Quick clay is also known as Leda Clay in Canada. It is a unique form of highly sensitive clay, that has been the underlying cause of many deady 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. [1]

Undisturbed Leda Clay resembles a water-saturated gel. It is a favourite trick of geotechnical engineers to place a nice block of this clay upon the hand, and then strike it. The clay instantly turns into a flowing ooze.

This type of clay is only found in the northern countries that have subjected to past glaciation (mainly Russia, Canada, Norway and Sweden). 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 floculation to take place. Essentially, this formed a strongly bonded soil skeleton, which was 'glued' by highly mobile sea-salt ions. [2]

At this point, there was only the formation of very strong marine clay, which is all over the world, highly stable, but with it's 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.

For various reasons, the rainwater in these northern countries was quite aggressive to these clays, either because it was softer (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 senstivity with high moisture content).

Add human habitation and the scene is set for catastrophe. The most disasterous such landslide occured in 1908 at Notre-Dame-de-la-Salette, Quebec, with the loss of 33 lives.

These landslides are progressive, meaning they usually start at the river, and progress upwards, at a slow walking speed. They have been known to penetrate kilometers inland, and consume everything in their path.