WATER AS A MECHANICAL AGENT. 193 



:flood at the same time, will have its mouth filled with sand-bars by the 

 greater river, and often, also, in spite of its floods. This subject is well 

 illustrated in Eeports on the Mississippi and its Tributaries by General 

 O. K. Warren. 



Sand-bars ; obliquely laminated structure. — A sand-bar, as shown by Gen- 

 eral Warren, has usually a slight pitch up stream and a steep one at the down- 

 stream extremity. The sand is carried on until the crest is reached, when 

 it falls over and stops in the still water below. The stratification will corre- 

 spond with the surface ; and as the sand-bar extends itself down stream by 

 the additions to its extremity, the pitch of the down-stream extremity will 

 determine oblique bedding parallel with it. The pushing of detritus along 

 the bottom of a river must result in similar oblique bedding. But in both 

 cases, oblique deposition will be followed by deposition in horizontal beds 

 when the floods are declining, so that combinations of the two, often of a 

 Tery irregular character, should exist in such deposits. 



(2) Over the flood-grounds. — The flood-grounds or river-flats are under 

 water only in times of floods. As the water rises in the channel, the velocity 

 slowly increases ; finally, where too great to be further withstood by the 

 earthy banks, the waters spread laterally to the limits of the flats. They 

 lose in velocity, and drop more or less of the material transported, resting 

 long after the flood ceases for such deposition wherever the surface is low. 

 At the same time, the upper or surface portion of the flood-waters may shear 

 off any accumulations above the general level, left by a former higher flood, 

 or may work with the outer margin to extend the limits of the flood-grounds. 

 The flood-grounds may thus lose from their surface, and, in parts, be cut 

 away to open new channels ; but they generally gain as much as they lose 

 or more. Along the sides of the channel they are often built up higher than 

 elsewhere, thus making high banks which may be emerged during an ordinary 

 flood. This raising of the margin takes place because of the deposition from 

 loss of velocity by friction against the banks, and because logs and debris 

 of other kinds are here stranded ; the debris serves to impede the velocity 

 still more and thus is buried by the sediment. Further, an emerging bank 

 often catches floating seed and grows shrubbery. These raised banks are 

 most common along the lower, less vigorous portions of a river. They give 

 the flood-plains a slope outward on one or both sides. Along the lower Mis- 

 sissippi the pitch from the river amounts, on an average, to seven feet for the 

 first mile. (H. & A.) As above explained, the deposits of the flood-grounds 

 may be the finest of silt, or the coarsest of gravel and stones, according to 

 the region and the pitch of the stream. The course of a tributary from a 

 mountain region over the flood-plain of the main stream may throw into 

 and across the earthy or sandy flats of the latter a wide thickening bed 

 of stones or gravel. 



A flood-ground is properly the surface of a terrace ; and it is the lowest 

 of the terraces where a valley has several. Terraces occur along nearly all 

 Dana's manual — 13 



