In Holes, what two important discoveries does Stanley make after tumbling into the gully?

After tumbling into the gully, Stanley finds water and an onion.
In chapter 38, Stanley and Zero continue climbing the mountain. During the exhausting climb, Stanley notices a large stone precipice ahead. He makes his way towards it but never reaches it. Instead, he falls into what appears to be a gully.
Inside the gully, Stanley notices that the ground is muddy. He is initially distressed at his predicament but soon cheers up. He concludes that there must be water in the gully, since water is needed to turn soil into mud.
So, Stanley begins digging into the soil with his hands. The more he digs, the more water fills up in the hole he has dug. Eventually, Stanley manages to dig a hole as deep as the length of his arm.
Both boys drink the water, and Stanley continues widening the hole. He soon discovers a round, smooth object hidden in the mud. When Stanley wipes the dirt off the object, he sees that it is an onion. Stanley then proceeds to eat half of the onion. He gives the other half to Zero.