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192 THE OaJAN
If the inclined plane which the dune turns toward the sea remained perfectly even, the zone of the shore would only present, in all its extent a
single rampart of sand gradually encroaching on the lands. But at length
the slope of each dune can not fail to offer some inequalities caused by foreign bodies, or by plants that take their origin in the sand. All the salient
points strong enough to resist the wind serve as supports to new dunes,
grafted, so to say, on the sides of the ancient one. These new dunes themselves bristle with irregularities, which other sand-hillocks soon cover, and
it is thus that all those ranges of moving hills arise, which are separated
by long and narrow valleys, called lettes or lldes by the peasants of the
French Landes. In certain places, especially between Biscarosse and La
Teste, these " lettes," for a length of several leagues, resemble the dried-up
beds of large rivers, surrounding'large islets of verdure with their sandy
waves.
Notwithstanding the apparent disorder of these hillocks, in the midst
of which an inexperienced traveler might easily lose his way, the general
disposition of the sands can always be referred to a uniform type, which
local geographical facts variously modify, such as the contours of the marine shore, the nature of the soil, the force and direction of the winds, the
presence or absence of vegetation. The dune nearest to the sea, and, in
consequence, the most recent, is less elevated than the older hillock situated immediately beyond; and this, in the same way, attains a less considerable height than the following hill. In a system of dunes, generally
each range which is developed farther inland exceeds the preceding ones
in elevation, and forms, as it were, a new step on the slope of the great
primitive dune which serves as an avant garde to the army of sands.
This last dune, the true crest of the entire system, enlarges itself, little by
little, with all the materials which have served for the formation of the
inferior dunes situated on the side nearest to the sea. The grain of sand
which the air carries to the summit of the first hillock, and which falls
afterward into a ravine, may remain during centuries under the superincumbent masses; but owing to the constant progress of the dune, the
superficial layers of which are swept by the wind and then let fall by it
farther down the talus, this grain of sand at last re-appears, is carried
anew to a summit, it descends again, and thus does not cease to travel
from dune to dune to the last. ,*
As the innumerable arenaceous particles are moved by virtue of rigorous laws, we can consequently measure the force of the winds, by the
height of the mass, and the rapidity of the displacement of the hillocks.
Attentive observation permits us, in the same way, to compare with each
other the various atmospheric currents which drive the sands onward,
and to indicate exactly the one whose action is the most energetic.
Thus, in the peninsula of Arvert or La Tremblade, situated between the
mouth of the Gironde and that of the Seudre, the chain of dunes rises
gradually in a northerly direction, and it is at the northern extremity
that the highest hillock is found. This phenomenon is explained by the
| Title | The ocean, atmosphere, and life |
| Creator | Reclus, Elisée |
| Publisher | Harper |
| Place of Publication | New York |
| Date | 1873 |
| Language | eng |
| Type | Books/Pamphlets |
| Title | 00000213 |
| Type | Books/Pamphlets |
| Transcript | 192 THE OaJAN If the inclined plane which the dune turns toward the sea remained perfectly even, the zone of the shore would only present, in all its extent a single rampart of sand gradually encroaching on the lands. But at length the slope of each dune can not fail to offer some inequalities caused by foreign bodies, or by plants that take their origin in the sand. All the salient points strong enough to resist the wind serve as supports to new dunes, grafted, so to say, on the sides of the ancient one. These new dunes themselves bristle with irregularities, which other sand-hillocks soon cover, and it is thus that all those ranges of moving hills arise, which are separated by long and narrow valleys, called lettes or lldes by the peasants of the French Landes. In certain places, especially between Biscarosse and La Teste, these " lettes," for a length of several leagues, resemble the dried-up beds of large rivers, surrounding'large islets of verdure with their sandy waves. Notwithstanding the apparent disorder of these hillocks, in the midst of which an inexperienced traveler might easily lose his way, the general disposition of the sands can always be referred to a uniform type, which local geographical facts variously modify, such as the contours of the marine shore, the nature of the soil, the force and direction of the winds, the presence or absence of vegetation. The dune nearest to the sea, and, in consequence, the most recent, is less elevated than the older hillock situated immediately beyond; and this, in the same way, attains a less considerable height than the following hill. In a system of dunes, generally each range which is developed farther inland exceeds the preceding ones in elevation, and forms, as it were, a new step on the slope of the great primitive dune which serves as an avant garde to the army of sands. This last dune, the true crest of the entire system, enlarges itself, little by little, with all the materials which have served for the formation of the inferior dunes situated on the side nearest to the sea. The grain of sand which the air carries to the summit of the first hillock, and which falls afterward into a ravine, may remain during centuries under the superincumbent masses; but owing to the constant progress of the dune, the superficial layers of which are swept by the wind and then let fall by it farther down the talus, this grain of sand at last re-appears, is carried anew to a summit, it descends again, and thus does not cease to travel from dune to dune to the last. ,* As the innumerable arenaceous particles are moved by virtue of rigorous laws, we can consequently measure the force of the winds, by the height of the mass, and the rapidity of the displacement of the hillocks. Attentive observation permits us, in the same way, to compare with each other the various atmospheric currents which drive the sands onward, and to indicate exactly the one whose action is the most energetic. Thus, in the peninsula of Arvert or La Tremblade, situated between the mouth of the Gironde and that of the Seudre, the chain of dunes rises gradually in a northerly direction, and it is at the northern extremity that the highest hillock is found. This phenomenon is explained by the |
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