Quick clay, also known as Leda clay and Champlain Sea clay in Canada, is a unique form of highly sensitive marine clay, with the tendency to change from a relatively stiff condition to a liquid mass when it is disturbed. Undisturbed quick clay resembles a water-saturated gel. When a mass of quick clay undergoes sufficient stress, however, it instantly turns into a flowing ooze, a process known as liquefaction. A small block of quick clay can liquefy from a stress as simple as a modest blow from a human hand, while a larger deposit is mainly vulnerable to greater stresses such as earthquake vibrations or saturation by excess rainwater. Quick clay behaves this way because, although it is solid, it has a very high water content, up to 80%. The clay retains a solid structure despite the high water content, because surface tension holds water-coated flocs of clay together in a delicate structure. When the structure is broken by a shock, it reverts to a fluid state.
Quick clay is only found in the northern countries such as Russia, Canada, Norway, Sweden, and Finland, and in the US state of Alaska, which were glaciated during the Pleistocene epoch. In Canada, the clay is associated primarily with the Pleistocene-era Champlain Sea, in the modern Ottawa Valley, St. Lawrence Valley and Saguenay River regions.
Quick clay has been the underlying cause of many deadly landslides. In Canada alone, it has been associated with more than 250 mapped landslides. Some of these are ancient, and may have been triggered by earthquakes.
Formation of quick clay 
At the height of the past glaciation (about 10,000 years ago), the land was 'pushed' down by the weight of the ice (isostatic depression). All of the ground-up rock was deposited in the surrounding ocean, which had penetrated significantly inland. The loose deposition of the silt and clay particles in the marine environment, allowed an unusual flocculation to take place. Essentially, this formed a strongly bonded soil skeleton, which was 'glued' by highly mobile sea-salt ions.
At this point, there was only the formation of very strong marine clay, which is found all over the world and highly stable, but with its own unique geotechnical problems. When the glaciers retreated, the land mass rose (post-glacial rebound), the clay was exposed, and formed the soil mass for new vegetation. The rainwater in these northern countries was quite aggressive to these clays, perhaps because it was softer (containing less calcium), or the higher silt content allowed more rainwater and snowmelt to penetrate. The final result was that the ionic 'glue' of the clay was weakened, to give a weak, loose soil skeleton, enclosing significant amounts of water (high sensitivity with high moisture content).
Quick clay deposits are rarely located directly at the ground surface, but are typically covered by a normal layer of topsoil. While this topsoil can absorb most normal stresses, such as normal rainfall or a modest earth tremor, a shock that exceeds the capacity of the topsoil layer — such as a larger earthquake, or an abnormal rainfall which leaves the topsoil fully saturated so that additional water has nowhere to permeate except into the clay — can disturb the clay and initiate the process of liquefaction.
Potential disaster 
Because the clay layer is typically covered with topsoil, a location which is vulnerable to a quick clay landslide is usually identifiable only by soil testing, and is rarely obvious to a casual observer. Thus human settlements and transportation links have often been built on or near clay deposits, resulting in a number of notable catastrophes:
- On 19 May 1893, a landslide in Verdal, Norway, killed 116 people and destroyed 105 farms. It left a crater several kilometers in diameter.
- The most disastrous such landslide to affect North America occurred in 1908, when a slide into the frozen Du Lièvre River propelled a wave of ice-filled water into Notre-Dame-de-la-Salette, Quebec causing the loss of 33 lives and the destruction of 12 homes.
- In 1955, a landslide affected part of the downtown of Nicolet, Quebec, causing $10 million in damages.
- On May 4, 1971, 31 lives were lost when 40 homes were swallowed up in a retrogressive flowslide in Saint-Jean-Vianney, Quebec, resulting in the relocation of the entire town when the government declared the area uninhabitable due to the presence of Leda clay.
- The experience of Saint-Jean-Vianney contributed to the abandonment of the town of Lemieux, Ontario in 1991, after a 1989 study showed it was also located on the same type of clay along the South Nation River. In 1993, those findings were borne out when town's abandoned main street was swallowed by a massive 17 hectare landslide.
- Another famous flow of quick clay at Rissa, Norway, in 1978 caused about 33 hectares (82 acres) of farmland to liquefy and flow into the lake Botn over a few hours, with the loss of one life. The Rissa slide was well recorded by local citizens and a documentary film was made about it in 1981.
- On May 11, 2010, quick clay took the lives of a family living in Saint-Jude, Quebec, when the land their house was built on suddenly tumbled down toward the Salvail River. The landslide was so sudden that the family members died where they sat; they had been watching an ice hockey game on television. The slide took out a portion of rural road which took a year to reinstate.
These landslides are retrogressive, meaning they usually start at a river, and progress upwards at slow walking speed. They have been known to penetrate kilometers inland, and consume everything in their path.
In modern times, areas known to have quick clay deposits are commonly tested in advance of any major human development. It is not always possible to entirely avoid building on a quick clay site, although modern engineering techniques have found technical precautions which can be taken to mitigate the risk of disaster. For example, when Ontario's Highway 416 had to pass through a quick clay deposit near Nepean, lighter fill materials such as polystyrene were used for the road bed, vertical wick drains were inserted along the route and groundwater cutoff walls were built under the highway to limit water infiltration into the clay.
- "Residents seek reassurance in wake of deadly slide". The Globe and Mail, May 12, 2010.
- Natural resources Canada - Landslides
- St-Jean-Vianney Landslide
- Lemieux Ottawa Valley Ghost Town - Canadian Geographic Magazine
- BFI | Film & TV Database | The RISSA LANDSLIDE (1981)
- "Family dead in basement after sinkhole ate home". CNN, May 12, 2010.
- Article, TVA, July 13, 2011
- "Conquering the Leda clay", Ontario Ministry of Transportation.