WT's are very often misleading in their terminology. Much as I hate the term "thingy", it is better than calling an object something it isn't.
In this case, there isn't a grain of sand anywhere near your "sandpit". Actually, that is a small pool formed into the rock. There is a valve-handle close-by that lets you fill it with water. The water shows a silhouette and the marble is the creature's eye.
In your WT with all the numbers, it is #34. That map doesn't help much but maybe, between the 2 WT's, you can find your way there.
Here's more from the Official Guide. (Continuing along the path after the sunning creatures.)
"There was a bounce in my step as I returned to the path and followed it deep into the rock.
. . .
"I emerged from a long tunnel to find myself on a walkway that I had observed from the Submarine Plateau. There was a guard tower straight ahead of me, but I saw no sign of the guard. From the end of the walkway, I had a clear view of the crown-topped suspension platform (Whark gallows).
. . .
"To my left I could see across to Submarine Plateau, but there was a wide chasm between me and the waiting machine. I followed a ladder down to a ledge with a small circular pool, which was completely dry. I could see one pipe leading into the pool, as well as several possible drainage holes. There was also another Eye."