Last fall several geologists went hunting behind the Riverdale subdivision in Whitehorse. Even though they had a shotgun along, they were not looking for anything to put on the dinner table; instead the group was stalking evidence buried deep beneath the earth.

The geologists would fire the shotgun straight down into the ground, setting off sound waves that bounced off of the layers of rock and soil below. When these waves reflected back towards the surface, they were picked up by a series of small microphones called geophones.

By recording the speed and direction of the sound waves, this seismic unit would help produce a picture of the underground world. With luck the sound waves would bounce off of bedrock and perhaps even indicate the presence of an ancient river channel called the Chadburn River Valley.

Forest Pearson, a geological engineering consultant, thinks that the Yukon River flowed through this channel until a lava flow filled it and forced the river to cut a new course to the east.

The basalt flows that once poured out of the slopes near the Mount Sima Road had recently been dated at 8.4 million years old. The molten lava flowed into the ancient Yukon River valley, spreading as far north as the downtown area.

The basalt is most dramatically visible in the walls of Miles Canyon, and also forms ledges along the river below the Whitehorse Dam. But most of the basalt is buried under thick layers of sand and gravel left behind by the glaciers that once covered this area.

The geologists had been given permission to use a firearm within city limits partly because applied and pure science meshed on this project. The City of Whitehorse has warm water wells in the area, and hoped the shotgun blasts might produce some useful information on groundwater there.

In winter, groundwater drawn from wells in the Riverdale area is combined with the frigid water from Schwatka Lake, the source of most of the City's water. The warmer groundwater raises the overall temperature of the water supply, thus reducing heating costs and the possibility of frozen water mains.

"The water is relatively warm because it is underground for about ten years," says Pearson. This is the estimated amount of time that it takes a drop of water to travel underground from the Hidden Lakes area north to where it is sucked up by the Riverdale wells.

The City wants to know how much groundwater it can pump from the wells each winter without removing water faster than it is naturally replaced. It also must guard against depleting the groundwater to the point that cold water from the Yukon River would be drawn up.

Two wells drilled by the City in the late 1970s first hinted at the existence of the Chadburn Buried Valley. A well drilled near the Chadburn Lake Road hit basalt, but no basalt was found when another well was drilled near the north end of Hidden Lake. Basalt is formed when lava cools and hardens into rock.

"Lava, when it comes out of the ground, can flow like water. It will seek its own level just like water," explains Pearson. "So when basalt filled the valley there should not have been any low spots."

When the wells showed that the basalt did not form an even layer across the valley, the geologists had to enter enter the realm of "geofantasy" to find answers.

"You look for other possible mechanisms for why this could be. The best we can come up with is that the basalt flows were like corn syrup plugging the valley. The river backed up and cut a new channel to the east," says Pearson.

In the Whitehorse area, it is not easy figuring out what lies underground. Millions of years after the lava had hardened into basalt, massive sheets of ice ground their way through the area, advancing and retreating on at least four different occasions, and leaving behind immense loads of gravel and other rocky debris.

Drilling, the best way to find out what lies underground, is very expensive. The shotgun experiment, a low cost alternative, was not completely successful as the shotgun blast was not loud enough to send sound waves all of the way down to bedrock. But the experiment did provide information on the upper surface of the basalt.

Other evidence for the Chadburn Buried Valley can be seen on the surface. "When you look at a map, you see that Chadburn Lake, Chadden Lake, Hidden Lake and the Riverdale valley all line up," says Pearson. He thinks this chain of lakes shows the channel through which the ancient river once flowed.

He thinks that huge blocks of ice left behind by the retreating glaciers formed the lakes. "The ice blocks sat there and got covered in gravel. The ice stagnated and melted and that is why you have the kettle topography in the Chadburn and Long Lake areas," he explains. A kettle refers to a lake formed where glacial ice has melted in place.

The current shape of the Yukon River Valley was formed when the last melting glaciers formed an immense body of water named Glacial Lake Champagne, which filled many of the big valleys in the southern Yukon. It drained suddenly when an ice dam at the head of the lake gave way, releasing a torrent of water that sliced through the soft sediments of the lake bottom.

For more information on the Chadburn Buried Valley, contact Forest Pearson at 633-6474.