This story took place in New Brunswick. In the 1870s, the area was plagued with turmoil- bad crops, tuberculosis, harsh weather, etc. Like most witch hunts and executions, these factors led to extreme paranoia. People felt the need to blame their hardships on something or someone, usually evil forces and witches.
The blame eventually turned to a local girl named Rebecca Lutes. She was hung and buried at the base of the tree from which she was executed on January 2, 1876. She was sixteen. The young farm girl’s body was placed face-down in the grave, so that if she were to dig herself out, she’d be digging downward- a slow route to hell. They also took the time to cover her with concrete.
There is much more to this story, and a LOT of suspicious details. You can read about it here: http://mynewbrunswick.ca/folklore/rebeccas-concrete-grave/