sed spur running out from the solid ground of older and higher land
into the water-soaked alluvium of the Netherlands. It was the most
important town of all this region before the arts of civilization began
the conquest by dike and ditch of the amphibian coastal belt which now
comprises one-fourth of the area and holds one-half the population of
the Netherlands.[413] So ancient London marked the solid ground at the
inner edge of the tidal flats and desolate marshes which lined the
Thames estuary, as the Roman Camulodunum and its successor Colchester on
its steep rise or _dun_ overlooked the marshes of the Stour inlet.[414]
Farther north about the Wash, which in Roman days extended far inland
over an area of fens and tidal channels, Cambridge on the River Cam,
Huntingdon and Stamford on the Nen, and Lincoln on the Witham--all river
seaports--defined the firm inner edge of this wide low coast. In the
same way the landward rim of the tidal waters and salt marshes of the
Humber inlet was described by a semicircle of British and Roman
towns--Doncaster, Castleford, Todcaster, and York.[415] On the flat or
rolling West African coastland, which lines the long shores of the Gulf
of Guinea with a band 30 to 100 miles wide, the sandy, swampy tracts
immediately on the sea are often left uninhabited; native population is
distributed most frequently at the limit of deep water, and here at head
of ship-navigation the trading towns are found.[416]

[Sidenote: Inner edge as head of sea navigation.]

While, on low coasts at any rate, the inner edge tends to mark the limit
of settlement advancing from the interior, as the head of sea navigation
on river and inlet it has also been the goal of immigrant settlers from
oversea lands. The history of modern maritime colonization, especially
in America, shows that the aim of regular colonists, as opposed to mere
traders, has been to penetrate as far as possible into the land while
retaining communication with the sea, and thereby with the mother
country. The small boats in use till the introduction of steam
navigation fixed this line far inland and gave the coastal zone a
greater breadth than it has at present, and a more regular contour. In
colonial America this inner edge coincided with the "fall-line" of the
Atlantic rivers, which was indicated by a series of seaport towns; or
with the inland limit of the tides, which on the St. Lawrence fell above
Quebec, and on the Hudson just below Albany.