THE WORK OF RUNNING WATER 83 



common among rivers than is generally known. In the Appala- 

 chian region, for example, there are few large streams which have 

 not either increased their waters by piracy, or suffered loss by the 

 piracy of others. Figs. 84 and 85 afford one illustration. Piracy 

 is favored by inequalities of hardness, for the streams which do not 

 cross hard rock deepen their channels more readily than those 

 which do. 



When a stream is diverted from a narrows, the water-gap be- 

 comes a wind-gap. Wind-gaps are common in most mountain 

 regions which have advanced to late maturity. Snickers Gap 

 (Fig. 85) is one example, and the Cumberland Gap, in the south- 

 western corner of Virginia, is another. Cumberland Gap afforded 

 the early emigrant the most available route across the mountains, 

 and during the last quarter of the eighteenth century, probably 

 more than 300,000 people passed through it to settle in Kentucky 

 and Tennessee. The numerous wind-gaps of the Blue Ridge Moun- 

 tains figured prominently in the westward movement of the people 

 in the early history of our continent, and again in the Civil War. 



DEPOSITION BY RUNNING WATER 



We have seen that rivers are always carrying mud, sand, gravel, 

 etc., from land to sea. We have seen also that these materials are 

 often dropped for a time on their way to the sea, to be picked up 

 again when the conditions for transportation are more favorable. 

 We have now to learn (1) the reasons why running water abandons 

 some of its load, temporarily at least, (2) the places where the ma- 

 terial is deposited, and (3) the effects of its deposition. 



Causes of Deposition 



When running water abandons some part of its load, it is gen- 

 erally because its current has been checked. 



1. The commonest cause of loss of velocity is decrease of slope 

 or gradient. This change may take place suddenly, as when run- 

 ning water passes from a steep slope to a flat, or when it enters 

 a lake or the sea; or it may take place slowly, as in flowing through 

 a valley whose slope becomes gradually less. 



