You think, perhaps, that an earthquake opened it?
My dear child, we must look before we guess.

Then, after we have looked a little, and got some grounds for guessing, then we may guess.

And you have no ground for supposing there ever was an earthquake here strong enough to open that glen.

There may have been one: but we must guess from what we do know, and not from what we do not.
Guess again.

Perhaps it was there always, from the beginning of the world?
My dear child, you have no proof of that either.