DEPOSITION 109 



If lake deposits are not available for study a great deal in the same 

 direction may be learnt from the deposits in a pond if it has been 

 drained. 



In the deposit of this fine-grained material, which is suspended 

 in water and can be carried great distances by it before sinking, 

 the prominent characteristic feature is the flatness, regularity, 

 parallelism, and wide extent of the laminae. 



In certain parts of its course a river is capable of depositing 

 coarser material such as pebbles and sand. While the fine material 

 is in suspension and may thus be spread over a wide area before 

 sinking to the bottom and finding rest, the coarser is rolled along 

 the bed of the stream just so long as the speed of the water is 

 sufficient. Directly the speed is checked the coarser material 

 must be dropped and, with further checking, that which is some- 

 what finer. Now the velocity changes with the gradient of the 

 river bed, and also as the river winds from side to side. Conse- 

 quently layers or spits of gravel are formed at these spots, and 

 the principal facts with regard to such deposits can be easily 

 observed there. Although pebbles are seen to be laid down with 

 their flatter sides and longer axes parallel to the surface on which 

 they are dropped (Fig. 25), such surface is usually inclined, and the 

 mass is heap-like in general shape. As the river winds from side to 

 side, or as it cuts its bed down deeper, it tends to destroy such gravel 

 masses and to move on their constituents to a lower level. Some 

 of the masses will generally survive, and it will often be found that 

 gravel pits have been opened in them, both when they are near the 

 river level and when they occur as terraces above the present level, 

 the river having deepened its valley since they were deposited. 



Examination of one of these gravel pits will show that the 

 deposit consists of irregularly alternating, bed-like ; masses of gravel 

 and sand (Fig. 17) ; their laminae are generally inclined and in 

 successive groups not parallel to one another. Such lamination is 

 called oblique, and it is an almost invariable accompaniment of the 

 deposit of coarse material rolled along and not held in suspension 

 by water. The process of formation will be understood if the 

 making of a quarry tip or a heap of pit waste or an embankment 

 is watched. The stuff is tipped down from wheel-barrows or 

 trucks on to the slope of the bank. Its successive layers, each a 



