16 Rev. J. F. Blahe — Form of Sedimentary Deposits, 



compensate this by spreading out transversely, so that its volume 

 may remain the same. In the present case the current will spread 

 out principally in a lateral direction, so that the sediment contained, 

 in any given volume of the current falls over a nearly constant 

 area, and the deposit from that volume will be of equal depth 

 throughout. This might seem at first to destroy the foregoing 

 argument, but the thickening therein referred to is produced by the 

 more rapid fall of the sediment in the higher layers into this volume, 

 so that the wider lateral spread of the deposit is an additional effect. 

 The constant widening of the current will deflect its margin to the 

 right and left, and the flow-lines will form spirals on each side of 

 the main axis, and there deposit some of the detritus they may carry. 



And this will have a curious result. The thicker deposits from 

 the slower moving water at the sides of the current will be laid 

 down adjacent to the thinner ones from the faster moving water 

 in the main stream. Thus there will be formed an apparent 

 depression of the sea-bed below the course of the current, though 

 the latter has never touched it. This will be marked by the higher 

 contour-lines bending towards the source of the current. On the 

 other hand, the detritus is carried farther out to sea in a straight 

 direction before deposition, and the lower contour-lines will bend 

 away from the source. 



Such, then, is the original form of a single deposit from a current 

 which starts with a given velocity and contains detritus of one rate 

 of sinking. It is thinnest near the source of supplj' and gradually 

 thickens to a maximum, until it ends seawards with a rapid slope ; 

 it expands at the sides with a curved boundary, convex seawards ; 

 and it has a depression opposite the source on its proximal, and 

 a prominence on its distal side. 



The total result from a current which varies in its velocity and 

 contents will be obtained by superimposing all the wedges thus 

 produced. If the variation be great the shorter wedges, corresponding 

 to the most rapidly sinking detritus, may overbalance the thin ends 

 of the wedges of finer detritus and make the total deposit thickest 

 near the shore ; but when the physical conditions remain constant 

 for a long period and the curi-ent brings uniformly fine detritus, all 

 the maxima will be added together and make the seaward end of 

 the deposit much thicker than the shoreward (see Fig. 2). 



The action of tidal and wind currents will modify these results. 

 The motion of such currents may be resolved into two components, 

 perpendicular and parallel to the shore respectively. The former, 

 mostly in connection with the tides, have as detritus carriers 

 a balance outwards, since the sea is essentially a denuding agent 

 on the shore ; they may therefore be included in the currents already 

 dealt with. The latter will tend to destroy the details of higher 

 contours, such as the bending shoi'ewards opposite the source of 

 an outward current ; also to elongate the original deposits throughout 

 at the expense of their thickness, and so join up the deposits from 

 adjacent outward currents and make the line of greatest thickness 

 a continuous one. Their action, however, is necessarily limited by 



