where their order is inverted, or where, through any 

 extent of country, they alternate with each other. The instances 

 adduced are not exceptions to, but examples of, the general 

 rule. There is not, I believe, a single river in England which 

 does not afford a more or less perfect illustration of some of the 

 phenomena above described. 



Perhaps the most important class of facts connected with 

 alluvial phenomena, and which at the same time very strikingly 

 exhibit their relation to all other deposits in this country, are to 

 be met with in the low marshy regions near the mouths of some 

 of our larger rivers. In proof of this assertion I shall proceed to 

 describe some of the physical characters of the fenny tract of 

 country which stretches from the south part of Lincolnshire to 

 the base of the chalk hills of Norfolk, Suffolk, and Cambridge- 

 shire, if a section be made through this region in a direction 

 which is transverse to the outfall of the waters, its profile will be 

 represented, first, by a line descending from the higher part of 

 Lincolnshire to the level of the fens ; secondly, by a succession 

 of horizontal lines exhibiting the several levels of distinct fenny 

 regions, interrupted heie and there by extensive protuberances 

 of diluvial gravel ; * lastly, by an undulating line ascending 

 from the alluvial region to the top of the hills which form its 

 .south-eastern boundary. If a section were made in a direction 

 transverse to the former, commencing at the south-west boun- 

 dary of the low lands, and ending in the sea, its profile would 

 be represented, first, by a hue showing the descent of the high 

 lands to the level of the fens ; secondly, by a long line extend- 

 ing almost at a dead level {except where it is interrupted by some 

 of the protuberances above-mentioned) to the eastern extremity 

 of the fens in the immediate vicinity of the coast; lastly, by a 

 line desceudiug rapidly from the level of the fens to low water 

 mark.t The singular contour indicated by the second section 

 has unquestionably arisen from the continued accumulation of 

 alluvial silt which has choked up the mouths of the rivers, and 

 raised their beds and all the contiguous country far above their 

 ancient level. ;{: 



• During great inundations these diluvial hiUs resemble islands rising out of an 

 inland sea. 3Iost of tlie towns and villages in the Isle of Ely are built upon them. 



t Thus from Peterborough to Sutton ^Vash below W'isbeach (a distance of more 

 than twenty miles), the fall of the water is on the average three inches and-a half for 

 each mile. But from Sutton N^'ash to low-water-mark at Crabhole, the fall is more 

 tlian three feet for each mile. — (See Rennie's Report on the Drainage of the Bedford 

 Level.) 

 - X Thus Thornjy north fen is thirteen feet ; Peterborough low fen twelve fee; six 



