The Canadian Shield's massive rolling hills of ancient bedrock cover almost two-thirds of Canada. As monolithic as the Shield may seem, it is actually made up of seven distinct geological "provinces." The world's oldest rocks are found on the Taiga Shield in the Slave Geological Province north of Great Slave Lake. They were formed near the dawn of the Earth's geological history 4 billion years ago.

During the Precambrian Era, Shield rocks were warped, folded, and faulted by violent spasms in the Earth's crust. Since their birth, relentless weathering and erosion from countless rainstorms, rivers, floods, and the annual freezing and thawing cycle have worn down the rocks. In places, the Shield was repeatedly plucked and scoured by the advance of glaciers, leaving areas now frequently infilled as lakes. Elsewhere it was blanketed by boulders, gravel, and sand released by glaciers in retreat.

This story of geological creation and change is plainly recorded in exposed bedrock and surface deposits of the Taiga Shield. Volcanic rocks testify to the earliest eruptions of lava that created the Earth's crust as we know it. Some of these eruptions occurred under extreme water pressure at the bottom of ancient seas, creating globular "pillows" of lava. Vast areas of granite, once buried deep beneath the rock surface, reveal the power of billions of years of erosion. And the passage of mighty glaciers can be read by recognizing the striations and grooves etched in the rocks. Soils are usually shallow and coarse and sometimes laced with patterns caused by the presence of permafrost.

The Taiga Shield Ecozone experiences considerable variation in daylight over the course of a year. Areas north of the Arctic Circle endure at least one day in which the sun never rises and at least one in which it never sets.

The subarctic climate is characterized by short, cool summers and long, cold winters. Precipitation is low to moderate, averaging 250 to 500 mm a year across much of the ecozone. On the Labrador coast, it ranges up to 800 mm annually. Snow and freshwater ice-cover persist for six to eight months. Dams and diversions have changed seasonal patterns of flow on several rivers in the eastern Taiga Shield.