Geological Time Scale

The geological formations in Canada's National Capital Area (NCA) represent to three distinct geological times. Each of the three differ substantially not only in age and time span, but also in the nature of the geological process involved and the type of material produced. The geological formations produced are: Precambrian metamorphic and igneous rocks of the Canadian Shield, Cambrian and Ordovician sedimentary rocks of the continental platform, and Quaternary unconsolidated sediments comprising the surface materials. Each of these formations were separated by long erosional periods as indicated in the Geological Time Scale. In order to understand how each geological unit was formed and how they are related, it is necessary to place each event in the global history of the Earth.

Since the formation of the Earth, approximately 4.6 Ga (billion years ago), the shape and position of continents have changed considerably. The earliest evidence of possible continental emergence from the melted mass of rocks can be traced to approximately 4.5 Ga, but the only preserved crustal rocks are in the range of 3.9 to 3.7 billion years. It is unlikely that the overall amount of continental crust at that time was greater than several percent, but by 2.5 Ga, as much as 50-75% of the continental crust may have been in place. The early fragments of continents were probably thin, but they gradually increased in thickness due to orogenic activities. Since the granitic material that formed the continents was lighter than the underlying magma and surrounding oceanic crust, they formed plates that emerged from the surrounding solidified crust. Convection movements in the magma under the crust caused the crust to crack and move laterally over the Earth's surface, this movement of the crust is at the origin of continental drift.

Very little is known of the size, shape and position of continents until the late Proterozoic 600Ma ( million years ago), but it is likely that the continents periodically collided to form super continents, called Pangea, and subsequently drifted apart. In the NCA the Precambrian metamorphic and igneous rocks that comprise of the Canadian Shield belong to that ancient and rather obscure portion of the Earth's history. However, as we move closer in geological times to the Late Paleozoic, the available geoscientific evidence increases substantially, permitting a better understanding of the formation and position of continents. That is why the formation of the Cambrian-Ordovician sedimentary rocks can be reconstructed with greater precision and detail (Bally et al.1989, McLennan, 1992). Much of the Cambrian-Ordovician formations and any rock deposited subsequent to the Ordovician Period were removed during the long erosion period that lasted until the Late Quaternary. For that time interval, no evidence of the geological events, other than tectonic, is known. Surficial deposits consist of Late Quaternary (<25 thousand years) glacial and related sediments, all other sediments having been removed by the last glacial advance.