Glacial till landform above Lake Ontario, Canada
The Oak Ridges Moraine is an ecologically important geological landform in the Mixedwood Plains of south-central Ontario, Canada. The moraine covers a geographic area of between Caledon and Rice Lake, near Peterborough. One of the most significant landforms in southern Ontario, the moraine gets its name from the rolling hills and river valleys extending east from the Niagara Escarpment to Rice Lake, formed 12,000 years ago by advancing and retreating glaciers (see "geological origins", below) during the last glaciation period. Below the approximately 200 metre thick glacial derived sediments of the moraine lies thick bedrock successions of Precambrian rocks and up to 200 metres of Ordovician aged rock (see "geology" below), capped by a regional unconformity of erosion and non-deposition to the Quaternary period. Rivers and lakes scatter the landscape and are important for creating habitat for the rich diversity of species of animals, trees and shrubbery (see "ecology"). These are also the supply of fresh water to aquifers in the moraine through complex subterranean connections (see "hydrology"). Construction development nearby, and with expansion of communities around the moraine in need of potable water, it is a contested site in Ontario, since it stands in the path of major urban development (see "political action"). Conservation of the moraine is thus an important step for keeping aquifers in a safe drinkable condition while also protecting the natural ecosystems surrounding and within the moraine (see "conservation"). This region has been subject to multiple decades of scientific research to study the origins of formation, and how early communities used the land. A larger focus currently is how to source potable water without removing the aquifer entirely (see "research" section).
Physiography.
The Oak Ridges Moraine is a pair of large ridges composed of four elevated wedges. It is bounded to the west by the Niagara Escarpment, a cuesta which was critical to the formation of the moraine, and to the east by the Trent River and Rice Lake. The four wedges ("Albion", "Uxbridge", "Pontypool" and "Rice Lake" from west to east) formed in stages, though some synchronous formation also occurred at an early period of formation. The moraine peaks at the Uxbridge wedge, generally rising from east to west, a result of the moraine's west-to-east formation. That is, the western portion of the moraine received earlier and more frequent sedimentary deposition than the eastern portion, as the ice lobes which controlled the moraine's eastern formation slowly retreated. The Rice Lake wedge is separated from the other wedges where the Oak Ridges Moraine intersects Rice Lake. As the moraine is a local topographic high, streams originating in the lower Oak Ridges Moraine sediment packages, by groundwater movement, either flow south into Lake Ontario or north toward Lake Simcoe or Georgian Bay through large river valleys carved during melting of glaciers.
To the north are drumlinized uplands, referred to as the "Peterborough drumlin field," which may continue south of Rice Lake and the Rice Lake wedge"." Near the Rice Lake wedge and Peterborough drumlin field are a couple of low-lying terrain plains and small hills on bedrock basement.
Geological origins.
The Oak Ridges Moraine probably formed in the Late Wisconsin glacial period. Ice melt from the Niagara Escarpment flowed into the western boundaries of the moraine, wherein conduits beneath the ice expanded to form a west-to-east passage between the main Laurentide Ice Sheet and a mass of ice in the Lake Ontario basin.
Stratified sediment was deposited rapidly on the high-relief erosional surface. Up to in some areas, the deposits occurred on surfaces defined by highly eroded channels and drumlin uplands. The channel tunnels were primarily created by the erosion from glacial rivers, a typical process before moraine formation.
Characteristic of this moraine is the transition of deposition layers from glaciofluvial to glaciolacustrine. Sedimentary glaciofluvial areas form the core of the Oak Ridges Moraine, though restricted to subglacial cavity fills of confined subaqueous fans. Overlaying this core is a glaciolacustrine sedimentary layer that is younger and topographically lower, principally in the delta and basin areas of the moraine.
In the late stages of its development, random unstratified accumulation (known as diamicton) occurred along the ice margins. Whereas the glacial river deposits were more substantial by volume, the diamicton deposits represent a greater portion of the moraine's exposed surface. Research suggests that sedimentation occurred in a west to east sequence along the four main sedimentary wedges: "Albion", "Uxbridge", "Pontytool" and "Rice Lake". The wedges may have formed in a relatively short period of time, perhaps no more than a few hundred years, as indicated by the annual deposition cycles in glacial lakes (known as varves) within the moraine sediments. However, the lowest beds in the moraine may have experienced synchronous sedimentation.
Geology.
Glacial sediments.
In the area of the Oak Ridges Moraine, glacially derived sediments, mainly of Pleistocene age, may overlie a regional unconformity and lower Newmarket till, up to 200 metres thick in some places. Extensive research in the moraine area indicate there may be 6 different periods of sedimentation from the onset of glaciation. There are "lower deposits" of sand, silt and clay, which rest directly above the regional time unconformity up to thick in some areas. These lower deposits are thought to be proglacial lake derived, and contain from bottom to top: York Till, Don Interglacial beds, Scarborough Formation, Sunnybrook Till and Thorncliffe Formation. Next comes the "Newmarket Till" of stony, silty sand and sandy-silt diamicton ranging from thick and is thought to extend southward under Lake Ontario. Cutting into the "Newmarket Till" is another regional unconformity, which led to coarse grained "channel fill" sediments. Some "channel fills" are 10 to 25 metres of gravels or sands that decrease in grain size during later deposition following a period of slowing meltwater runoff. "Channel fill" sediments comprise the base of "Oak Ridges Moraine sediments", atop the unconformity. Lower deposits of the "Oak Ridges Moraine sediments" stratigraphic package start as coarse sands and gravels whereas upper sediments are interbedded fine sands and silts with "channel fill". There may also be sequences of sediments decreasing in size from medium sands to silts and clays up to ten's of metres thick. Above these sediments lie clayey-silt to silt till of the "Halton Till", at thicknesses reaching . The "Halton Till" is not present over the entire expanse of the moraine, cropping out locally and interspersed. 
Glacial lake sediments cap the entire sequence of "Oak Ridges Moraine sediment" stratigraphy in the region, namely from two glacial lakes, Lake Algonquin and Lake Iroquois. The main depositional ingredient from these lakes is silt with some clay content, as layers of thick in places. Glacial sediments are subject to erosional forces and valley cutting processes, removing previously laid sediments and placing them further along the river path.
Unconformity.
At the Oak Ridges Moraine system, rocks from the Ordovician Period underlie the post-glacial sediment and are a large part of the bedrock geology, aside from the presence of some older basement rock beneath. From the basement rocks to the overlying stratigraphic successions is an unconformity (a gap in the geologic record) where many millions of years of depositional or crystalline record are erased (mostly Silurian to Pleistocene rocks) by regional erosion processes and continent building phases. A large time gap is important in this context to correlate where and when this event occurred with respect to both local and surrounding geology of the lithostratigraphic and geographical record. Cause of this stratigraphic gap may originate from a couple large scale meltwater events, the older Algonquin and younger Ontarian erosional floods from melting glaciers of the Late Wisconsinan glaciation.
Paleozoic bedrock.
Basement layers are composed of Early Ordovician aged dolostone as a first sequence only to be stripped away by erosion in the Middle Ordovician. Sea transgression during continent reformation in the Middle Ordovician brought in a second sequence of deposition as clastic and carbonate sediments above this new unconformity. A third depositional sequence is found in the Late Ordovician, separated from the Middle Ordovician by local erosion and clastic sediments, consists of a shale base topped with more shale interbedded with limestone and calcareous siltstones. Above this is yet another unconformity, this time due to sea regression in the Late Ordovician to Early Silurian time. Large gaps in deposition from the Early Silurian through Mesozoic time allowed large scale erosion to occur in this geographic region, to the onset of Quaternary glaciation and subsequent sediment deposition. A 1997 report from the Geological Survey of Canada estimates the bedrock thickness to range between thick, below approximately of Pleistocene aged sediments. This broad range of thickness is representative of the unequal effects of regional erosion, uplift, sedimentary deposition and continental processes (tectonics, building and destroying of continents).
Precambrian bedrock.
Rocks from the Grenville Province of Precambrian age lie below Ordovician aged limestones and shales in much of the Oak Ridges Moraine area. These come from a period of continental scale orogeny in eastern North America about 1.2 to 1.3 billion years ago. This area of the Grenville Province is mainly composed of metamorphosed gneiss and mylonite, while also including intrusions of granitic composition as well. Both the metamorphic and igneous rocks are able to better withstand erosional forces from the advancing ice sheets compared to some softer rock types. Softer metamorphic rocks of the Precambrian basement include marbles or metaturbidites, both from sedimentary origin.
Ecology.
The ecologically diverse moraine is the water source for many headwaters streams flowing south into rivers that drain into Lake Ontario and north into rivers that drain into Lake Scugog and Lake Simcoe. Varied landforms on the moraine, such as woodlands, wetlands, watercourses, kettle lakes and bogs, have provided an environment suitable for significant flora and fauna communities to develop and thrive. The moraine contains one of the last large contiguous forested spans in southern Ontario.
Some of the wide variety of plant and animal species found in the moraine are "species at risk" in Canada and Ontario, including the West Virginia white butterfly, Jefferson salamander, and red-shouldered hawk. Unique ecosystems in the moraine include wetlands similar to those of boreal forests in Northern Ontario, and remnants of tallgrass prairie and oak-pine savanna that are globally threatened ecosystems. A representative portion of the moraine may be found in the Happy Valley Forests, in York Region, north of Toronto.
Hydrology.
A number of features comprise the hydrological system of the Oak Ridges Moraine:
This hydrological system is inter-twined with a regional flow systems not bound by the morphological limits of the moraine. For this reason, environmentalists and researchers promote an aggressive protection strategy extending beyond the moraine, thus ensuring a contiguously protected hydrological system. Preservation of aquifers under the moraine is especially important, since they are used as a primary water source by some municipalities located on or near the moraine. These aquifers also discharge into tributaries that are the headwaters of approximately 65 creeks and rivers which eventually flow into Lake Simcoe, Lake Scugog and Lake Ontario or Georgian Bay.
The "Oak Ridges Moraine Conservation Act" stipulates that any development which targets the moraine or nearby areas must satisfy several conditions, most prominently that each development leave a buffer zone of between it and any hydrological feature; for a kettle lake, this measure is from the edge of the lake's catchment area.
The Oak Ridges Moraine's hydrological system is a major constituent of the Humber Watershed, so that any factors affecting the moraine may affect the connected systems. A specific concern is urbanization, which affects water quality by increasing its load of metals and organic contaminants. A study by Cook et al. (1985) found an increase in mean annual runoff, instantaneous discharge, and hydrograph peak flow as a result of urbanization: "...changes in land use coincided with changes in volumetric and time distribution aspects of hydrologic response".
Research on the moraine.
In 1829, John Bigsby conducted the first investigation of the moraine. He noted the elevation, and styled the area "Oak Ridge", identifying the portion of moraine north of Toronto. The moraine's extent was not established until 1863 when William Logan conducted the Geological Survey of Canada.
Taylor formally defined the landform as the Oak Ridges Moraine in 1913. He described its extent to be from King and Maple in the west to the Trent River in the east. He also proposed that its origin was overlapping, interlobate glaciation retreat, between the Lake Ontario Lobe and the older Lake Simcoe Lobe.