598 



THE NATURE BOOK 



"THE SWAMPY FLAT INDICATES THE FORMER EXTENSION OF THE 



WATER." 

 Head cf Haweswater. 



lessened \\hen its motion is arrested b}' 

 the tranquil waters, and banks of sand 

 and mud form a delta where the current 

 enters the lake. The finer sediments dis- 

 colour the water far into the lake, but they 

 gradually sink to the bottom until finally 

 only the minutest specks of ultra-micro- 

 scopic dimensions are left suspended in 

 the hquid. A tumbler of water taken 

 from this zone would appear to our coarse 

 senses clear and spotless, but the tiny 

 particles become apparent when the more 

 dehcate effects of colour are produced by 

 the filtering of the sun's rays through the 

 medium. The wonderful shades of blue 

 and green which delight the eye when 

 gazing into the depths of lakes fed by 

 turbid streams are the direct products of 

 the interference of these particles on the 

 sunlight which falls on their surfaces and 

 penetrates into the depths. The constant 

 deposition of material causes the delta to 

 spread, and the tract of low marshy 

 ground at the head of all lakes fed by 

 muddy streams is a measure of the infill- 

 ing which has taken place. Through this 



swampy flat which indicates the former factor in the infilling 

 extension of the water, a narrow channel 

 is kept clear in the direct hne of flow. 



Tributary 

 streams, too, en- 

 tering at the 

 sides deposit 

 the debris they 

 carry in suspen- 

 sion, and fan- 

 shaped deltas 

 grow out from 

 the banks to- 

 wards the mid- 

 dle. On the 

 western shore of 

 Tal-y-Llyn, near 

 Cader Idris, one 

 of these fans has 

 crept far into 

 the lake. In 

 the Straits at 

 H a \\" e s w a t e r, 

 only a narrow 

 channel inter- 

 %"enes between 

 the delta of the 

 Measand Beck 

 and the oppos- 

 ing shore. In 

 course of time the lake may be spanned 

 from shore to shore ; thus it becomes 

 divided into upper and lower parts, 

 whose waters are only connected by 

 a sluggish stream which breaches the 

 barrier on the side opposite to the 

 entering tributary. With this in view 

 it is not difficult to account for the 

 fact that a great number of our lakes 

 occur in pairs. Derwent Water and 

 Bassenthwaite Water, once continuous, 

 have been severed by the \\-aste brought 

 down by the Greta, Xewland's Beck, and 

 other minor streams. Grasmere and R\'dal 

 Water are spanned by the delta of the 

 Dunney Beck ; Buttermere and Cnmimock 

 \\'ater b}- Sail Beck and the neighbouring 

 brooks. In North Wales we have Llyn 

 Peris and Llyn Padarn, and in Ireland the 

 two lakes in the \'alley at Glendalough. 



Water-loving plants soon get a foothold 

 in shallows of the deltas, and the rate of 

 deposition is rapidly increased as the 

 muddy water is strained through the 

 matted sieve of roots and stems. The 

 growth of water plants is an important 



not only at the head 

 but also on the shaUow margins of the 

 lake. The plants inarch in regular order 



