Peacock : The Fenland Soils. 



i85 



highly-charged waters to a flat but fairly deep saucer lake in the 

 peat, lying about the Boulder Clay, to have all the circumstances 

 which are required for the formation of Shell Marl. I have kept 

 you rather long over this matter, for the 'Chalk Marl,' as it is 

 locally called, proves a very valuable dressing for peat and silt 

 either in the raw state or burnt. It contains 65 per cent, of 

 pure carbonate of lime. Kiln lime can be bought with a much 

 higher percentage, as high as 95, but a specialist alone knows 

 where to obtain it. 



The silt of the Fenland, which varies in depth from 8 to 18 

 feet, is particularly interesting to field-geologists, for it is an 

 estuarine formation which we can observe being made to-day 

 by the joint action of fresh and salt water. * 



Against a natural bank of peat, or one raised by man on the fore- 

 shore, first sand, and then fine silty sand brought down by the Fen- 

 land rivers is cast up by the sea, the lighter particles of the sand- 

 locally called ' warp' — being carried away by the wash of the waves. 

 When the bank is from 14 to 15 feet above low water mark, 

 Marsh Samphire [Salicornia herbacea) begins to grow, when the 

 surface is just covered at neap-tides. It disappears again when 

 the level of the soil is 16 feet above low water mark, or two feet 

 above ordinary spring tides. The Samphire from this point is 

 gradually replaced by grass. Now both the Samphire and 

 grasses act as a strainer for the muddy tidal water, and 

 gradually filter out the fine silt or warp to a higher and yet 

 higher level. At last only the highest spring tides can fully 

 cover the marsh which is formed. Old high marsh is 20*47 ^ eet 

 above low water, and an ordinary spring tide only 0*19 feet 

 higher. My friend, Mr. W. H. Wheeler, M.Inst.C.E., of Boston, 

 is the greatest living authority on silt and warp formation, t He 



"This is easily proved by its molluscan and mammalian fauna, which is 

 as follows : — Mollusca, Cardium edide, Mytiliis edidis, Ostrea edulis, Pisidium 

 amnicum, Scrobicidaria piperata, Bithinia tentactdata , Helix pulchella, 

 Hydrobia Ulvce, Limncea peregra, L. stagnalis, Planorbis carinatus, P. com- 

 plauatus, P. Icevis, P. vortex, Physa fontinalis, and Valvata cristata. The 

 Mammalia are as follows: — Balcena mysticetus, Delphinus iiirsio, Orca 

 gladiator, Phoca vitulina, Phoccena crassidens, and Trichecus rosmams. If 

 this is not an estuarine list, what would be? 



fWhen fine sand is mixed with a sufficient amount of chalk or clay 

 detritus to give the particles a certain amount of adherence, the mixture is 

 termed silt in this lecture. The warp, on the other hand, consists of finer 

 particles than those of which silt is composed. When the size oi the 

 particles suspended in water is so small as to be from ¥ £ 5 to ., r ^ r , of an inch 

 down to sizes so minute as not to be counted except by the aid of a pow er 

 microscope, they constitute the richest kind of 'warp' or tine alluvium. 

 When settled on the seabed this material is known as ooze. See \\ heeler ^ 

 'The Sea-Coast,' London, 1902, pp. 57-62, and his Brit. Assoc. Ad< 

 (Glasg-ow), 1901. 

 1902 June 1. 



