It also drove me crazy that Gavin was in such a hurry to jump down the sinkhole, that he didn't bother to get any supplies. They didn't say anything about the portal closing soon, so they should have just taken 5 minutes to gather a tent, sleeping bags, food, clothes, matches, a knife, weapons... from the surrounding area.
I was wondering that too. Sea levels were lower back then, so if anything, it would have made more sense for the 2021 sink hole to be by the ocean, and the 10,000 BC one to be in the woods. Maybe it's just confirmation that the portals can dump you out anywhere.