In chapter 39, Stanley awakes in a meadow. Towering over him is the rock formation known as God's Thumb, or Big Thumb, so called because it looks like a closed fist with the thumb pointing upwards. Stanley looks up at the massive rock formation and feels weak. He thinks to himself that "the insides of his mouth and throat were coated with sand." He then rolls over and sees the water hole from which he and Zero drank in the previous chapter. At the bottom of this hole Stanley can see "two inches of very brown water." The color of the water indicates that the water is not clean, but rather full or mud or sand, or both. The inside of Stanley's mouth therefore feels as if it is coated in sand partly because it probably is, and partly simply because his mouth is dry because he hasn't had a drink of clean water for so long.
Stanley's thirst, and thus the dry feeling inside his mouth, is also exacerbated by the physical exertion he endured on the previous night, when he carried Zero up the mountain. To combat his thirst, Stanley thinks about going back down the mountain to retrieve the shovel he left there. He can then use this shovel to dig deeper in the water hole. Stanley hopes that if he digs deeper, the water might become cleaner. Stanley, however, doubts whether he has the strength to go down the mountain and then come back up again, and he probably knows that the effort of this journey would only make him even more thirsty and his mouth even more dry.