F. Hardy — Mineral Covijwsition of Fenland Silt. 545 



joined the Ujoper Cretaceous highlands of East Yorkshire with those 

 of Norfolk. The agents which effected this operation were the rivers 

 of the early Fenland, combined with the usual agents of subaerial 

 weathering. The breach was probably completed in pre-Pleistocene 

 times, but prior to the oncoming of the Glacial period the covering 

 ■of Cretaceous strata was completely removed from the area, so that 

 the underlying Jurassic clays were exposed. During the Ice Age 

 a thick covering of glacial deposits was spread over the whole or 

 greater part of the area. Subsequent depression of the land-surface 

 allowed the sea to extend inland, so that a great bay resulted. It 

 was in this bay that the true Fen-beds were deposited. The inflowing 

 rivers caused an accumulation of alluvium near the shores of the 

 bay. and- the sea-currents brought doAvn material from the coasts 

 of East Yorkshire and North Lincolnshire. Gradually the bay 

 became filled with sediments, and in the south and south-eastern 

 regions swamp-plants began to grow The remains of these 

 subsequently gave rise to the peat lands of the Fens. 



The silts of the north-western parts of the Fenland are chiefly of 

 marine origin. They are still being deposited along the coast of the 

 Wash, where they give rise to salt-marsh. As the tidal water is 

 checked in its flow, the silt falls to the bottom, and thus a bank is 

 gradually formed against the shore. As soon as this bank rises to 

 a certain height (estimated at 8'6 feet above ordnance datum), 

 Salicornia begins to grow on it. The stems of this plant assist in 

 the further development of the marsh by checking the flow of the 

 water, and thus allowing a longer time for the silt to settle, and also 

 by holding up much of the fine material when the tide begins to ebb. 

 As the marsh increases in height, other plants establish themselves, 

 and " green marsh " results. Green marsh is only covered by sea- 

 water at the higher tides. It marks the stage when embanking 

 becomes practicable. 



The material of which the Fen silt is composed has been derived, 

 as already mentioned, from the soft strata (mainly glacial) of the 

 Yorkshire and North Lincolnshire coasts, which are rapidly being 

 eroded by the sea. The glacial deposits themselves are of the Purple 

 and Hessle boulder-clay type. They have been derived from the 

 rocks over which the ancient North Sea glacier passed in its move- 

 ment from the Scandijiavian highlands to the coast of England, 

 and they therefore perhaps include material taken from the bed 

 of the North Sea. 



III. MINERALOGICAL EXAMINATION OF A TYPICAL 

 SAMPLE OF MODERN FENLAND SILT. 



The sample examined and described below was taken from the 

 ioreshore of the north-west coast of the Wash immediately to the 

 seaward of the village of Friskney. It yielded the following results 

 when analysed : — 



VOL. LVII. — NO. XII. • 35 



