CONSTRUCTION OF DIKES. 
341 
brought down by the great rivers I have mentioned, and 
either directly deposited by them upon the sands of the bot¬ 
tom, or carried out to sea by their currents, and then, after a 
shorter or longer exposure to the chemical and mechanical 
action of salt water and marine currents, restored again to the 
land by tidal overflow and subsidence from the waters in 
which it was suspended. At a very remote period, the coast 
flats were, at many points, raised so high by successive allu- 
vious or tidal deposits as to be above ordinary high water 
level, but they were still liable to occasional inundation from 
river floods, and from the sea water also, when heavy or long;- 
continued west winds drove it landward. The extraordinary 
fertility of this soil and its security as a retreat from hostile 
violence attracted to it a considerable population, while its 
want of protection against inundation exposed it to the devas¬ 
tations of which the chroniclers of the Middle Ages have left 
such highly colored pictures. The first permanent dwellings 
on the coast flats were erected upon artificial mounds, and 
many similar precarious habitations still exist on the unwalled 
islands and shores beyond the chain of dikes. River embank¬ 
ments, which, as is familiarly known, have from the earliest 
antiquity been employed in many countries where sea dikes 
are unknown, were probably the first works of this character 
constructed in the Low Countries, and when two neighboring 
streams of fresh water had been embanked, the next step in 
the process would naturally be to connect the river walls 
together by a transverse dike or raised causeway, which would 
serve to secure the intermediate ground both against the back¬ 
water of river floods and against overflow by the sea^ The 
oldest true sea dikes described in historical records, however, 
are those enclosing islands in the estuaries of the great rivers, 
and it is not impossible that the double character they possess 
as a security against maritime floods and as a military ram¬ 
part, led to their adoption upon those islands before similar 
constructions had been attempted upon the mainland. 
At some points of the coast, various contrivances, such as 
piers, piles, and, in fact, obstructions of all sorts to the ebb of 
