(grammar help needed) How do I understand "upon which" in this context?
Rip’s story was soon told, for the whole twenty long years had been to him but as one night. The neighbors stared when they heard it; some were seen to wink at each other, and put their tongues in their cheeks: and the self-important man in the cocked hat, who, when the alarm was over, had returned to the field, screwed down the corners of his mouth, and shook his head—***upon which there was a general shaking of the head throughout the assemblage.***

What does "which" refer to here? and what is linked to "upon"? and what's the meaning of "upon which"? how do I interpret the whole sentence?

it's always hard to understand "upon". Thanks in advance.

Question

(grammar help needed) How do I understand "upon which" in this context?
Rip’s story was soon told, for the whole twenty long years had been to him but as one night. The neighbors stared when they heard it; some were seen to wink at each other, and put their tongues in their cheeks: and the self-important man in the cocked hat, who, when the alarm was over, had returned to the field, screwed down the corners of his mouth, and shook his head—***upon which there was a general shaking of the head throughout the assemblage.***

What does "which" refer to here? and what is linked to "upon"? and what's the meaning of "upon which"? how do I interpret the whole sentence?

it's always hard to understand "upon". Thanks in advance.

Answer

'upon which' is a concise way of saying, 'immediately after/following this action ...'.  
In this text, 'which' is referring back to 'shook his head'.

Paraphrasing, "After he shook his head there followed an immediate general shaking of peoples heads throughout the assemblage/group."

'upon which' also implies that this 'immediate action that followed' was a result of 'which' (shaking his head)

Answer

"Upon" has two uses:

(1) Indicate position: 
He put the flowers upon the table.

(2) Indicate that one action is linked to another closely in time (such as "during" or "immediately after"). 
Upon entry to the airplane, customers must show their passport.
Upon hearing the news, she fainted.

To paraphrase your example: 
When Rip tells his story, the neighbors stare at him and some don't believe it (they wink at each other). Another man returns to the field, and shakes his head--upon which all the neighbors (the assemblage) also shake their head.

In this sentence, "upon which" means "after which".

Answer

It means “during” the assemblage there was a great shaking of his head or “at that moment” (of the assemblage)