During the period from 1906 to 1914, Leffingwell spent nine summers and six winters on the Arctic coast of Alaska, making 31 trips by dog sled or small boats.

Leffingwell's father, who had become wealthy from his ownership of a fruit ranch in California, contributed $5000, and Mikkelsen raised a comparable amount in England and New York.

Their ship, the Duchess of Bedford, was locked in pack ice and destroyed, but they salvaged the wood to build a cabin which Leffingwell used intermittently through 1914.

He returned to the North Slope 1909–1912 and 1913–1914, working with one assistant to map 250 km of the arctic coast, and the Canning River valley .

[1][5][11] After spending a year and a half writing up his results at the United States Geological Survey in Washington, Leffingwell retired to Whittier, California, listing his occupation in 1917 as citriculturist.

[1][5][6] The Leffingwell Camp Site located on a remote barrier island off Alaska, was declared a National Historic Landmark in 1978.