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 355-453
feet 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 VALLEY
OF THE RIVER SANTA CRUZ, HIGH UP ITS COURSE.