Rideau 
Geology 

Thirteen thousand years ago, a blink of the eye in geologic time, the Rideau region was sitting under a glacier, over a kilometre in thickness. There were no lakes, no trees, no wildlife, and no people. As the glaciers receded they exposed the bedrock, striated (ice gouged) in many places, and they also left behind trillions of tonnes of glacial rubble. The bedrock, which had been depressed by the weight of the ice, now started to rise up (isostatic rebound). Vegetation started to take hold and eventually the region took on the appearance it has today.

Geology has played an important role in affecting the appearance of the area and influencing man's uses of the region. The shape of some of the lakes is tectonically influenced, an example being the Rideau Lakes (Big, Upper and Lower) which follow the trace of the aptly named Rideau Lake Fault. Fishing in the region is excellent since the lakes are unaffected by so called "acid rain", due to the fact that most of the lake basins are composed of limestone, which acts as a buffer to any acidic precipitation. Areas where the glaciers left thicker cover (gravels and soils) are now primarily agricultural regions, areas that have thin cover and exposed bedrock are generally wooded with little agricultural development.

THE GLACIERS

Notwithstanding the arguments over man's role in global warming, it exists, and has existed in the Rideau region for the last 13,000 years. There have been four periods of glaciation in recent times, each lasting about 100,000 years, and each separated by an interglacial period with temperatures as warm or warmer than they are now. At the height of each glacial period, over 30% of the earth's surface was covered by ice (compared to 10% ice coverage today).

The most recent period of glaciation in North America is known as the Wisconsin Period. In the Rideau area, it first took hold about 90,000 years ago, and ended about 13,000 years ago. As the ice retreated, the landscape underwent progressive changes:

• Thirteen thousand (13,000) years ago, the whole region, including present day Lake Ontario was under ice.
  
  
• Twelve thousand, five hundred (12,500) years ago, Lake Iroquois had formed where present day Lake Ontario is located. It was quite a bit bigger than Lake Ontario, and considerably higher (about 335 feet above sea level compared to today's 243 feet), since the water outlet to the east was blocked by the retreating glaciers. Lake Iroquois drained southeast into present day New York State.
  
  
• Twelve thousand (12,000) years ago, Lake Iroquois covered most of the Rideau region, stretching all the way north to Ottawa.
  
  
• By eleven thousand, five hundred (11,500) years ago, the scene had changed quite dramatically. The glaciers had retreated far enough to open up the St. Lawrence Valley lowland and let the ocean flood in. The Champlain Sea extended west, all the way up the Ottawa River Valley, past Ottawa. The southwestern margin of this sea was close to the present day height of land that traverses the Lower Rideau Lake area. At this time, the drainage from Lake Iroquois/Lake Ontario was east, down the channel of the present day St. Lawrence River, into the Champlain Sea. This new drainage resulted in a lake that was a bit smaller than present-day Lake Ontario.
  
  Fossil evidence of beluga whales and several kinds of arctic marine pelecypods (clams) have been found in the Ottawa area. The Champlain Sea would have been cloudy with rock-flour, ground by the glaciers whose meltwater poured into this sea. This rock-flour settled to the bottom of the sea and is known today as Leda Clay.
  
  
• Changes continued, driven to a large degree by isostatic rebound. Now that the weight of the ice was gone, the land was rising. This lifting of the land eventually pushed back the Champlain sea. For instance, in the western part of the Merrickville area the sea departed about 11,100 years ago and in the eastern part it was gone by about 10,500 years ago
  
  
• By 6,000 years ago isostatic rebound was complete and the region looked very much like it does now. It should continue to do so for some time.
  
  

The last interglacial period in this region lasted over 150,000 years, so we certainly don't have to worry about glaciers in our lifetime.

THE ROCKS

There is a basic division in the geological time scale that occurs at 600 million years ago. Everything prior to that date is known as the Precambrian. Most of the currently known fossil record of the earth comes after that, and there are many geological divisions from 600 million years ago to the present. Many of the rocks in the Rideau Region belong to the Precambrian.

As can be seen from the simplified geological map of the Rideau Lakes region, most of the region consists of Precambrian limestone (marble), gneiss, quartzite, granulite, migmatite and granitic (quartz monzonite) plutons. These rocks are overlain in many parts by more recent, Palaeozoic sedimentary rocks. A description of these rocks can be found in the next section. There is not a great deal of relief in the area, but much of what can be seen is a result of differential erosion. Softer rock, such as the limestone, are eroded more quickly, and now form the basins of many of the lakes. Harder rocks such as the gneisses, quartzites, and granites tend to form many of the hills and ridges in the region.

In a regional sense, the Precambrian rocks in the Rideau region form part of what is known as the "Frontenac Axis", a narrow section of the Canadian Shield that links the Laurentian Plateau with the Adirondack Mountains of New York. In the landsat image you also can clearly see these two physiographic features, and if you look closely, you can see the physical expression of the Frontenac Axis.

Most of the Precambrian rocks have been metamorphosed (altered from their original state by pressure and heat) and belong to the Grenville Metamorphic Series. They are in excess of one billion years old. These metamorphic rocks were intruded by younger igneous rocks (granites and gabbros), shown in pink on the map. These in turn were cut by more recent intrusive rocks, pegmatites and diabase dykes.

These Precambrian rocks are uncomformably (separated by a long erosional period) in contact with overlying Palaeozoic sedimentary rocks, shown in grey on the map. These include sandstone, conglomerate, dolomite, dolomitic limestone, sandy shale, fine grained grey limestone, and sandy limestone. It is from this series of rocks that the blocks for the Rideau locks and dams were cut.

RIDEAU ROCK TYPES

The main rock types are listed below.

Limestone - Known as crystalline limestone it is really a coarsely crystalline calcite marble. This rock is soft and has been eroded more deeply than other rocks, and therefor forms the basins for most of the large lakes of the region. In many areas it is almost pure calcium carbonate, although inclusions are very common. These can include varying amounts of graphite, and silicate minerals such as serpentine, quartz, phlogopite and feldspar. In some areas, dolomitic limestone is present. In the region around Charleston Lake, silicious crystalline limestone can be found, usually adjacent to the quartzite units.

Gneiss and Quartzite - Quartzite is a "silica flooded" metamorphic rock. The high silica content and the way it is incorporated into the rock makes it very hard. It is the hardest rock in the region and therefore forms many of the ridge tops in areas where it is present. Its origins are sedimentary, most likely sandstone, and in some places original sedimentary features such as bedding and crossbedding can be observed.

Gneiss is a generic descriptive term for a metamorphic rock with discontinuous visible layering (foliation) which occurs when groups of platy or elongate grains in separate lenses or streaks. It will often had differential colouring with alternating streaks/bands of light and dark minerals. In the Rideau Lakes region it is the most abundant rock after crystalline limestone. The most common units are a quartz-feldspar gneiss and a quartz-biotite-feldspar gneiss. In many areas, the metamorphism has introduced metamorphic minerals such as garnet (pink to dark red in crystals up to 2 inches in diameter) and cordierite (dark blue pods and segregations about 1 inch long).

Granulite and Migmatite - A granulite is an almost structureless quart-feldspar rock. It can be distinguished from gneiss by its generally lighter colour (almost no dark minerals) and the almost complete absence of foliation (layering). A migmatite is a metamorphic rock with a mixed structure, that usually results from the mixing of metamorphic and igneous material. Thus it is commonly found in proximity to the intrusive units (granites) and may represent re-melting of the original gneissic units.

Granite and Gabbro - Several igneous rock units occur in the area, the main ones being granitic units which range in composition from monzonite to quartz-monzonite. They are light in colour, generally coarse grained, and contain reddish-pink feldspar.
The other igneous rocks are gabbro and diorite, both dark coloured intrusive rocks. The diorite is a greenish-grey medium grained rock, although coarse grained sections can occur. The gabbro is generally medium grained, with a greenish brown colour.

Palaeozoic Sedimentary Rocks - These are the youngest rocks in the area, of Upper Cambrian to Middle Ordovician age (450 to 500 million years ago). Most of the units are flat lying, often lying unconformably over the tilted and folded Precambrian rock units. The lowermost unit is the Nepean Sandstone, a buff coloured sandstone which in some places contains a quartz-pebble conglomerate. It is overlain by the March and Oxford formations consisting of sandy dolomite, dolomite, and limestone. The uppermost unit is the Ottawa formation, consisting of sandy shale, fine grained grey limestone, and sandy limestone.

ECONOMIC GEOLOGY

No large scale mining has ever been conducted in this region, but many small, short term operations at various times mined a variety of minerals including; galena, magnetite, hematite, mica, apatite, graphite, barite, celestite, quartz crystals, feldspar, building stone, and gravel.

Galena (lead sulphide) was mined near Lyndhurst in the mid to late 1800s. A couple of very small scale operations mined barite, and several mined celestite (strontium sulphate).

Iron was mined in several localities. From 1858 to 1871, iron, in the form of titaniferous magnetite, hosted by gabbro, was mined near Newboro Lake (Chaffey Mine and Matthews Mine). Some small scale mining of hematite (iron ore), starting as early as 1810, was done near Delta. The original mine supplied a smelter at Furnace Falls (now Lyndhurst).

There are several deposits of apatite and mica which have been mined. Apatite was a prime source of phosphate for fertilizer prior to 1890. The largest apatite mine was the Opinicon Rock Lake mine which operated from 1888 to 1892. Mica was mined from several localities. It often occurs with apatite, so some of the mines produced both products. The main mica mines were the Sand Lake mine, which opened in 1870 and was mined intermittently until 1912, and the Stoness mine, near Buck Lake which mined mica from 1894 to 1902.

Graphite was mined intermittently in the Timmins Mines, north of Big Rideau Lake, from 1918 to 1923.

Quartz crystals were mined near Black Rapids (Lansdowne Township), starting in 1943. The crystals are found in vugs, some up to a foot long and doubly terminated.

Several granite quarries were opened. Some attractive pink and red quartz monzonite was mined (known as Lyndhurst Rose Granite and Rideau Red Granite). Unfortunately, much of this rock contains inclusions and irregular fractures, and the amount of waste in the mines could be high at times.

Sandstone was quarried for use in building the Rideau Canal. The sandstone used in the construction of the locks and dam at Jones Falls was mined near Elgin. The churches in Elgin are also built of this material.

Limestone continues to be quarried to this day. Much limestone was quarried for the use in constructing stone houses, particularly near Ottawa and near Kingston. The latter has so many buidlings made of limestone that it is known as the "Limestone City".

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CREDITS: The descriptions of the detailed geology of the region were taken from Geological Survey of Canada Memoir 346, Geology of the Westport Map Area, by H.R. Wynne-Edwards, published in 1967.

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