I took archaeology as an elective for my Associates Degree in Oregon. The Professor was a very entertaining guy and was very blunt in talking about the reality of archaeology in Oregon. He stated point blank that, “there is no way of knowing when humans first inhabited North America, because all of their first settlements are under the ocean. During the ice ages they followed the coastline. That’s all well under the ocean now. During that class I think I remember the oldest artifact in Oregon was a woven grass sandal from 14,000 years ago near Fort Rock. The class was from nearly 30 years ago so excuse my memory if I got that wrong.
That’s definitely a factor, and not just in the Americas — during much of the past 2 million years the continental shelf as we know it today was at least partially above sealevels due to so much of the waters of the oceans being locked up in glaciers. Kudos to your prof.