CHAPTER VIII 

 RECONSTRUCTIVE PROCESSES — LAKE AND ICE DEPOSITS 



IV. Lacustrine or Lake Deposits 



Lakes are important places of sedimentary accumulation, for 

 the streams that flow into them deposit all their loads of suspended 

 material, and however turbid the inflowing streams, the outlet is 

 clear and sparkling, the lake acting as a settling basin. A most 

 striking instance of this is the Rhone, which enters Lake Geneva 

 a muddy, glacial torrent, but flows out in a state of exquisite 

 purity. The ' Niagara, as it leaves Lake Erie, is likewise beauti- 

 fully clear. Occasional exceptions to this rule may occur, as when 

 a shore current of the lake washes sediment into the outlet, but 

 for the most part, the lake intercepts and holds all the sediments 

 carried by its tributary streams. 



i. Fresh-water Lakes, a. Mechanical Deposits. — The me- 

 chanical sediment which accumulates in a lake basin is of two 

 kinds, (i) that which is brought in by tributary streams, and 

 (2) that which the lake itself acquires by cutting into its shores. 

 In most cases the stream-borne sediment is much the more im- 

 portant of the two classes. The rivers which empty into lakes 

 nearly all form deltas ; if a strong shore current sweeps past 

 the mouth of the stream, it will distribute part of the materials 

 along the shore, and the waves will act as transporting agents to 

 the same effect. The deltas spread out in a fan-shape from the 

 mouths of the inflowing streams, and if they are numerous enough, 

 may surround the entire lake with delta deposits. In each delta 

 the successive layers of sediment will have an inclination dependent 

 upon the depth of water into which the stream debouches and the 

 coarseness of the debris carried. If in deep water, the beds may 



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