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. We are here in a sort of amphitheatre, or half a one, with the limestone hills around us, and the new red sandstone plastered on, as it were, round the bottom of it inside.
But what is this high bit with E against it?
Those are the high hills round Bath, which we shall run through soon. They are newer than the soil here; and they are (for an exception) higher too; for they are so much harder than the soil here, that the sea has not eaten them away, as it has all the lowlands from Bristol right into the Somersetshire flats.
* * *
There. We are off at last, and going to run home to Reading, through one of the loveliest lines (as I think) of old England.
And between the intervals of eating fruit, we will geologize on the way home, with this little bit of paper to show us where we are.
What pretty rocks!
Yes. They are a boss of the coal measures, I believe, shoved up with the lias, the lias lying round them. But I warn you I may not be quite right: because I never looked at a geological map of this part of the line, and have learnt what I know, just as I want you to learn simply by looking out of the carriage window.