The formation of the Temiskaming Rift Valley was a singularly defining event for this region.
The following text is taken from "Impressions of Temiskaming", by Michael Werner, 2013
Temiskaming is built on the same Archean basement rock that underlies almost all of Canada, the Precambrian or Canadian Shield. It also lies within an ecoregion known as the Boreal Forest, which covers almost 60% of Canada’s land area, forming a continuous belt from the east coast to the Rockies. Scientists call the area where the Canadian Shield and the Boreal Forest overlap the Boreal Shield, the largest of Canada's 15 terrestrial ecozones.
Newly emerged from the glacial might of the ice ages after the retreat of the Wisconsin Ice Sheet only 10,000 years ago, the mere blink of an eye in geological time, it is an extremely young landscape. Though the rock is unimaginably old, formed one to two billion years ago, the current surface features have been so influenced by the recent continental glaciations that it almost seems as if nothing that came before matters. From Newfoundland in the east to British Columbia in the west, there is a commonality to the physical geography of the landscape that, in some enigmatic but distinct way, tells you that you are standing in some part of the vastness of Canada's Boreal Shield country.
Because of this common glacial history, Temiskaming shares a great deal of its character with the rest of northern Canada. And yet in the underlying details there is still much to distinguish her. Though the glaciers have certainly left their mark, the geological phenomena that have conspired to produce the most distinctive landform endowment of Temiskaming began much, much longer ago.
About 450 million years ago, near the end of the Ordovician period, a series of already existing, even older faults were reactivated along a weak zone of the North American tectonic plate. As the underlying tectonic forces attempted to stretch the plate, these parallel faults resulted in a block of land subsiding by about 300 meters to produce a deep rift valley 50 kilometers wide and 400 kilometers long. The Temiskaming Rift Valley was born.
The Temiskaming Rift Valley, or Temiskaming Graben, is the northern extension of the Ottawa-Bonnechere Graben, which is part of the Saint Lawrence rift system.
The development of the Temiskaming Rift Valley was a singularly defining event for this region. It has had a controlling influence on all of the surface processes that followed, including even the glaciers themselves. It was not, however, a simple, one-time occurrence. The full geological picture is somewhat more complicated and spans hundreds of millions of years. The Temiskaming Rift Valley is actually a succession of nested rift valleys, assembled along several series of parallel fault lines, each becoming progressively deeper the closer they are positioned to the centre of the main formation. The deepest, and perhaps most recently active, is a long trench about 3 kilometers wide that forms the deepest parts of Lake Temiskaming.
In addition, during some of the later subsidence events, there remained raised blocks between some of the subsiding blocks. The ridge from Dawson Point to Earlton, down the centre of which runs a section of Highway 11, is one such block that remained raised when blocks on either side of it subsided further. This limestone escarpment, a remnant of Paleozoic sedimentary sea-floor deposits, is itself a portion of the dropped block from the earlier main subsidence event. All Paleozoic sediments on the surrounding landscape, for 225 kilometers in every direction, that were not protected as part of this subsided block within the Temiskaming Rift Valley, have long ago eroded away into complete oblivion.
The ancient faults that delimit the Temiskaming Rift Valley and its substructures are still active today, and recent studies of the post-glacial sediments at the bottom of Lake Temiskaming indicate that there has been significant and ongoing faulting and subsidence within even the last few thousand years. The magnitude 6.2 Temiskaming earthquake of November 1, 1935, the magnitude 5.2 Kipawa earthquake of January 1, 2000, and the magnitude 5.0 earthquake centred 60km northeast of Ottawa on June 23, 2010 are all associated with the release of stresses along this same system of faults. These faults form part of a zone of earthquake activity known as the Western Quebec Seismic Zone, the most seismically active area in eastern North America. Indeed, the Temiskaming Rift Valley, and associated rift valleys along the Ottawa and St. Lawrence Rivers, seem to represent a persistent weak zone in the otherwise rigid North American plate.
The formation of the Temiskaming Rift Valley 450 million years ago is responsible for much of that which distinguishes Temiskaming District from the rest of northern Ontario today. From the agricultural plains of the Little Clay Belt to the mineral deposits that have given us such a rich mining history, this system of faults and rifts has provided an enduring and unique legacy. There are "kimberlite pipes" within the rift valley that are considered to be diamond bearing.
A rift valley is a linear-shaped lowland between several highlands or mountain ranges created by the action of a geologic rift or fault. A rift valley is formed on a divergent plate boundary, a crustal extension or spreading apart of the surface, which is subsequently further deepened by the forces of erosion. When the tensional forces are strong enough to cause the plate to split apart, a center block drops between the two blocks at its flanks, forming a graben. The drop of the center creates the nearly parallel steeply dipping walls of a rift valley when it is new. That feature is the beginning of the rift valley, but as the process continues, the valley widens, until it becomes a large basin that fills with sediment from the rift walls and the surrounding area.