Formation

Canadian Shield

Canadian Shield

The Canadian Shield, also called the Laurentian Plateau, or Bouclier Canadien , is a vast geological shield covered by a thin layer of soil that forms the nucleus of the North American or Laurentia craton. It is an area mostly composed of igneous rock which relates to its long volcanic history...

 rock forms the nucleus of the North American continent. Other geological

Geology

Geology is the science comprising the study of solid Earth, the rocks of which it is composed, and the processes by which it evolves. Geology gives insight into the history of the Earth, as it provides the primary evidence for plate tectonics, the evolutionary history of life, and past climates...

 structures assumed positions around or on top of the Shield millions of years after it was formed. The Rocky Mountains

Rocky Mountains

The Rocky Mountains are a major mountain range in western North America. The Rocky Mountains stretch more than from the northernmost part of British Columbia, in western Canada, to New Mexico, in the southwestern United States...

 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 great rolling plain of ancient bedrock

Bedrock

In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...

. During the late Precambrian era, fierce convulsions in the Earth’s crust

Crust (geology)

In geology, the crust is the outermost solid shell of a rocky planet or natural satellite, which is chemically distinct from the underlying mantle...

 resulted in a warped, collapsed Shield. The foundation of much of the ecozone is now metamorphic gneiss

Gneiss

Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...

, a highly banded rock formed by intense pressure and heat. Many of the minerals that contribute to the Shield’s economy may have formed during these geologically turbulent times.

During the last ice age

Ice age

An ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...

, which ended 10 000 years ago, the advance of glacier

Glacier

A glacier is a large persistent body of ice that forms where the accumulation of snow exceeds its ablation over many years, often centuries. At least 0.1 km² in area and 50 m thick, but often much larger, a glacier slowly deforms and flows due to stresses induced by its weight...

s continuously plucked and scoured the Boreal Shield, forming striation

Striation

Striations means a series of ridges, furrows or linear marks, and are used in several ways* Glacial striation* Striation , a striation as a result of a geological fault* In medicine, striated muscle...

s in the bedrock and carrying large boulders many kilometers. In retreat, massive glaciers enveloped most of the landscape with great amounts of glacial deposits including gravel, sand, shale, and numerous sediments. The vast majority of inadequately drained depressions that were 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.

Changing conditions

As for the boreal forest, fire suppression, insect control, clear-cutting and single-species tree farming are widespread. These practices may, over the long term, reduce the diversity of both plant and animal species and increase the forest’s vulnerability to disease.

Many Shield lakes and soils are extremely sensitive to changes in pH

PH

In chemistry, pH is a measure of the acidity or basicity of an aqueous solution. Pure water is said to be neutral, with a pH close to 7.0 at . Solutions with a pH less than 7 are said to be acidic and solutions with a pH greater than 7 are basic or alkaline...

. Acid rain

Acid rain

Acid rain is a rain or any other form of precipitation that is unusually acidic, meaning that it possesses elevated levels of hydrogen ions . It can have harmful effects on plants, aquatic animals, and infrastructure. Acid rain is caused by emissions of carbon dioxide, sulfur dioxide and nitrogen...

from local sources and from the long-range transport of airborne pollutants has already taken an ominous toll. It may be weakening the general vigour and growth rate of trees, as well as of aquatic species, in sensitive areas.

External links

• Boreal Shield (The Canadian Atlas)