Mount Churchill is most noteworthy as the source of the White River Ash, deposited during two of the largest volcanic eruptions in North America during the past two millennia. This twin-lobed tephra deposit covers more than of eastern Alaska and northwestern Canada, with the northern lobe deposited 1900 years ago and extending over and the larger eastern lobe about 1,250 years ago and stretching over . The total volume of the ash exceeds , or roughly 50 times the volume of the 1980 eruption of Mount St. Helens, and ash layers up to thick can be seen just below the surface in many roadcuts along the Alaska Highway.