so nearly a circle that its course seems to have been
"described by a compass," Caesar says, while fortifications across the
isthmus made the position of the town almost impregnable.[723] Verona,
lying at the exit of the great martial highway of the Brenner Pass,
occupies just such a loop of the Adige, as does Capua on the Volturno,
and Berne on the Aare. Shrewsbury, in the Middle Ages an important
military point for the preservation of order on the marches of Wales, is
almost encircled by the River Severn, while a castle on the neck of the
peninsula completes the defense on the land side.[724] Graaf Reinett, at
one time an exposed frontier settlement of the Dutch in Cape Colony, had
a natural moat around it in the Sunday River, which here describes
three-fourths of a circle.

[Sidenote: River islands as sites of trading posts and colonies.]

The need of protection felt by all colonists in new countries amid
savage or barbarous people whom encroachment sooner or later makes
hostile, leads them if possible to place their first trading posts and
settlements on river islands, especially at the mouth of the streams,
where a delta often affords the site required, and where the junction of
ocean and river highway offers the best facilities for trade. A river
island fixed the location of the English settlement at Jamestown in
Virginia, the French at Montreal and New Orleans, the Dutch at Manhattan
and Van Renssellaer Island in the Hudson, the Swedes at Tinicum Island
in the Delaware River a few miles below the mouth of the Schuylkill.[725]
St. Louis, located on a delta island of the Senegal River, is one of the
oldest European towns in West Africa;[726] and Bathurst, founded in 1618
on a similar site at the mouth of the Gambia, has for centuries now been
the safe outlet for the trade of this stream.[727] Such island
settlements at river mouths are a phenomenon of the outer edge of every
coastal region; but inland stations for trade or military control also
seek the protection of an island site. The Russians in the seventeenth
century secured their downstream conquest of the Amur by a succession of
river island forts,[728] which recall Colonel Byrd's early frontier post
on an island in the Holston River, and George Rogers Clark's military
stockade on Corn Island in the Ohio, which became the nucleus of the
later city of Louisville.

[Sidenote: Swamps as barriers and boundaries.]

More effective than rivers in the protection