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Geological Overview
Green Spaces of Northwestern Ontario


Geological Overview

The mesas of rock that surround Thunder Bay are among the oldest on earth - some 600 million years old.

A bed of relatively soft sedimentary rock, known as shale, was laid down by shallow seas nearly 1.5 billion years ago. Shortly after the shale was deposited, molten diabase, a much harder volcanic rock, was forced upward, eventually cooling off as horizontal sheets called sills or perpendicular intrusions called dikes.

With the eventual erosion of the overlying and surrounding shale, many of the sills remained standing, some rising more than 200 metres above the surrounding landscape. Today they are defined by steep rock cliffs. The lower portions are marked by embankments of accumulated rock fragments, called talus, that have fallen from the rock face. Diabase sills form the Sleeping Giant and the low mountains that extend along the north shore of Lake Superior from Nipigon to the Minnesota border.

Glacial History

The effects of glaciation are dramatically evident upon the Northwestern Ontario landscape. For more than 50,000 years thick accumulations of ice mantled much of the northern two-thirds of North America. As recently as 13,500 years ago all of Northwestern Ontario was deeply buried under ice. As the pace of global deglaciation accelerated, the region was gradually freed of its icy mantle between 11,600 and 8,400 years ago.

As the ice melted, enormous volumes of meltwater, earth and rock debris were released. The so called glacial till either mantled the newly exposed land or was transported by flowing meltwater. Much of this meltwater remained trapped between unglaciated uplands and the glacial fronts to form large meltwater lakes. One of the largest of these lakes was Lake Agassiz which encompassed a vast area throughout northwestern Ontario, northern Minnesota, the eastern Dakotas, south-central Manitoba and east-central Saskatchewan. Another major glacial lake (named Lake Minong) overfilled the current Lake Superior basin, resulting in many ancient beach features found many kilometres inland from the current Lake Superior shoreline.

Human History

Within a few hundred years after deglaciation, the land began to support hardy pioneer plants which, in turn, attracted arctic-adapted animals. This emerging post-glacial habitat also attracted humans northward. The first human inhabitants are referred to by archaeologists as Palaeo Indians (or Old Indians). These societies are particularly noted for their great skill in producing stone projectile points which likely date between 9,000 and 6,500 years ago. These first inhabitants probably hunted and gathered a wide range of plants and animals, but likely focused heavily upon herds of caribou. In the Thunder Bay area many Palaeo Indian archaeological sites have been discovered along the abandoned ancient shores of Lake Minong, especially where bedrock exposures of stone suitable for tool production are found.

As time passed the tools and economies of successive generations of Aboriginal people changed from those associated with the Palaeo Indians into those associated with the Archaic cultural traditions (ca 6,500 to 2,000 years ago). These people were also hunters and gatherers, but in an environment which had gradually become mantled with coniferous forest. The tools associated with these people included a range of notched projectile points and other cutting, piercing and grinding tools. Particularly notable is the development of a copper working industry utilizing nodules of Native Copper found throughout the western Lake Superior basin. This involved heating (but not melting) copper nodules and hammering them into complex tools that were laboriously finished by grinding and polishing.

Beginning as early as 2,000 years ago, new cultural traditions and technologies deriving from the eastern woodlands of the USA began to appear in Northwestern Ontario. Associated with probable increased population densities, these societies were also highly efficient hunters and gatherers. The Middle Woodland society archaeologically known as Laurel is primarily distinguished as being the first Boreal Forest producers of earthenware ceramics. Throughout the past 2,000 years there has been a succession of ceramic producing hunting and gathering societies observed in the archaeological record. Written documentation of the land and people of northwestern Ontario, available after about AD 1600, permits identification of Native groups, many of whom are the ancestors of the contemporary Ojibwe and Cree inhabitants of the region.


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