I remember reading a time travel book when I was young, which means it would have been written no later than the '80s, but probably much before that.
It featured an adult and some kids back in time and them trying to get back to the present. I recall that the time travel effect was a ring, as at one point the ring opened up and a "brontosaurus" hit it and burned it's leg sending chunks of it back through the ring.
I recall a section where they were with "cave people" who had just hunted and killed some animal, and they used their pocket knife to extract more meat from the carcass, and actually cooking the meat unlike their paleolithic 'friends'. (I seem to recall that one of the paleolithic girls seemed to fancy the boy time traveler, much to his discomfort.)
I am pretty sure that at one point they were watching the time ring appear over water, knowing they could not reach it, and waiting for it to be reachable.
I think the close of the book was them freezing to death in the ice age, commenting on how in the future when their bodies are found how it would be hard to explain how they got there, and then finally being rescued before it was too late.
I hope that's enough to go on? Any ideas?