I went back recently to Lemieux, Ontario, a village that stood on the South Nation River south of Ottawa. I'd first gone there in 1990 when it was being demolished because it had been built over a layer of Leda clay, and there had already been several landslides along the river. At the time, the residents thought the town was safe because it had been there for a century.
Leda clay is a clay that is formed in salt water. The St. Lawrence valley and area was covered by the Champlain Sea after the last Ice Age and there are Leda clay deposits in many places in the St. Lawrence, Saguenay, Ottawa and Richelieu valleys. There have been many landslides in this area - in 1971, 31 people were killed in one in Saint-Jean-Vianney, Quebec, which had been built on the same type of clay.
Leda clay is held stable by the salt in it, but as the ground water leaches out the salt, it becomes unstable and can liquify suddenly. That happened in Lemieux on June 20, 1993, and fortunately there was nothing there anymore. One man was hurt after his truck hurtled into the 30 foot deep crater that had suddenly formed where the road had been moments before. The clay liquified and flowed into the river, taking the fields and trees that were on top of it along with it. There was a grove of birch trees that remained standing straight up when they moved like a ship with masts into the place below where the road had been. In many places, it just looked like the ground had suddenly sunk down 30 feet.
A lady let me onto her land to take a picture of another landslide along the river which had happened years earlier that shows what Lemieux looked like after the landslide - she said she was also offered money to move off by the government but she said it wasn't enough and got a geologist to write a report saying that the area I was standing on was still stable:
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