A synclinal trough or valley; a synclinal fold.
These valleys are not synclinal foldings of rocks; they seem to be erosion-valleys.
These bands are often concealed by more recent deposits, but it is clear that in this region the Devonian beds form a basin or synclinal with the Amazon for its axis.
In the basin of the Lower Amazon the Carboniferous beds lie within the Devonian synclinal and crop out on both sides of the river next to the Devonian bands.
The western half of the basin occupies a trough of synclinal structure; but the making of this syndine is so ancient that it cannot be directly connected with the occurrence of the lake to-day.