The plain at the head of the valley is tolerably level, but water-worn, and with many sand-dunes on it like those on a sea-coast.At the highest point to which we ascended, it was sixteen miles wide in a north and south line;and forty-five miles in length in an east and west line.It is bordered by the escarpments, one above the other, of two plains, which diverge as they approach the Cordillera, and consequently resemble, at two levels, the shores of great bays facing the mountains; and these mountains are breached in front of the lower plain by a remarkable gap.The valley, therefore, of the Santa Cruz consists of a straight broad cut, about ninety miles in length, bordered by gravel-capped terraces and plains, the escarpments of which at both ends diverge or expand, one over the other, after the manner of the shores of great bays.Bearing in mind this peculiar form of the land--the sand-dunes on the plain at the head of the valley--the gap in the Cordillera, in front of it--the presence in two places of very ancient shells of existing species--and lastly, the circumstance of the 355-453feet plain, with the numerous marine remains on its surface, sweeping from the Atlantic coast, far up the valley, I think we must admit, that within the recent period, the course of the Santa Cruz formed a sea-strait intersecting the continent.At this period, the southern part of South America consisted of an archipelago of islands 360 miles in a north and south line.We shall presently see, that two other straits also, since closed, then cut through Tierra del Fuego; I may add, that one of them must at that time have expanded at the foot of the Cordillera into a great bay (now Otway Water) like that which formerly covered the 440 feet plain at the head of the Santa Cruz.
(DIAGRAM 6.NORTH AND SOUTH SECTION ACROSS THE TERRACES BOUNDING THE VALLEYOF THE RIVER SANTA CRUZ, HIGH UP ITS COURSE.
The height of each terrace, above the level of the river (furthest to nearest to the river) in feet:
A, north and south: 1,122
B, north and south: 869
C, north and south: 639
D, north: not measured.D, north? (suggest south): 185E: 20
Bed of River.
Vertical scale 1/20 of inch to 100 feet; but terrace E, being only twenty feet above the river, has necessarily been raised.The horizontal distances much contracted; the distance from the edge of A North to A South being on an average from seven to ten miles.)I have said that the valley in its whole course is bordered by gravel-capped plains.The section (Diagram 6), supposed to be drawn in a north and south line across the valley, can scarcely be considered as more than illustrative; for during our hurried ascent it was impossible to measure all the plains at any one place.At a point nearly midway between the Cordillera and the Atlantic, I found the plain (A north) 1,122 feet above the river; all the lower plains on this side were here united into one great broken cliff: at a point sixteen miles lower down the stream, I found by measurement and estimation that B (north) was 869 above the river: very near to where A (north) was measured, C (north) was 639 above the same level: the terrace D (north) was nowhere measured: the lowest E (north) was in many places about twenty feet above the river.These plains or terraces were best developed where the valley was widest; the whole five, like gigantic steps, occurred together only at a few points.The lower terraces are less continuous than the higher ones, and appear to be entirely lost in the upper third of the valley.Terrace C (south), however was traced continuously for a great distance.The terrace B (north), at a point fifty-five miles from the mouth of the river, was four miles in width; higher up the valley this terrace (or at least the second highest one, for I could not always trace it continuously) was about eight miles wide.This second plain was generally wider than the lower ones--as indeed follows from the valley from A (north) to A (south) being generally nearly double the width of from B (north) to B (south).Low down the valley, the summit-plain A(south) is continuous with the 840 feet plain on the coast, but it is soon lost or unites with the escarpment of B (south).The corresponding plain A(north), on the north side of the valley, appears to range continuously from the Cordillera to the head of the present estuary of the Santa Cruz, where it trends northward towards Port St.Julian.Near the Cordillera the summit-plain on both sides of the valley is between 3,200 and 3,300 feet in height; at 100 miles from the Atlantic, it is 1,416 feet, and on the coast 840 feet, all above the sea-beach; so that in a distance of 100 miles the plain rises 576 feet, and much more rapidly near to the Cordillera.The lower terraces B and C also appear to rise as they run up the valley; thus D (north), measured at two points twenty-four miles apart, was found to have risen 185 feet.From several reasons I suspect, that this gradual inclination of the plains up the valley, has been chiefly caused by the elevation of the continent in mass, having been the greater the nearer to the Cordillera.