Metcalfe Geoheritage Park was made possible by the support of the Mississippi River Power Corp., the Municipality of Mississippi Mills, the Canadian Geological Foundation, the APGO Education Foundation, and the Metcalfe Geoheritage Park Committee.
Reading the Rocks
The 22 samples on display are sedimentary rocks of Ordovician age from our area, about 490 to 440 million years old, along with older igneous and metamorphic rocks of Precambrian age, from about 2.5 billion to 1 billion years ago. They collectively illustrate many of the features that allow geoscientists to unravel Earth’s remarkable geological history.
During the Ordovician Period, about 490 to 440 million years ago, our part of the world was closer to the equator. Laurentia, the ancestral continent of North America, was covered by the warm shallow waters of the Iapetus Ocean. It was a rich environment for early forms of marine life. Their fossils are found in the sedimentary rocks of the Almonte region.
Precambrian Super Eon
About a billion years ago, during the Precambrian Super Eon, our part of the world was much smaller than the present day continent of North America. Off the coast, to the east, were microcontinents of volcanic islands with intervening seas and sedimentary basins. A modern example might be Indonesia with its many volcanic islands.
The westward moving microcontinents collided with the mainland, one after the other. The repeated collisions thrust the volcanic and sedimentary rocks up and over the continental edge. The thrust sheets of the separate landmasses were pushed together, folded, buckled, invaded by magma, and pushed skyward to form the Grenville Mountain chain along the eastern coast of the continent. A modern example is the Himalayas formed by India colliding with Asia.
Under conditions of high temperature and pressure, volcanic and sedimentary rocks were transformed into metamorphic rocks. As the mountain chain eroded away over hundreds of millions of years, these rocks became exposed. They form the rolling hills west of Almonte we know geologically today as the Grenville Province of the Canadian Shield.