312 THE EEGOLITH 



shallow shores, still further dam its sluggish current till now, 

 during summer months, it is little more than a stagnant pond 

 full of rank vegetation, and a source of odors foul and atmos- 

 pheres enervating. The so-called ''Potomac Flats" south of the 

 city of Washington owe their origin and unhealthy conditions 

 to similar processes. 



The method of alluvial deposition in the flood plain or delta, 

 of the lower Mississippi has been worked out by McGee,^ whom 

 we cannot do better than quote in considerable detail. 



In length this flood plain reaches from the mouth of the Ohio 

 1100 miles measured along the river, or half as far measured 

 in an air line, to the Gulf, and is bounded on the east by the 

 bluff rampart separating it from the contiguous district; it is 

 bounded on the west by a less continuous and less conspicuous 

 rampart crossing the Arkansas River at Little Rock and gradu- 

 ally failing southward until this district and its more westerly 

 neighbor nearly blend. The surface of this otherwise monoto- 

 nous district is relieved by a few small tracts of higher land. 

 Most conspicuous of these is Crowley Ridge in eastern Arkansas, 

 a long belt of upland stretching from the southeastern Missouri 

 southward between the "White and St. Francis rivers to the 

 Mississippi at Helena. This belt of upland rises 100 or 200 feet 

 above the insulating flood plain, and in its steepness of slope 

 and rugosity of outline fairly simulates the eastern rampart 

 overlooking the ''delta" in corresponding latitudes. 



The vast lowland tract comprised in and constituting most of 

 this district is at once the most extensive and most complete 

 example of a land surface lying at base-level, or a trifle below, 

 that the continent affords. 



It is trenched longitudinally by the Mississippi, and trans- 

 versely by the White, Arkansas, Red, and other large rivers; 

 between these greater waterways it is cut into a labyrinth of 

 peninsulas and islands by a network of lesser tributaries and 

 distributaries, the former gathering the waters from its own 

 surface and from adjacent country, and the latter aiding the 

 main river to discharge its vast volume of water and its immense 

 load of detritus into the Gulf. The whole surface lies so low 

 that it is flooded by periodic overflows of the Mississippi and 

 its larger tributaries, and with each flood receives a fresh coat- 

 ing of river sediment ; and much of the flood plain, fertilized by 



^ The Lafayette Formation, Ann. Eep. U. S. Geol. Survey, 1890-91. 



