It's all about time

The Earth formed about 4.6 billion years ago.  Geologists have broken this enormous length of time into major divisions using fossil assemblages and radiometric dating of rocks.  During the vastness of Precambrian time, the Earth's crust developed, and early life evolved.  The Paleozoic began with the first appearance of fossils with hard parts and ended with the largest extinction in Earth's history.  The Mesozoic was the Age of Dinosaurs and ended with their extinction.  The Cenozoic, which includes the present, is the Age of Mammals and includes the evolution of man and a major Ice Age.

Beneath our feet

The late Precambrian, early Paleozoic, and late Cenozoic (Quaternary) are recorded in the rocks beneath our feet.  Each rock-building interval was followed by a long interlude of erosion, spanning hundreds of millions of years, that we see as a gap in the rock record.

Ancient mountains

The oldest rocks in the region are the Precambrian marble, quartzite, and granite of the Gatineau Hills and parts of Carp-Kanata area. These rocks are the deeply eroded roots of ancient mountains that were once as tall as the present-day Himalayas.  Between 1.2 and 1.0 billion years ago, sedimentary and volcanic rocks, originally deposited along the margin of ancient North America, were deformed, metamorphosed, and intruded by magma as a result of collision with another continent. This collision ceased about 1.0 billion years ago and the Precambrian mountains began to slowly wear down.

Tropical Ottawa-Gatineau

In the Paleozoic, between 510 and 440 million years ago, a warm tropical sea flooded the region. (We were near the equator then!) The oldest Paleozoic rock (the Nepean sandstone) was an ancient Cambrian beach. In the Ordovician, an ocean covered this beach and limestones and shales were deposited. Trilobites, cephalopods, crinoids, corals, snails, and other shelled animals that lived in the ancient coral reefs can be found by the thousands in the shales and limestones under our feet.

Breaking apart

About 175 million years ago, in the Mesozoic, the Ottawa-Bonnechere graben formed when the land surface moved downward between two major fault zones. These ancient faults are occasionally reactivated today, releasing crustal stress in the form of earthquakes.

Did you know? ... The dramatic escarpment that forms the southern edge of the Gatineau Hills between Quyon and Gatineau is a fault scarp along the northern side of the Ottawa-Bonnechere graben.

The big chill

During the Quaternary, great ice sheets covered northern North America several times.  The loose sediments that blanket bedrock in much of the Ottawa-Gatineau area were left by these glaciers or deposited in the Champlain Sea at the end of the Ice Age.