LAKES. 603 



often separated from them by a high bank of reeds, sedges, and 

 rushes, through which one or more narrow channels have been 

 artificially cut. The depth of the broads does not appear to ex- 

 ceed fifteen feet ; as a rule it is about eight feet. In Hickling 

 Broad the depth is for the most part sufficiently shallow to allow 

 any one to walk upon its gravelly bottom. All observers agree 

 that the broads are slowly becoming shallower, partly by the 

 growth and decay of certain plants, partly by the deposit of earthy 

 sediment. The valleys marked out by the rivers were by recent 

 depression extensively flooded, and widened, and deepened by 

 estuarine action ; but the mouth of the estuary became choked up 

 by sandbanks about a.d. 1000 (see p. 594). The character of the 

 area was thereby changed ; the waters, now entirely fresh, could no 

 longer fill the valleys, and Broads were left in the hollows here and 

 there. To some extent their preservation has been due to the 

 retentive character of the sub-strata, as most of them are based on 

 the laminated clays of the Norwich Crag Series or on Lower 

 Glacial brickearth. Some Broads have been artificially enlarged 

 by the digging of peat, and others, like Fritton and Ormesby, are 

 preserved by artificial dams.^ 



The alternation of hard and soft strata along a river course may 

 lead to the formation of a lake, the softer strata being worn away, 

 and the harder forming barriers at either end. Lakes, indeed, 

 may be due to various causes or a combination of causes, such as 

 depression, and to faulting and disturbance of strata. Sir A. C. 

 Ramsay regards Llanberis lake and Llyn Ogwen, as well as some 

 other lakes in North Wales and Anglesey, to be due to Glacial 

 erosion,- while the Rev. J. Clifton Ward has advocated the Glacial 

 origin of some of the rock-bound lake-basins of Cumberland and 

 Westmoreland.^ Moraines may form barriers and produce lakes 

 (see Fig. 84, p. 491); and the sea also may raise up accumulations 

 that check the drainage of the land and cause flooding over low- 

 lying areas. Small sheets of water may also collect among hillocks 

 of Blown Sand where these accumulations are based on low-lying 

 and more or less retentive strata ; and meres may be formed by 

 subsidence due to the dissolution of strata. (See pp. 596, 597.) In 

 other countries, and in ancient times in this country, volcanic 

 action and the unequal elevation of parts of the sea-bottom have 

 produced extensive lacustrine areas. 



Waterfalls are sometimes produced by the passage of a stream 

 or river along its channel, from hard rocks to soft, the latter being 

 worn away, while the former beds stand out as ledges ; they may 

 also be formed where a stream runs out to sea over a cliff. 

 Sedgwick pointed out that in mountains composed of horizontal 

 strata of varied induration, waterfalls are abundant, and he 



1 Trans. Norf. Nat. Soc. iii. 458 ; S. Woodward, Hist. Norwich Castle, 1847. 

 ^ Q. J- xviii. 202. 



3 Q. J. XXX 96, xxxi. 157 ; see also W. Hopkins, Q. J. iv. 75, and R. Harkness, 

 Q. J. xxii. 485. 



