Their dinosaur fossils are primarily teeth recovered from microvertebrate fossil sites. Additional geologic formations that have produced only few fossils are the Belly River Group and St. Mary River Formations of the southwest and the northwestern Wapiti Formation. The Wapiti Formation contains two "Pachyrhinosaurus" bone beds that break its general trend of low productivity, however. The Bearpaw Formation represents strata deposited during a marine transgression. Dinosaurs are known from this formation, but represent specimens washed out to sea or reworked from older sediments. Paleo-Indians arrived in Alberta at least 10,000 years ago, toward the end of the last ice age. They are thought to have migrated from Siberia to Alaska on a land bridge across the Bering Strait and then possibly moved down the east side of the Rocky Mountains through Alberta to settle the Americas. Others may have migrated down the coast of British Columbia and then moved inland. Over time they differentiated into various First Nations peoples, including the Plains Indian tribes of southern Alberta such as those of the Blackfoot Confederacy and the Plains Cree, who generally lived by hunting buffalo, and the more northerly tribes such as the Woodland Cree and Chipewyan who hunted, trapped, and fished for a living. After the British arrival in Canada, approximately half of the province of Alberta, south of the Athabasca River drainage, became part of Rupert's Land which consisted of all land drained by rivers flowing into Hudson Bay. This area was granted by Charles II of England to the Hudson's Bay Company (HBC) in 1670, and rival fur trading companies were not allowed to trade in it. The Athabasca River and the rivers north of it were not in HBC territory because they drained into the Arctic Ocean instead of Hudson Bay, and they were prime habitat for fur-bearing animals. The first European explorer of the Athabasca region was Peter Pond, who learned of the Methye Portage, which allowed travel from southern rivers into the rivers north of Rupert's Land. Fur traders formed the North West Company (NWC) of Montreal to compete with the HBC in 1779. The NWC occupied the northern part of Alberta territory. Peter Pond built Fort Athabasca on Lac la Biche in 1778. Roderick Mackenzie built Fort Chipewyan on Lake Athabasca ten years later in 1788. His cousin, Sir Alexander Mackenzie, followed the North Saskatchewan River to its northernmost point near Edmonton, then setting northward on foot, trekked to the Athabasca River, which he followed to Lake Athabasca.