What leads Nelly to visit the Heights in Chapter 11? What does she encounter when she arrives there?

One day, Nelly is on her way to Gimmerton when she is seized with an urge to visit the Heights; she is suddenly afraid that something is wrong with little Hareton. When Nelly reaches the gate, she sees that the boy is physically fine, but mentally and emotionally stunted; he does not remember her and throws a rock at her.