Canadian Shield rock forms the nucleus of the North American continent. Other geological structures assumed positions around or on top of the Shield millions of years after it was formed. The Rockies are relative newcomers on the geological stage, having risen a mere 60 million years ago. Most Shield rocks were formed well over a billion years earlier, during the very first chapter of the planet's history known as the Precambrian era.
What once may have been a towering mountain chain is today a massive rolling plain of ancient bedrock. During the late Precambrian era, violent spasms in the Earth’s crust warped, folded and faulted the Shield. The foundation of much of the ecozone is now metamorphic gneiss, a highly banded rock formed by intense pressure and heat. Many of the minerals that contribute to the Boreal Shield’s economy may have formed during these geologically turbulent times.
During the last ice age that ended 10 000 years ago, the advance of glaciers repeatedly plucked and scoured the Shield, carving striations in the bedrock and carrying large boulders many kilometres. In retreat, glaciers blanketed much of the landscape with gravel, sand and other glacial deposits. The many poorly drained depressions left behind, as well as natural faults in the bedrock, now form the millions of lakes, ponds and wetlands that give this ecozone its distinctive character and charm.
The climate of the Boreal Shield is generally continental with long cold winters and short warm summers. Cold air masses over Hudson Bay bring relatively high levels of precipitation to much of the area, from 400 mm in the west to 1 000 mm in the east. The average midwinter temperature is -15°C, while in midsummer it hovers around 17°C. The typical year sees between 60 and 100 frost-free days. Regions bordering the Great Lakes and the Atlantic tend to be warmer in winter and cooler in summer thanks to the moderating influence of large water bodies.