No survivors in Quebec slide – The Globe and Mail.
The ancient ice age has taken its modern victims. Only the family’s golden retriever, tied outside to a tree, survived.
The article linked above describes the land left behind when the inland Champlain Sea receded, and how the clay shifted, creating the sinkhole. The clay itself is well described in a Wikipedia article. The link follows.
The Champlain Sea extended to the Upper Ottawa Valley. A map is available here:
Minor earth tremors, human drilling, and excessive rain and snow, are among the factors that can change the quality of the clay in an instant. I grew up in the Ottawa Valley, not knowing the land below could be so treacherous.