142 PHYSIOGRAPHY. [chap. 



purged of its impurities. During its passage through the 

 lake, the mud which it held in suspension is deposited upon 

 the bottom ; and, accordingly, at the entrance of the river, new- 

 land is being slowly formed by the growth of this sediment. 

 In fact, Port Vallais, the Fortus Vaksia of the Romans, 

 which was originally situated on the margin of the lake, is 

 now nearly two miles inland ; the intervening ground having 

 been formed, at the expense of the lake, by accumulated 

 sediment delivered by the river. In this way, a lake may 

 grow shallower and smaller, until at length it becomes com- 

 pletely silted up ; and a marshy tract is formed, through 

 which the river flows in a meandering course. Such land 

 is generally called alluvium} 



It often happens that, without flowing into a lake, a river 

 may get relieved of much of its burden of sedimentary 

 matter. When an unusual supply of water is suddenly 

 delivered into a stream, by heavy rainfall or by rapid thaw 

 of snow, the swollen stream rises above its banks, and floods 

 the adjacent land. In a flood, or freshet, the water is always 

 highly charged with detritus ; and, on the overflow of the 

 river, some of this is deposited as a fine layer of mud evenly 

 spread over the flooded soil. The overflow being repeated 

 season after season, the layers of mud accumulate until they 

 form a low alluvial tract on each side of the stream. Most 

 rivers are bordered by strips of rich meadow-land, which 

 have been formed in this way. Such low- lying alluvial 

 meadows are common along the banks of the Thames \ and 

 in the lower part of the basin, where the river is broad, 

 especially between London and Tilbury, there are great 

 expanses of flat marshy ground : the Isle of Dogs, for 

 example, is such an alluvial tract. Periodical deposition of 



^ Alluvium, from the Lat. ad. and lno, I wash ; land which is 

 added to by the wash or flow of water. 



