From late 1852 until January 1855, Allen remained in Washington Territory. In June 1853 he surveyed the Indian trail over Naches Pass in the Cascade Mountains as a potential route for a wagon road to divert incoming emigrants from the Willamette Valley to Puget Sound. From July through October he led a construction crew that began building the wagon road. That winter he explored Puget Sound to its northern reaches by whaleboat, ran for a position in the territorial senate (he lost), and shared his cabin with Captain George B. McClellan and George Gibbs (geologist), both on assignment to conduct various surveys of the newly created Washington Territory. Allen wrote up their reports and also created a dictionary of the Chinook Jargon at least as comprehensive as the better-known Gibbs dictionary. (This unpublished dictionary may be found in Hervey Allen Papers, Special Collections Department, University of Pittsburgh.) In 1854 Allen and his crew returned to the mountains to improve the wagon road. He took time out from his road building to make the first ascent of Mount Adams.