350 NATURAL SCIENCE. 



JULY, 



watercourses, the acres of flat-leaved pond weeds and water lilies 

 that cover the Broads, and the dense tussocks of the sedge {Carex 

 paniciilata) that stand up like buoys to warn the sailor of the shoals, 

 together form to the ordinary townsman a revelation of the charm 

 and beauty of our water flora. 



Reluctant though one may be to leave the water, it is necessary 

 to do so if we desire a closer acquaintance with the geology of the 

 district. Landing on the flat tract of meadow that borders the rivers, 

 one gets an idea of its formation : the soil is everywhere damp and 

 studgy, the surface is broken up by holes between the roots of the 

 rushes or marsh plants, by the growth and decay of which these 

 "rands" have been mainly formed. The hill-slopes, on the other 

 hand, are composed of gravels and pebble beds, with occasional beds 

 of fossiliferous ironstone, belonging to the Bure Valley beds, a series 

 now of more than local interest since Professor Prestwich correlates 

 it with the isolated patches of hill-gravel that stretch away to the 

 south-west of England. Whatever may be the verdict upon this con- 

 clusion, there can be no doubt that the beds are here marine in origin, 

 and that they were deposited in shallow water, probably in an 

 estuary. Above these Bure Valley beds rests the series of sands, 

 gravels, clays, and loams belonging to the Glacial Period : the chief 

 of these is the Chalky Boulder Clay formed by the great ice-sheet 

 that once covered East Anglia. The only bed later than these is the 

 Alluvium, which forms the plains at the foot of the hill-slopes : but 

 the glacial beds occur on the summits of the hills on either side of the 

 valleys and rarely at lower levels within these ; hence it is obvious that 

 they once spread as a sheet over the whole district before the 

 existence of the present valleys, only occasionally cutting down to 

 lower levels. Now these valleys are of great depth ; thus a boring at 

 Yarmouth passed through 170 feet of drifts before it struck the 

 London Clay, thus proving the existence there of a buried river 

 channel 160 feet below the sea level, and therefore indicating a subsi- 

 dence of the area to this extent. This might have been inferred, 

 without any direct proof, from the fact that the valleys could not 

 have been cut out by streams flowing with their present sluggish 

 course. The whole fall of the Bure from Wroxham Bridge to 

 Yarmouth is only about 2i inches to the mile. An elevation of 160 

 feet would have turned the Bure into a torrent, with ample power for 

 the erosion of its channel to this depth. As the excavation of the 

 valleys commenced almost immediately after the close of the Glacial 

 Period, it is quite probable that the elevation thus indicated was part 

 of the great elevation which seems always to have accompanied, if 

 not directly caused, the glaciation of temperate regions. The valleys 

 were, however, certainly formed before the subsidence, so that when 



33. V. Wood has suggested that part of this may be really Crag {Geol. 

 Mag., iv., 1867, p. 560). 



