Geology

How Geology Shaped Hastings County, by Chris Fouts

The bedrock geology of Hastings County represents a long span of geological history. The southern end, from Lake Ontario northward roughly to Highway 7, is underlain by flat-lying "layer cake" stratigraphy of limestones and shales of the Black River Formation. These date to the middle Ordovician Period and have been identified as about 460 million years old, when the area was covered by warm shallow seas and was located in a tropical latitude. The limestone makes good building stone (see old buildings and foundations in Belleville, Stirling, Tweed and Madoc), as well as aggregate for road building, cement and concrete. It also provides lime-rich soils for farming and many types of agriculture. Fossils of corals, brachiopods (shellfish), gastropods (snails) and trilobites can be found at a number of locations.

Looking at a road map of Hastings County one notices that the grid-work of concession roads in the southern part of the county gives way to winding, and at times seemingly randomly directed, roads north of Highway 7. Leaving the flat-lying sedimentary rock one moves onto the ancient craton, or core, of the North American continent, the Canadian Shield. Of necessity, roads meander around the gently sloping hills and valleys of Precambrian rock.

Many people consider the Canadian Shield as one single unit of granite rock but many years of study have allowed geologists to recognize distinct units, or provinces, within the Shield which are characterized by certain rock types and structure. Centre and North Hastings are underlain by a part of the Shield called the Grenville Province, so called because it is characterized by rock units that were first described along the Ottawa River at the town of Grenville. These rock units include metamorphosed sediments (marbles and gneisses), metamorphosed volcanic rock flows and ash fall, plus intrusions of many different plutonic rocks, and skarn deposits (rock formations caused by the mixing of magma with sedimentary rock). All these rocks are between 900 million to 1.5 billion years old (about one quarter of the way back to the formation of the earth).

The terrain around the Madoc-Marmora-Bannockburn area is dominated by marbles, volcanic rock and waterlain volcanic ash sediments. The many colours of marbles found here have been used for building stone, landscaping aggregate, terrazzo aggregate and poultry grit. Magma intrusions into sedimentary rock have produced the talc deposit mined in Madoc and the iron ore deposits (magnetite) found at Marmora and in the surrounding area. Roofing granules are produced from basalt (black volcanic rock) quarried at Madoc and Havelock, and gold has been recovered from greenstone (basalt) found near Eldorado, Bannockburn and Gilmour. Crustal deformation and faulting sometime in the late Paleozoic era led to the development of fluorite-bearing veins in the Madoc area, around Moira Lake. First worked in 1905, activity in these veins peaked during the periods 1916-1920 and 1940-1951, during which time 39 deposits were mined and material was used to supply much of North America's needs for this commodity.

Moving northward toward Bancroft there is a subtle change in geology to highly metamorphosed sediments which have been greatly deformed by extreme pressure and intruded by various types of magma, ranging from granite and syenite (light-coloured quartz and feldspar-rich rock) to basalt and gabbro (dark-coloured metal-rich rock).

Bancroft is regarded as the "Mineral Capital of Canada" because of the great variety and quality of mineral specimens found there. This wide range of mineral types is a result of the many different geological environments that have occurred in this region. Approximately one billion years ago this region was the eastern edge of the continent. Plate tectonic activity caused the formation of a mountain range through this area (roughly running through Parry Sound, Haliburton, Bancroft and Pembroke). The plate below the Proto-Atlantic Ocean moved westward and dipped under, or subducted under, the North American plate. This action pushed sedimentary rock from the coast back onto the continent and folded the coastal rock, a process we can see in the Rocky Mountains today. Magma from the melting plate descending under the continent rose up as intrusions into this mixture, where in places it melted and mixed with sedimentary rocks.

The great variety of minerals found in the Bancroft region was a wonderful boon to the pioneers of the area who developed mines to extract many of these minerals: feldspar (ceramic products, abrasives and fillers), corundum (abrasives), fluorite (flux and chemicals), apatite (fertilizer phosphate), mica (insulation and roofing granules), nepheline (ceramics and glazes), marble and granite (building stone), garnet (abrasives), graphite (refractory products), molybdenite (steel alloy and high-temperature lubricant), sodalite (ornamental stone and jewellery), gold, iron ore, and much later, uranium. With the exception of uranium almost all of Bancroft's mining activity took place roughly between 1880 and 1930.

The mineral deposits of the North Hastings region are numerous, and generally show good ore grades, but are characteristically small in size, and are uneconomic by today's mining standards. Uranium provided a second mining boom for the area during the 1950s and 1960s as governments scrambled to find material to feed newly developed power plants and weapons. Bancroft was a known source of uranium and as such attracted numerous companies and prospectors who scoured the countryside in search of the element. Four mines were eventually developed: Bicroft (1956-1963), Canadian Dyno(1954-1960), Greyhawk (1957-1959) and Faraday/Madawaska Mines (1957-1964 and 1976-1982).

Today the former mine sites and prospect pits make excellent mineral collecting sites which feed a major part of the North Hastings tourism industry. The annual Rockhound Gemboree, held in Bancroft during the Civic Holiday weekend, is Canada's largest gem and mineral show, attracting in excess of 15,000 people each year.