coarser material than that on either side. This is a lag deposit of 
material large enough in particle size so that, if lifted by turbulent 
forces, it drops back into the trough without being carried beachward by 
the uprushing tongue of water, or seaward by diffusion. 
The cross-section of Figure 2 is properly supplemented by the beach 
map of Figure 3, in which waves approach a straight shore at an angle. 
The refracted wave crests, the surf zone, and the momentary water edge 
are shown schematically. The breaking waves, with their resulting uprush 
surge, give rise to swash marks that indicate limits of uprush, normally 
on the seaward side of the berm crest. For completeness, a backshore of 
moderate width, with dunes in the hinterland, is included in this map, 
Waves that approach at an angle to the shore develop a longshore 
current as indicated by the arrow in Figure 3. This current normally 
transports sand, and introduces a component of movement parallel to the 
shoreline. On the other hand, the uprush-backwash zone represents a 
component of movement essentially normal to the shoreline. Thus two 
phenomena occur simultaneously: a downbeach movement of material from 
left to right on the map, and a crossbeach movement of material between 
the plunge point and the berm, For any stretch of shore some miles in 
length, the downbeach movement results in a gradual but systematic change 
in the average size, shape, and density of the beach particles. For any 
one cross-section normal to the shore, the nearly continuous uprush and 
backwash tends to develop a pattern of relatively rapid change in beach 
particle properties across the foreshore. Thus, the rates of change in 
beach particle properties are much greater along a traverse normal to the 
shoreline than in a direction parallel to the shoreline. 
The tide, in essence, moves the water's edge to and fro across the 
foreshore, allowing the uprush to extend to greater or lesser distances 
from the normal position of the berm. The plunge point, for a given 
state of the waves, also moves with the water edge, resulting in con- 
siderable reworking of the foreshore material as the tide rises and falls. 
The tide also contributes a component to the shore current that may act 
with or in opposition to the shore current generated by the waves. 
Where the amount of sand moving along a shore is relatively large, 
the foreshore may be aggraded, with the result that the berm moves seaward. 
This movement of the berm produces a wider backshore, and thus adds sand 
to the backshore "storage bin", free from subsequent wave work as long as 
the beach foreshore remains stable, or continues to aggrade. The backshore 
sand, removed from the zone of hydraulic forces, becomes subject to the 
geologic work of wind. The combination of shifting wind directions, and 
the selective transport of materials from backshore to dunes, tends to 
blur any gradients or trends that may have been inherited from the hy- 
draulic conditions operative as the berm moves seaward. 
