CHAPTER IV 
Submerged River-Valleys off the Coast of Portugal 
EXPLANATION TO ACCOMPANY PLATE IV 
‘HE Continental Platform off the coast of Portugal is throughout its length comparatively 
narrow, resembling that of the coast of Spain along the southern margin of the Bay of Biscay, 
with which it is continuous. It seldom exceeds 30 miles in width ; and its descent into the 
abyss is generally steep, descending from 120 to 1200 fathoms (720-7200 feet) in a short space, 
and from this going down to 2000 fathoms on the ocean floor. 
It is incised, however, by several well-defined cafions ; most of which are connected with the rivers 
now entering the ocean at the borders of the land; but owing to the narrowness of the platform, and the 
consequent silting up of the channels within the limited distance, there is much difficulty in tracing 
the connection between the existing rivers and the channels by which they entered the ocean. : 
The Arosa.—In the description of some of these streams we may commence with the Arosa, near the 
margin of Spain and Portugal. This river probably formed a junction with the river Lerezo Veda before 
entering the head of the deep and wide ravine which descends from the Continental Platform at the 
200-fathom contour, and opens out on the abyssal ocean at about the 1000-fathom line after a course of 
35 miles, in long. 9° 35’ W. and lat. 42° 35’ N. The form of the isobaths indicates the occurrence 
of wide platforms bounded by cliffs on either side of the cafion. 
The Lima.—The platform at this part of the coast being narrow, the head of this gorge (long. 
g° 5’ W.) approaches to within 20 miles of the mouth of the river Lima at Viana. At its upper part 
the gorge is narrow, and descends within a distance of ro miles to a depth of 1137 fathoms (6822 feet) 
below the surface of the ocean. Absence of soundings prevents any attempt to connect the outlet with 
the river itself across the plateau ; but there can be little doubt of the continuity of the channel. 
The Douro.—Proceeding southwards along the coast, the next important river is the Douro, which 
has its source in the Sierra de Gata and enters the ocean at Oporto. The inward bend of the 200-fathom 
contour for a distance of 8 or 10 miles opposite the mouth of this river leaves no doubt that we are here 
in the presence of its submerged channel; and the curves of the 50- and 100-fathom contours enable us 
to trace the channel across the platform to a distance of only 14 miles from the mouth of the river itself 
below Oporto. Owing, however, to the absence of soundings in the deeper portions of the submerged bay, 
the exact form of this part of the channel cannot be determined ; but sufficient remain to show that the 
river formerly entered the outer ocean through a wide bay and with rapid descent at a depth of about 
1500 fathoms. 
The Mondego.—The submerged mouth of this river is the most remarkable of any off the coast of 
Portugal for the precipitate form of descent from the margin of the platform to its base on the edge of 
the abyssal ocean. For, within a distance of about 8 miles, the channel descends from the 100-fathom 
contour to that of 1200 fathoms, which works out at 6600 feet; with a descent of 825 feet per mile. 
The walls on either side rise almost vertically several thousand feet, and enclose an exceedingly narrow 
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