The geology of the Yukon Territory tells an amazing story of change and transition. There are many great museums and road signs along the highways which do a fantastic job of describing the geological features its historical and ecological significance.
Many scientists believe Yukon time began 250 million years ago. The rocks and sediments on the Whitehorse valley have been formed through the last 250 million years. They tell remarkable stories about tropical oceans and reefs, granite magmas and copper rich fluids deep in the Earth, lava flows, violent volcanic ash eruptions and great glaciations. Canyon Mountain, locally known as Grey Mountain, consists of limestone that formed as a reef in a tropical ocean. About 170 million years ago, the ocean basin closed, burying and squeezing the reef. Fossils of sponges and other reef animals were destroyed by the pressure and heat. In the recent geological past, water dissolved the limestone creating crevices and shallow caves.
The copper deposits of the Whitehorse Copper Belt occur at the boundary between granite and limestone. These deposits formed about 110 million years ago, during the age of dinosaurs. Granite magma moved upward through deeply buried crustal rocks, including layers of limestone. Copper-bearing fluids released from the crystalizing granite reacted vigorously with limestone, causing copper minerals to precipitate. Subsequent erosion of about 10 kilometres of overlying rocks has brought these copper deposits near the surface, where they can be mined.
Fast forward to 20,000 years ago. Ice sheets up to 2 kilometres thick covered southern Yukon Territory. Glacier ice flowed northward along the Yukon River valley, overtopping Mount McIntyre, Canyon Mountain and Golden Horn Mountain. Rock surfaces were scratched and scraped by stones frozen into the base of the glacier.
Five thousand years later the glaciers covering southern Yukon Territory thinned and retreated to the south. Less than 2000 years later, glacier ice began to disappear from the Yukon River valley at Whitehorse. North-flowing meltwater however, was dammed by a remnant mass of glacier ice near present-day Lake Laberge and a large glacial lake developed in the Takhini and Yukon River valleys. Streams flowing from melting ice transported large amounts of silt and sand into the lake. The silt deposits are exposed in the bluffs which border the Yukon River and surround downtown Whitehorse. They form the flat surface on which the Whitehorse airport is located.
Approximately 12000 years ago, the glacial lake drained and the Yukon River reclaimed the valley. The river eroded into the soft lake silt, creating the cliffs that border the downtown area. Downtown Whitehorse is built over sand and gravel that were deposited by the shifting channels and periodic floods of the Yukon River over thousands of years.
As the rivers erode the banks, these layers are exposed helping geologists and archaeologists piece together the stories of the past.
Questions: To log this cache go to the posted coordinates and answer the following questions from the signage. Please take the time to read all the signs and enjoy the great views.
1. After topsoil, which layer is relatively small, and often contains bones?
2. A tephra is a layer of volcanic ash and because of its size, is often easy to recognize and date. Which tephra is above the paleosol?
3. Which layer is made up of light-coloured, mineral-rich particles blown off streambeds left by retreating glaciers?
4. Optional: Take a picture of some of the great landscapes near the sign. Post a note and wait for our affirmative reply to log as found. We will try to be quick.