St. Peter's Mission was founded approximately in 1874 when Sister Amadeus and three other Ursuline nuns came to Cascade. St. Peter's Mission was a boarding school/academy for Native Americans. When Sister Amadeus became ill, Mary Fields also known as Black Mary, an indentured servant of her family, came to Montana to live with her. Black Mary stayed in the area and there are quite a few stories written and told about her.
At the time of Black Mary's arrival, St. Peter's Mission buildings were in need of repair. Mary, was six feet tall, heavy, tough and as strong as any man and soon she became the foreman of the project. However, after an altercation with a fellow worker Mary was asked to leave the Mission. In 1895 she found a job that suited her, as a U.S. mail coach driver for the Cascade County region of central Montana. She never missed a day, and it was in this position that she became a legend in her own time known as Stagecoach Mary, for her ability to deliver on a regular schedule. She died in 1914 in Cascade, Montana. Neighbors buried her in the Hillside Cemetery in Cascade, marking the spot with a simple wooden cross which may still exist today.
Basically, St. Peter's Mission remains today only as a landmark in the area, although there is still an occasional mass said to mark special events. Still visible on the hill above the church is the old cemetary.