The strata must once have risen above the present surface, and in many cases may be found descending to the surface again with a contrary dip, the intervening portion of the undulation having been worn away.
Climatic agencies have smoothed and modified everything rugged or abrupt, until an impression of gentle undulation rather than of grandeur is suggested.
North of the Congo basin and separated from it by a broad undulation of the surface is the basin of Lake Chad - a flat-shored, shallow lake filled principally by the Shari coming from the south-east.
Who knows but if our instruments were delicate enough we might detect an undulation in the crust of the earth?
Several pretty large logs may still be seen lying on the bottom, where, owing to the undulation of the surface, they look like huge water snakes in motion.