Slope County was formed in 1915; it was originally a part of Billings County. However, when Billings grew too large, it was divided into three counties: Billings, Golden Valley and Slope. Therefore, if you're looking for information and can't find it in Slope -- particularly if it was before 1915, try Billings and/or Golden Valley. As well, Bowman was also originally part of Billings so you might want to check there too.
According to the U.S. Census Bureau and the U.S. Geological Survey, as of the 1990 Census, the population of Slope County was 907. The towns located within Slope County are Amidon, DeSart, Marmarth, Mazda, Pierce, Three V Crossing and Walser Crossing.
If you have resources for Slope County or would like to volunteer to help with look-ups, please e-mail me at Tim Stowell / Chattanooga, TN