Why Past Perfect?

Hello, everybody!

Could you please help me understand why the Past Perfect was used in the following passage?

«Under the violent Provencal sun, the elms and beeches looked exotic trees, and in the early morning, when the mists were thick, the hills _had put on_ an unearthly shape.»

Thank you in advance.

Thanks you, Jim!

Here goes the whole paragraph:

«Day after day, through all that August, morning and evening were wrapped
in haze; day after day the earth shimmered in the heat, and the air was
strange, unfamiliar. As he wandered in the lanes and sauntered by the
cool sweet verge of the woods, he saw and felt that nothing was common or
accustomed, for the sunlight transfigured the meadows and changed all the
form of the earth. Under the violent Provencal sun, the elms and beeches
looked exotic trees, and in the early morning, when the mists were thick,
the hills had put on an unearthly shape.»

Maybe it'll help you to help me.

Thank you, CJ.

Well, I perceive it this way: put on the garment of mist thus assuming a strange shape. Since the mist sort of envelops the hills, "put on" seems ok to me.

P.S.: That's from Arthur Machen's "The Hill of Dreams".

Nothing supernatural. I was scanning Wikipedia for info on the Hermetic tradition, met a section called "Hermetic art in popular culture and entertaniment" (usually I don't read such sections) which mentioned Lovecraft, and decided to have a look... In Lovecraft's "The Call of Cthulhu" there was a reference to Machen.

To me, Machen is harder to read than Lovecraft.