352 NATURAL SCIENCE. 



July, 



The same southerly drift has affected other parts of the eastern 

 coast : thus to it is due the great projection of Spurn Head ; but 

 in the case of the Humber, the river is sufficiently large to prevent 

 its mouth from being seriously interfered with, and thus it has been 

 but slightly deflected to the south. In the case of the Thames, the 

 land to the north could not yield such masses of glacial boulders and 

 other debris as the sea had to work with in Norfolk. 



The influence of this barring of the rivers would be to at once 

 check the velocity of the current at the mouth : the water striking the 

 bar would immediately drop its load of debris, and thus strengthen 

 the barrier by forming a shoal on the inner side. This would be 

 built up further during floods, and by the growth of plants which 

 would bind the loose sediment into a coherent mass, and by their 

 decay gradually raise it above the water line. Thus, what would happen 

 would be simply the silting up of the estuary at its mouth, instead of 

 at its head as in the Wash, or along lines of still or " dead " water 

 as in the Thames. This tract of silted land would, by the continua- 

 tion of these operations, work its way gradually backward up the 

 estuary, leaving a great sheet of water separated from the sea by a 

 band of alluvium ; of this Breydon Water may be the diminished 

 representative. But as the land worked further backward, it would 

 cross the entrance of branches of the estuary ; the sediment would be 

 carried along the central channel, upon the sides of which it would be 

 deposited : it could thus cut off the branches either entirely, as in 

 the case of Fritton Lake, or connected by a channel just sufficient 

 for the escape of the surplus rainfall, as does the memorable Muck 

 Fleets for the three great sheets of Rollesby, Ormesby, and Filby 

 Broads. 



In such a way may be explained the origin of those Broads 

 which lie in valleys off the main courses of the three rivers, but this 

 will not suffice for those which lie along the rivers, and wholly 

 surrounded by the alluvium. Let us take the most complex case. 

 At the head of the alluvial plain of the Bure, at its junction with the 

 narrow valley above Wroxham, there is a series of seven Broads, 

 separated from one another and from the gravel hills on either side 

 by tracts of ordinary "rand." The origin of the marginal rand is 

 very simple : under the influence of the wind, rain, and frost, the 

 pebbles constantly tend to roll down the hill sides into the Broad ; 

 upon the flat submerged bank thus formed, bulrushes and other 

 plants would take root and help to raise it into a low terrace running 

 at a uniform height along the foot of the hills. 



The Bure must at this time have swept down the valley above 

 Wroxham with considerable power, carrying with it much mud and 

 sand. When the river struck the great Broad, which covered the 



5 " Fleet," it may be remarked, means ditch, and has no reference to the rate at 

 which this Muck may be ascended. 



