The Belt Supergroup is an assemblage of primarily fine-grained sedimentary rocks and mafic intrusive rocks of late Precambrian (Mesoproterozoic) age. It was named after the Big Belt Mountains in west-central Montana. It is present in western Montana and northern Idaho, with minor occurrences in northwestern Washington and western Wyoming. It extends into Canada where the equivalent rocks are called the Purcell Supergroup and are exposed in southeastern British Columbia and southwestern Alberta. Spectacular outcrops of these rocks can be seen in Glacier National Park in northwestern Montana and in Waterton Lakes National Park in southwestern Alberta.