when did the ice-free corridor open in north america allowing one path for human migration from asia
Settlement of the Americas The onset of the Last Glacial Maximum after 30,000 years BP saw the expansion of alpine glaciers and continental ice sheets that blocked migration routes out of Beringia. By 21,000 years BP, and possibly thousands of years earlier, the Cordilleran and Laurentide ice sheets coalesced east of the Rocky Mountains, closing off a potential migration route into the center of North America.[8][9][10] Alpine glaciers in the coastal ranges and the Alaskan Peninsula isolated the interior of Beringia from the Pacific coast. Coastal alpine glaciers and lobes of Cordilleran ice coalesced into piedmont glaciers that covered large stretches of the coastline as far south as Vancouver Island and formed an ice lobe across the Straits of Juan de Fuca by 15,000 14C years BP (18,000 cal years BP).[11][12] Coastal alpine glaciers started to retreat around 19,000 cal years BP [13] while Cordilleran ice continued advancing in the Puget lowlands up to 14,000 14C years BP (16,800 cal years BP)[12] Even during the maximum extent of coastal ice, unglaciated refugia persisted on present-day islands, that supported terrestrial and marine mammals.[10] As deglaciation occurred, refugia expanded until the coast became ice-free by 15,000 cal years BP.[10] The retreat of glaciers on the Alaskan Peninsula provided access from Beringia to the Pacific coast by around 17,000 cal years BP.[14] The ice barrier between interior Alaska and the Pacific coast broke up starting around 13,500 14C years (16,200 cal years) BP.[11] The ice-free corridor to the interior of North America opened between 13,000 and 12,000 cal years BP.[8][9][10] Glaciation in eastern Siberia during the LGM was limited to alpine and valley glaciers in mountain ranges and did not block access between Siberia and Beringia.[7]
Northern Pacific Railway In 1873, Northern Pacific made impressive strides before a terrible stumble. Rails from the east reached the Missouri River on June 4. After several years of study, Tacoma, Washington, was selected as the road's western terminus on July 14. However, for the past three years the financial house of Jay Cooke and Company had been throwing money into the construction of the Northern Pacific. As with many western transcontinentals, the staggering costs of building a railroad into a vast wilderness had been drastically underestimated.