That stands for the Mendip Hills to the west, which are made of old red sandstone, very much the same rock (to speak roughly) as the Kerry mountains.
And why are the lines in it twisted?
To show that the strata, the layers in it, are twisted, and set up at quite different angles from the limestone.
But how was that done?
By old earthquakes and changes which happened in old worlds, ages on ages since.

Then the edges of the old red sandstone were eaten away by the sea--and some think by ice too, in some earlier age of ice; and then the limestone coral reef was laid down on them, "unconformably," as geologists say--just as you saw the new red sandstone laid down on the edges of the limestone; and so one world is built up on the edge of another world, out of its scraps and ruins.
Then do you see B.

With a notch in it?
That means these limestone hills on the shoulder of the Mendips; and that notch is the gorge of the Avon which we have steamed through.
And what is that black above it?
That is the coal, a few miles off, marked C.
And what is this D, which comes next?
That is what we are on now.

New red sandstone, lying unconformably on the coal.

I showed it you in the bed of the river, as we came along in the cab.