I’m not sure, but I think Bilbo Baggins mentioned this spot in his memoirs.
Actually, it’s the remains of an old mill sitting on the bank of the Ribeira do Vascão, right on the border between the Algarve and the Alentejo. If you look carefully, you can see one whole and one partial mill stone amongst the rocks at lower left.
I took this photo on the way to Mértola a couple of weeks ago, and yes, the grass really was that green. Another reason to love the Algarve (or Alentejo) in winter-slash-spring…
(Click the image to biggify.)