Summit County, Colorado was one of the original 17 Colorado counties created by the First Territorial Legislature on November 1, 1861. It
was named for the many mountain summits within the county's borders (along the lines of how
Boulder County was named after its boulders). In 1874, the northern
half of the county was split off to form
Grand County, and the creation of
Garfield and
Eagle counties in 1883 finalized the borders which Summit
County has retained to this day.
Source: wikipedia.org