190 THE WORK OF RUNNING WATER 



ment, too, is sorted by the water, and the finer material is 

 carried far out from the river mouth. Formations of this 

 kind are called deltas, from the Greek capital letter Delta 

 (A), which has the shape of a triangle. Few deltas have 

 this ideal shape, but there is a general correspondence to it. 

 Deltas have rich, fine-textured soils and are very fertile. 

 The Nile delta during all history has been noted for its 

 fertility. But they are treacherous places, as they are liable 

 to inundations by the overflowing of the river at time of 

 flood. Because they are pushed out into the sea, they 

 are peculiarly exposed to the sweep of the waves in great 

 storms. The delta of the Mississippi is more than 200 miles 

 long and has an area of more than 12,000 square miles. The 

 Po in historic time has built a delta more than 14 miles 

 beyond Adria, a former port which gave its name to the 

 Adriatic Sea. 



Inland Waterways and History. From earliest times 

 rivers have played a most important part in the world's 

 history. At first almost all human movement was along 

 river valleys, as they offered the easiest routes of travel. 

 Here, too, men found the fertile and easily worked land so 

 necessary in their primitive agriculture. Thus their settle- 

 ments were usually placed upon the banks of rivers. In war 

 the river offered a means of defense, as the Tiber so often 

 did to Rome. 



Before the time of railways, rivers and lakes supplied 

 almost the only means of inland commerce. In our own 

 country the hundred and fifty miles of unobstructed river- 

 way stretching from New York to the north was the great 

 road from Canada and the Lakes to the sea, fought for 

 persistently in French and Indian Wars as well as in the 



