PROFILE OF THE SUBAQUEOUS SHORE TERRACE 29 
distinctly greater in a zone adjacent to the land, a smaller 
terrace will rest upon the larger. If transportation parallel to 
the shore line be distinctly greater in a zone off-shore, and the 
supply of drift be at hand, a ridge will be built along the line of 
this more effective current. 
Barriers —It has been shown above that when the off-shore 
slope is too low for equilibrium, and there are no currents 
alongshore, steepening is effected, in the main, by accumulation 
at the water’s edge, though there may be some small tendency 
to accumulation at or just within the breaker line. When cur- 
rents are flowing, they have a zone of greater efficiency along 
this same line or just outside. This is because the material 
which they transport is more agitated by wave action, and is to 
some extent lifted into the current. Excessive transportation 
along this zone initiates the ridge which may continue to grow 
until it assumes the functions of the beach. It this then called 
a banhien, 
The essential function of the barrier is to steepen the bottom 
slope by carrying the shore line farther out. If the slope is not 
abnormally low, the barrier is not needed; nor are the con- 
ditions present which make its formation possible, one of these 
conditions being that the agitation on the bottom at the breaker 
line should exceed that nearer shore. It was seen above that 
this condition is present, only on a deficient slope. 
The slope may become deficient in several ways. The cur- 
rents themselves might be the cause; or it may result from the 
sediments delivered by streams, as at many places on our 
Atlantic coast; or the gentle slope may have belonged to the 
original bottom over which the waters rose, as seems to have 
been the case with Lake Michigan in its former extension in 
the vicinity of Chicago. Doubtless far the most frequent 
occasion of deficient slope is the falling of the water level or 
the rising of the shore. That the immediate off-shore slope 
should in this case be too low, is the necessary consequence of 
the concavity of the normal slope near shore. The slope from 
the Atlantic shore line, where well removed from rivers, as on the 
