The Yukon has two major geological components. The area mainly northeast of Tintina Trench formed the west coast of ancient North America until about 200 million years ago. The remaining half of the Yukon originated elsewhere and was transported, via plate tectonics, to its present position. These transported Yukon terranes can be divided into about ten large blocks, each of which has distinct geological features. You are standing in what geologists call the Whitehorse Trough.
This area consists of 200-to 170-million-year-old sedimentary and volcanic rocks that formed in a deep-sea basin and adjacent chain of volcanic islands. The rounded conglomerate boulders making up Conglomerate Mountain above you came from the now-eroded volcanic islands laying to the west, and were deposited as gravelly rivers and fan-deltas that developed toward the east. Similar conglomerate deposits extend from south of Atlin, British Columbia to north of Carmacks, indicating that the Whitehorse Trough basin was at least 600 km long. Fossils in the rocks show that this basin was covered by sea water until about 170 million years ago.
The layer of white ash in this rest stop was laid down during the eruption of Mount Churchill, at the headwaters of the White River near the Yukon/Alaska border, over 1,100 years ago.