Origin of the Oak Ridges Moraine
Encyclopedia
The Oak Ridges Moraine is a geological landform that runs east-west across south central Ontario
, Canada
. It developed about 12,000 years ago, during the Wisconsin glaciation
in North America
. A complex ridge of sedimentary material, the moraine
is known to have partially developed under water. The Niagara Escarpment
played a key role in forming the moraine in that it acted as a dam for glacial meltwater
trapped between it and two ice lobes.
In order to understand the process by which the moraine formed, it is necessary to understand the physical geography in which it currently exists (that is, its physiography), and the layers of sediments found within it (the stratigraphy of the moraine).
These wedges are separated by narrow east to west ridges, and formed via sedimentation. The hummock
y terrain, small elevated plains and narrow ridges suggest that its formation is of glacial origin.
The moraine is nearly bisected in the east; the Rice Lake
wedge is isolated from the three western wedges since the moraine is absent directly south of Rice Lake.
Four primary geophysical structures are intimately tied to the formative processes of the Oak Ridges Moraine:
, a prominent cuesta
that was fundamental to the development of the moraine. Specifically, the escarpment's channel
s provided a route for drainage of ice-marginal meltwater; the channel system was eroded into the cuesta, and along its exposed eastern facing. Meltwater drained southwest and northeast along these corridors during the formation of the Oak Ridges Moraine. The Niagara Escarpment probably controlled regional water levels throughout the formation of the moraine.
field is a large physiographic feature to the north of the Oak Ridges Moraine. Having a northeast to southwest orientation, it is cut by a network of deep valleys with wide, flat floors. The valleys are steep-sided, and have a branching pattern. The valley floors also exhibit inset esker
s. The feature also extends south of the moraine.
ary material, for some of which the origin is unknown. The base of the moraine touches bedrock
; "it is not clear if the bedrock valley itself was sculpted by a subglacial meltwater event, or was of a preglacial fluvial origin, or was possibly a combination of both." (Davies and Holysh, 2004, pg 27-5) Above the bedrock are the Lower Sediments (as styled by the Geological Survey of Canada), consisting of diamict
and fine-grained glaciolacustrine sediments at its base, and an upper sandy aquifer
layer. (Davies and Holysh, 2004, pg 27-5). Overlaying this is a thick sequence of Newmarket Till, a sandy loam sedimentary layer common in the area.
Each wedge exhibits different characteristics and sedimentary patterns, thus displaying many incongruities and depositional unconformities. The Rice Lake wedge, for instance, achieves a peak of 180 m above sea level, with a 20 m layer of shallow surface Oak Ridges Moraine sediments, underlain by the Newmarket Till. The other wedges, which peak at up to 335 m, have thicker and more varied surficial deposits, and the Newmarket Till lies deeper below the surface. Moreover, the sedimentary deposits are underlain by Halton Till on these wedges, which is not present in the Rice Lake wedge.
Thus, from top to bottom, the stratigraphy of the moraine is:
The four distinct stages of the model are:
However, the process by which each of these stages occurred has not been fully investigated, and is not completely understood.
Moreover, the ice sheet's constant retreat created fractures where it met and scoured the bedrock. These fractures allowed the development of sub-glacial rivers (adj. glaciofluvial) and lakes (adj. glaciolacustrine).
With constant exposure to the sun, the surface of the ice sheet continued to melt; the meltwater flowed from high points to the north- and south-east, and from the Niagara Escarpment to the west, into the cavity created by the fissure. The fast-flowing meltwater rapidly eroded the surface of the cavity, and created channels beneath the surface of the ice.
The channel fills along the core of the Oak Ridges Moraine are primarily coarse to fine, though the channel bases exhibit a thick sequence of coarse sediments.
Moreover, analysis of eskers within the deep valleys to the north of the moraine show that they tend to broaden to form gravel sheets. Accordingly, the "sedimentological data support the interpretation that the underfit valleys are tunnel channels eroded and filled with high-velocity. perhaps subglacial, meltwater." (Barnett et al., 1998, pg 1161)
Because of the continued erosional effects of the rapidly flowing meltwater, a passage opened on the Niagara Escarpment, creating a channel to the Campbellville outlet. The opening of the outlet allowed significant water discharge from the area, causing the transition from a glaciolacustrine fan sedimentation (from the glacial lake filling the original cavity) to deltaic sedimentation. Additionally, observations demonstrate that the Uxbridge wedge, which sits at an elevation of 335 m, was produced by an ice-confined sedimentation pattern in a lake which water level was controlled by the Campbellville outlet.
basin, "interbedded glaciolacustrine stratified sediments and massive to bedded diamicton" occurs in the upper deposition layers (Barnett et al., 1998, pg 1162). Some of these were deposited ice-marginally. In the east, deposition predominantly focused along the glacial margin, for example at the Palgrave Moraine.
Ontario
Ontario is a province of Canada, located in east-central Canada. It is Canada's most populous province and second largest in total area. It is home to the nation's most populous city, Toronto, and the nation's capital, Ottawa....
, Canada
Canada
Canada is a North American country consisting of ten provinces and three territories. Located in the northern part of the continent, it extends from the Atlantic Ocean in the east to the Pacific Ocean in the west, and northward into the Arctic Ocean...
. It developed about 12,000 years ago, during the Wisconsin glaciation
Wisconsin glaciation
The last glacial period was the most recent glacial period within the current ice age occurring during the last years of the Pleistocene, from approximately 110,000 to 10,000 years ago....
in North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...
. A complex ridge of sedimentary material, the moraine
Moraine
A moraine is any glacially formed accumulation of unconsolidated glacial debris which can occur in currently glaciated and formerly glaciated regions, such as those areas acted upon by a past glacial maximum. This debris may have been plucked off a valley floor as a glacier advanced or it may have...
is known to have partially developed under water. The Niagara Escarpment
Niagara Escarpment
The Niagara Escarpment is a long escarpment, or cuesta, in the United States and Canada that runs westward from New York State, through Ontario, Michigan, Wisconsin and Illinois...
played a key role in forming the moraine in that it acted as a dam for glacial meltwater
Meltwater
Meltwater is the water released by the melting of snow or ice, including glacial ice and ice shelfs over oceans. Meltwater is often found in the ablation zone of glaciers, where the rate of snow cover is reducing...
trapped between it and two ice lobes.
In order to understand the process by which the moraine formed, it is necessary to understand the physical geography in which it currently exists (that is, its physiography), and the layers of sediments found within it (the stratigraphy of the moraine).
Regional physiography
The Oak Ridges Moraine consists of four elevated, wedge-shaped structures; from west to east, they are:- The Albion Hills wedge,
- the Uxbridge wedge,
- the Pontypool wedge, and
- the Rice Lake wedge.
These wedges are separated by narrow east to west ridges, and formed via sedimentation. The hummock
Hummock
A hummock is a boss or rounded knoll of ice rising above the general level of an ice-field, making sledge travelling in the Arctic and Antarctic region extremely difficult and unpleasant....
y terrain, small elevated plains and narrow ridges suggest that its formation is of glacial origin.
The moraine is nearly bisected in the east; the Rice Lake
Rice Lake (Ontario)
Rice Lake is a lake located in south-eastern Ontario, in Northumberland County, south of Peterborough and the Kawartha lakes and north of Cobourg. The lake is part of the Trent-Severn Waterway, which flows into the lake by the Otonabee and out via the Trent. The lake is 32 km long and...
wedge is isolated from the three western wedges since the moraine is absent directly south of Rice Lake.
Four primary geophysical structures are intimately tied to the formative processes of the Oak Ridges Moraine:
- the Niagara Escarpment to the west
- drumlinized uplands in the north, with extensions south of the Oak Ridges Moraine
- wide, flat-floored valleys, primarily in the north
- low-relief plains in the south.
Niagara Escarpment
The western margin of the moraine is delineated by the Niagara EscarpmentNiagara Escarpment
The Niagara Escarpment is a long escarpment, or cuesta, in the United States and Canada that runs westward from New York State, through Ontario, Michigan, Wisconsin and Illinois...
, a prominent cuesta
Cuesta
In structural geology and geomorphology, a cuesta is a ridge formed by gently tilted sedimentary rock strata in a homoclinal structure. Cuestas have a steep slope, where the rock layers are exposed on their edges, called an escarpment or, if more steep, a cliff...
that was fundamental to the development of the moraine. Specifically, the escarpment's channel
Channel (geography)
In physical geography, a channel is the physical confine of a river, slough or ocean strait consisting of a bed and banks.A channel is also the natural or human-made deeper course through a reef, sand bar, bay, or any shallow body of water...
s provided a route for drainage of ice-marginal meltwater; the channel system was eroded into the cuesta, and along its exposed eastern facing. Meltwater drained southwest and northeast along these corridors during the formation of the Oak Ridges Moraine. The Niagara Escarpment probably controlled regional water levels throughout the formation of the moraine.
Peterborough drumlin field
The Peterborough drumlinDrumlin
A drumlin, from the Irish word droimnín , first recorded in 1833, is an elongated whale-shaped hill formed by glacial ice acting on underlying unconsolidated till or ground moraine.-Drumlin formation:...
field is a large physiographic feature to the north of the Oak Ridges Moraine. Having a northeast to southwest orientation, it is cut by a network of deep valleys with wide, flat floors. The valleys are steep-sided, and have a branching pattern. The valley floors also exhibit inset esker
Esker
An esker is a long winding ridge of stratified sand and gravel, examples of which occur in glaciated and formerly glaciated regions of Europe and North America...
s. The feature also extends south of the moraine.
Stratigraphy
The Oak Ridges Moraine consists of numerous layers of sedimentSediment
Sediment is naturally occurring material that is broken down by processes of weathering and erosion, and is subsequently transported by the action of fluids such as wind, water, or ice, and/or by the force of gravity acting on the particle itself....
ary material, for some of which the origin is unknown. The base of the moraine touches bedrock
Bedrock
In stratigraphy, bedrock is the native consolidated rock underlying the surface of a terrestrial planet, usually the Earth. Above the bedrock is usually an area of broken and weathered unconsolidated rock in the basal subsoil...
; "it is not clear if the bedrock valley itself was sculpted by a subglacial meltwater event, or was of a preglacial fluvial origin, or was possibly a combination of both." (Davies and Holysh, 2004, pg 27-5) Above the bedrock are the Lower Sediments (as styled by the Geological Survey of Canada), consisting of diamict
Diamicton
In geology, diamicton is a very poorly sorted sediment. Large sedimentary grains gravel size and larger are set in a matrix of fine grains....
and fine-grained glaciolacustrine sediments at its base, and an upper sandy aquifer
Aquifer
An aquifer is a wet underground layer of water-bearing permeable rock or unconsolidated materials from which groundwater can be usefully extracted using a water well. The study of water flow in aquifers and the characterization of aquifers is called hydrogeology...
layer. (Davies and Holysh, 2004, pg 27-5). Overlaying this is a thick sequence of Newmarket Till, a sandy loam sedimentary layer common in the area.
Each wedge exhibits different characteristics and sedimentary patterns, thus displaying many incongruities and depositional unconformities. The Rice Lake wedge, for instance, achieves a peak of 180 m above sea level, with a 20 m layer of shallow surface Oak Ridges Moraine sediments, underlain by the Newmarket Till. The other wedges, which peak at up to 335 m, have thicker and more varied surficial deposits, and the Newmarket Till lies deeper below the surface. Moreover, the sedimentary deposits are underlain by Halton Till on these wedges, which is not present in the Rice Lake wedge.
Thus, from top to bottom, the stratigraphy of the moraine is:
- Oak Ridges Moraine sediments
- Halton Till (not present at Rice Lake)
- Channel sediments
- Newmarket Till
- Lower sediments (sandy aquifer above, diamict and glaciolacustrine units below)
- Bedrock
Formation of the moraine
The most accepted theory of the development of the Oak Ridges Moraine is that proposed by Barnett et al. (1998). "Moraine development is highlighted in four stages and evolved rapidly from a glaciofluvial-dominated core to flanking glaciolacustrine-dominated wedge sediments." (Barnett et al., 1998, pg 1160)The four distinct stages of the model are:
- subglacial sedimentation,
- subaqueous fan sedimentation,
- fan to delta sedimentation, and
- ice-marginal sedimentation.
However, the process by which each of these stages occurred has not been fully investigated, and is not completely understood.
Pre-formative history
During the Late Wisconsinan period, the central part of Ontario was under thick ice. An east-west fissure split the ice sheet at the western periphery of the Oak Ridges Moraine, creating the Simcoe ice lobe to the north, and the Ontario lobe to the south.Moreover, the ice sheet's constant retreat created fractures where it met and scoured the bedrock. These fractures allowed the development of sub-glacial rivers (adj. glaciofluvial) and lakes (adj. glaciolacustrine).
With constant exposure to the sun, the surface of the ice sheet continued to melt; the meltwater flowed from high points to the north- and south-east, and from the Niagara Escarpment to the west, into the cavity created by the fissure. The fast-flowing meltwater rapidly eroded the surface of the cavity, and created channels beneath the surface of the ice.
Subglacial sedimentation
The meltwater eventually created a network of subglacial channels; many subglacial cavities also formed throughout the ice in the region. The sediment-laden meltwater travelled along these channels, increasingly depositing material in the cavities, and in the channels as the flow waned.The channel fills along the core of the Oak Ridges Moraine are primarily coarse to fine, though the channel bases exhibit a thick sequence of coarse sediments.
Moreover, analysis of eskers within the deep valleys to the north of the moraine show that they tend to broaden to form gravel sheets. Accordingly, the "sedimentological data support the interpretation that the underfit valleys are tunnel channels eroded and filled with high-velocity. perhaps subglacial, meltwater." (Barnett et al., 1998, pg 1161)
Fan to delta sedimentation
The sedimentary deposits characteristic of this stage were lateral to, and overlying the deposits from the previous stage. Moreover, "[fan] sedimentation in this stage is distal to primary sediment input sources and occurs in a large ice-controlled lake." (Barnett et al., 1998, pg 1162) The most prominent process throughout this stage is the transition from fan to fan delta, and hence to deltaic sedimentation patterns, with extensive deposition of fine sand and silt rhythmites.Because of the continued erosional effects of the rapidly flowing meltwater, a passage opened on the Niagara Escarpment, creating a channel to the Campbellville outlet. The opening of the outlet allowed significant water discharge from the area, causing the transition from a glaciolacustrine fan sedimentation (from the glacial lake filling the original cavity) to deltaic sedimentation. Additionally, observations demonstrate that the Uxbridge wedge, which sits at an elevation of 335 m, was produced by an ice-confined sedimentation pattern in a lake which water level was controlled by the Campbellville outlet.
Ice-marginal sedimentation
In the Humber RiverHumber River (Ontario)
The Humber River is one of two major rivers on either side of Toronto, Ontario, Canada, the other being the Don River to the east. It was designated a Canadian Heritage River on September 24, 1999....
basin, "interbedded glaciolacustrine stratified sediments and massive to bedded diamicton" occurs in the upper deposition layers (Barnett et al., 1998, pg 1162). Some of these were deposited ice-marginally. In the east, deposition predominantly focused along the glacial margin, for example at the Palgrave Moraine.