A distinctive and widespread tephra bed is a very useful stratigraphic tool, especially if its age is accurately and precisely known. However, distal tephra beds, like those in the Klondike region of the Yukon, are difficult to date because of their fine grain size and contaminated character. Grain-specific methods, such as the fission-track and 40 Ar/39 Ar techniques, are essential for reliable results.
Three rhyolitic tephra beds, each with a homogeneous glass composition, have been dated by the glass-fission-track method. Mosquito Gulch tephra is 1.42 ± 0.16 Ma and dates an intermediate-level terrace on Bonanza Creek. Quartz Creek tephra is 3.00 ± 0.33 Ma and confirms a late Pliocene age for the upper part of the White Channel gravel. Furthermore, the presence of the Quartz Creek tephra in an ice-wedge cast indicates that permafrost conditions must have existed in west-central Yukon at this time. The Jackson Hill tephra is 0.13 ± 0.03 Ma.