Introduction
15
is taken as the roc-fathom (600—feet) line. The continental shelf is
covered by marine sediments which may be thousands of feet thick.
lt varies considerably in width and occasionally is absent altogether.
The end of the shelf is generally marked by an abrupt increase in
gradient to 1 in 40. Dramatic though this change from shelf to
continental slope may seem, the eye would hardly detect it. In
places the surface of the slope is cut into by shallow valleys and
by canyons some of which, like the Hudson Canyon off the New
York coast, rival the Grand Canyon and stretch back to the con—
tinental shelf itself. Beyond the slope occurs the continental rise
which slopes more gently down to the abyssal plain of the deep
ocean. Some of the attest areas on the Earth’s surface occur here
where sediment collecting over millions of years has lled in any
depressions. Only sea—mounts, some with at tops known as guyots,
rise above the plain the surface of which may be etched with wide
and shallow ‘canyons’ thought to have been created by rapidly
moving ‘rivers’ of sediment known as turbidity currents.
The abyssal plains with their sea—mounts, guyots and shallow
canyons and valleys are not the only features in an ocean basin; one
of the most dramatic pieces of oceanic scenery must be the midocean ridges. These craggy chains of rock tower towards the surface
and sometimes rise above it producing strings of islands—the
Tristan da Cunha group mark one area where the mid—Atlantic
Ridge clears sea level. In direct contrast to these heights are the
deep trenches, generally found along the margins 'of the continents.
Off the west coast of South America, one such trench merges with
the continental slope which, because the continental shelf is very
narrow, leads almost directly to the adjacent mountains of the
Andes. From the peaks of the Andes down to the depths of this
trench involves a descent of about eight miles!
For the most part this is a world of darkness—a place that has
remained unlit by the sun since the time when the oceans formed.
The reason for this is that water acts as a lter, removing rst the
longer wave—lengths of visible light such as red and yellow and
nally the shorter ones of green and blue. This effect is apparent
even in the comparatively shallow water of a small lake which usually
seems to be tinged with blue particularly at its deepest part. It is
more obvious still to someone swimming some feet below the sur—
face of the sea; when he looks up
at it the sunlight appears
to be blue
or blue—green. Colour photographs taken underwater conrm these
15
is taken as the roc-fathom (600—feet) line. The continental shelf is
covered by marine sediments which may be thousands of feet thick.
lt varies considerably in width and occasionally is absent altogether.
The end of the shelf is generally marked by an abrupt increase in
gradient to 1 in 40. Dramatic though this change from shelf to
continental slope may seem, the eye would hardly detect it. In
places the surface of the slope is cut into by shallow valleys and
by canyons some of which, like the Hudson Canyon off the New
York coast, rival the Grand Canyon and stretch back to the con—
tinental shelf itself. Beyond the slope occurs the continental rise
which slopes more gently down to the abyssal plain of the deep
ocean. Some of the attest areas on the Earth’s surface occur here
where sediment collecting over millions of years has lled in any
depressions. Only sea—mounts, some with at tops known as guyots,
rise above the plain the surface of which may be etched with wide
and shallow ‘canyons’ thought to have been created by rapidly
moving ‘rivers’ of sediment known as turbidity currents.
The abyssal plains with their sea—mounts, guyots and shallow
canyons and valleys are not the only features in an ocean basin; one
of the most dramatic pieces of oceanic scenery must be the midocean ridges. These craggy chains of rock tower towards the surface
and sometimes rise above it producing strings of islands—the
Tristan da Cunha group mark one area where the mid—Atlantic
Ridge clears sea level. In direct contrast to these heights are the
deep trenches, generally found along the margins 'of the continents.
Off the west coast of South America, one such trench merges with
the continental slope which, because the continental shelf is very
narrow, leads almost directly to the adjacent mountains of the
Andes. From the peaks of the Andes down to the depths of this
trench involves a descent of about eight miles!
For the most part this is a world of darkness—a place that has
remained unlit by the sun since the time when the oceans formed.
The reason for this is that water acts as a lter, removing rst the
longer wave—lengths of visible light such as red and yellow and
nally the shorter ones of green and blue. This effect is apparent
even in the comparatively shallow water of a small lake which usually
seems to be tinged with blue particularly at its deepest part. It is
more obvious still to someone swimming some feet below the sur—
face of the sea; when he looks up
at it the sunlight appears
to be blue
or blue—green. Colour photographs taken underwater conrm these