In The Chrysalids, what is the plot and setting of Chapter 16?

David and the other telepaths, including Rosalind, fled to the Fringes once they were discovered after the incident with Petra’s horse.  She called to them and they came without her making any noise, causing suspicion.  They end up in a cave, where David has to deal with the meeting between his girlfriend Rosalind and his former girlfriend Sophie. 

David and Sophie were young and when they were friends. Then her extra toes were discovered and she was sterilized and sent to the Fringes society. Sophie and Rosalind are clearly jealous of one another. 

Sophie's eyes travelled over Rosalind, in her russet woollen dress with its brown cross applique, and rested for a moment on her leather shoes. She looked down at her own soft moccasins, then at her short, tattered skirt. (Ch. 16) 

Sophie tells Rosalind that she has to remove the cross that is sewn to her bodice.  The cross is required of all women in Waknuk, a strictly religious society, but is a vestige of the restrictive society they have left.  Sophie tells Rosalind that the men resent the cross, because the women are sterilized before being sent to the Fringes. 

Rosalind tries to treat Sophie with caring and compassion.  However, Sophie resents Rosalind.  When she tries to help Sophie, Sophie lashes out at Rosalind while David looks helplessly on, waiting to see what will happen. 

'Damn you!' she said again. 'Go on, laugh at me, God damn your lovely face. Laugh at me because I do want him, me!' She gave a queer, choked laugh herself. 'And what's the use? Oh, God, what's the use? If he weren't in love with you, what good would I be to him — like this?'  (Ch. 16) 

In the end, it passes because David and Rosalind are on the outside too now.  Since they have been exposed as telephaths, they are also deviants.  They can’t return.  The problem is that they ran away before they could be caught, so unlike all of the other women there Rosalind is intact.  She has not been sterilized.  Seeing her, Sophie imagines what she might have had.  To make matters stranger, Sophie is with Spider, who is the leader of the Fringe group and David's uncle.  His arms were too long.

David awakens, and communicates telepathically with Michael.  They are being chased by Waknuk hunting parties.  Petra, meanwhile, is communicating with distant Sealand, where someone is coming to rescue them.  There, telepaths are not considered deviants.

Before the Sealanders can rescue them, they have to battle the religious fanatics of Waknuk, including David and Petra's father.  The Fringe people fight back.  Spider, David's uncle, shoots David's father.  Sophie is killed in the chaos.  The Sealanders land with their "gleaming white fish-shape" and rescue the telepaths.