What is the main event in chapter 5 The toil of trace and trail

Hal and his friends know nothing about sledding or dogs. They mistreat the dogs and bungle the journey. As Hal beats Buck, John Thornton springs upon Hal and drives him back, telling him that he will kill him if he hits the dog one more time. Hal tries to retaliate and gets his knuckles slashed for his troubles. John cuts Buck's traces, and the team proceeds. A quarter of a mile away, the ice breaks and the sled goes down, taking humans and dogs with it.