AQUEOUS AGENCIES. 



39 



the mouths of all rivers by the fan-like spreading of the 

 currents and the consequent checking of the velocity by 

 contact with still water of the sea or lake. They are usu- 

 ally of semicircular or horseshoe-like form, as shown in 

 Figs. 16, 17. In rivers forming deltas (Fig. 16) this is 

 the only bar ; but in rivers forming estuaries (Fig. 17) 

 there are two bars one at the mouth of the estuary, and 

 the other at its head. The bar at the mouth is formed 

 in the usual way, by the spreading of the current of the 

 outgoing tide, the consequent 

 checking of its velocity, and de- 

 posit of its sediment. It has, 

 therefore, the usual semicircular 

 or horseshoe form. There are 

 usually passes or deeper channels 

 through which the tides ebb and 

 flow. 



The bar at the head of the 



estuary or bay is formed by the meeting of two opposing 

 sediment-laden currents, viz., the up-flowing tide and the 

 down-flowing river. The meeting of these at every tide 

 makes still water at that place, and the sediments from 



FIG. 16. 



both are dropped there. In fact, at the head of the 

 estuary there are three associated phenomena, all pro- 

 duced by the meeting of these opposing currents : 1, 



