第28章 ON THE ELEVATION OF THE WESTERN COAST OF SOUTH AME
On the syenitic ridge, which forms the southern boundary of Herradura Bay and Plain, I found the Concholepas and Turritella cingulata (mostly in fragments), at the height of 242 feet above the sea.I could not have told that these shells had not formerly been brought up by man, if I had not found one very small mass of them cemented together in a friable calcareous tuff.I mention this fact more particularly, because I carefully looked, in many apparently favourable spots, at lesser heights on the side of this ridge, and could not find even the smallest fragment of a shell.This is only one instance out of many, proving that the absence of sea-shells on the surface, though in many respects inexplicable, is an argument of very little weight in opposition to other evidence on the recent elevation of the land.The highest point in this neighbourhood at which I found upraised shells of existing species was on an inland calcareous plain, at the height of 252 feet above the sea.
It would appear from Mr.Caldcleugh's researches, that a rise has taken place here within the last century and a half ("Proceedings of the Geological Society" volume 2 page 446.); and as no sudden change of level has been observed during the not very severe earthquakes, which have occasionally occurred here, the rising has probably been slow, like that now, or quite lately, in progress at Chiloe and at Valparaiso: there are three well-known rocks, called the Pelicans, which in 1710, according to Feuillee, were a fleur d'eau, but now are said to stand twelve feet above low-water mark: the spring-tides rise here only five feet.There is another rock, now nine feet above high-water mark, which in the time of Frezier and Feuillee rose only five or six feet out of water.Mr.Caldcleugh, I may add, also shows (and I received similar accounts) that there has been a considerable decrease in the soundings during the last twelve years in the Bays of Coquimbo, Concepcion, Valparaiso, and Guasco; but as in these cases it is nearly impossible to distinguish between the accumulation of sediment and the upheavement of the bottom, I have not entered into any details.
VALLEY OF COQUIMBO.
(FIGURE 9.EAST AND WEST SECTION THROUGH THE TERRACES AT COQUIMBO, WHERETHEY DEBOUCH FROM THE VALLEY, AND FRONT THE SEA.
Vertical scale 1/10 of inch to 100 feet: horizontal scale much contracted.
Height of terrace in feet from east (high) to west (low):
Terrace F.364
Terrace E.302
Terrace D.shown dotted, height not given.
Terrace C.120
Terrace B.70
Terrace A.25 sloping down to level of sea at Town of Coquimbo.)The narrow coast-plain sends, as before stated, an arm, or more correctly a fringe, on both sides, but chiefly on the southern side, several miles up the valley.These fringes are worn into steps or terraces, which present a most remarkable appearance, and have been compared (though not very correctly) by Captain Basil Hall, to the parallel roads of Glen Roy in Scotland: their origin has been ably discussed by Mr.Lyell.("Principles of Geology" 1st edition volume 3 page 131.) The first section which I will give (Figure 9), is not drawn across the valley, but in an east and west line at its mouth, where the step-formed terraces debouch and present their very gently inclined surfaces towards the Pacific.
The bottom plain (A) is about a mile in width, and rises quite insensibly from the beach to a height of twenty-five feet at the foot of the next plain; it is sandy, and abundantly strewed with shells.
Plain or terrace B is of small extent, and is almost concealed by the houses of the town, as is likewise the escarpment of terrace C.On both sides of a ravine, two miles south of the town, there are two little terraces, one above the other, evidently corresponding with B and C; and on them marine remains of the species already enumerated were plentiful.
Terrace E is very narrow, but quite distinct and level; a little southward of the town there were traces of a terrace D intermediate between E and C.
Terrace F is part of the fringe-like plain, which stretches for the eleven miles along the coast; it is here composed of shingle, and is 100 feet higher than where composed of calcareous matter.This greater height is obviously due to the quantity of shingle, which at some former period has been brought down the great valley of Coquimbo.
Considering the many shells strewed over the terraces A, B, and C, and a few miles southward on the calcareous plain, which is continuously united with the upper step-like plain F, there cannot, I apprehend, be any doubt, that these six terraces have been formed by the action of the sea; and that their five escarpments mark so many periods of comparative rest in the elevatory movement, during which the sea wore into the land.The elevation between these periods may have been sudden and on AN AVERAGE not more than seventy-two feet each time, or it may have been gradual and insensibly slow.From the shells on the three lower terraces, and on the upper one, and I may add on the three gravel-capped terraces at Conchalee, being all littoral and sub-littoral species, and from the analogical facts given at Valparaiso, and lastly from the evidence of a slow rising lately or still in progress here, it appears to me far more probable that the movement has been slow.The existence of these successive escarpments, or old cliff-lines, is in another respect highly instructive, for they show periods of comparative rest in the elevatory movement, and of denudation, which would never even have been suspected from a close examination of many miles of coast southward of Coquimbo.
(FIGURE 10.NORTH AND SOUTH SECTION ACROSS THE VALLEY OF COQUIMBO.
>From north F (high) through E?, D, C, B, A (low), B?, C, D?, E, F (high).
Vertical scale 1/10 of inch to 100 feet: horizontal scale much contracted.