Slope County is a county in North Dakota, United States, on the state’s badlands border with Montana. It had a population of 706 at the 2020 census and covers 1,219 square miles (3,159 km2) centred near 46°27′N 103°28′W, making it both the least populous county in North Dakota and one of the least populous in the country.
Amidon is the county seat, and its postal code is 58620. The county has two incorporated cities (Amidon and Marmarth), a handful of unincorporated communities, and no organized civil townships. Slope County lies in the Mountain time zone.
Towns & Villages in Slope County
Slope County’s population is spread across two small cities and several unincorporated places and ghost towns scattered along the Little Missouri badlands. Amidon, the seat, and Marmarth, the largest city, are the only incorporated places; everything else below is unincorporated (Unincorp.).
| Place | Type | Pop. | ZIP |
|---|---|---|---|
| Amidon | City · seat | 24 | 58620 |
| Marmarth | City | 101 | 58643 |
| Pierce | Unincorp. | — | 58620 |
| De Sart | Unincorp. | — | 58620 |
| Mazda | Unincorp. | — | 58620 |
| Mound | Unincorp. | — | 58620 |
| Three V Crossing | Unincorp. | — | 58620 |
| Walser Crossing | Unincorp. | — | 58620 |
De Sart, Mazda, Mound, Three V Crossing and Walser Crossing are historic rural place names on maps and in county records rather than active settlements with services; none carries its own post office today, so mail for that side of the county is addressed through Amidon.
ZIP Codes
- 58620 — Amidon (also serves Pierce and the county’s eastern communities)
- 58643 — Marmarth (also serves the county’s southwest corner near the Montana line)
These two post offices cover the entire county; there is no third ZIP code inside Slope County’s borders.
Geography
Slope County sits in the North Dakota badlands, cut by the Little Missouri River and its tributary breaks, with rugged buttes, grassland and exposed lignite outcrops typical of the region. Of the county’s 1,219 square miles, 1,215 square miles are land and about 4 square miles are water. Elevations range from river-bottom lowlands to the high buttes near the Montana border, and much of the terrain is unsuited to row-crop farming, supporting cattle ranching instead.
Slope County borders Billings County to the north, Golden Valley County to the northwest, Stark County to the northeast, Hettinger County to the east, Adams County to the southeast, Bowman County to the south, and Fallon County, Montana, to the west.
Demographics
The county’s 706 residents at the 2020 census are concentrated in its two cities: Marmarth, the largest, and Amidon, the seat. Slope County’s median household income runs below the North Dakota median of $76,525, reflecting its ranching-dependent, sparsely populated economy. Population density is under 1 person per square mile, among the lowest of any county in the contiguous United States.
Economy & Local Life
Cattle ranching is the backbone of the Slope County economy, with large grazing operations spread across the badlands terrain. Oil and gas activity tied to the wider Bakken and Williston Basin region has touched the county at its margins, though development here is lighter than in counties farther north. Amidon’s courthouse and a handful of small businesses anchor the seat, while Marmarth retains a rail-era Main Street and hosts the annual Marmarth Mineral, Fossil and Gem Show, drawing visitors interested in the area’s well-known badlands fossil beds.
History
Slope County was created on November 3, 1914, and organized on January 14, 1915, carved from the badlands terrain west of the Little Missouri River. It takes its name from the Missouri Slope, the broad geographic incline that defines much of western North Dakota. Marmarth grew as a Milwaukee Road railroad division point in the early 1900s, briefly outgrowing Amidon before the rail traffic declined; Amidon was retained as county seat and today remains one of the smallest county seats in the United States.
Getting There
U.S. Highway 12 crosses the northern part of the county through Marmarth, and U.S. Highway 85 runs along the eastern edge; North Dakota Highways 21 and 67 provide the remaining local connections.
- Dickinson, ND — about 55 miles northeast
- Bowman, ND — about 25 miles south
- Baker, MT — about 35 miles west
- Belfield, ND — about 45 miles north
The nearest commercial airport is Dickinson Theodore Roosevelt Regional Airport (DIK).
Frequently Asked Questions
Where is Slope County?
It is in southwestern North Dakota, United States, bordering Montana along the badlands of the Little Missouri River.
What is the population of Slope County?
706 people, per the 2020 census — the least populous county in North Dakota.
How big is Slope County?
1,219 square miles (3,159 km2), of which about 1,215 square miles are land.
What is the Slope County seat?
Amidon is the county seat.
How many towns are in Slope County?
Two incorporated cities, Amidon and Marmarth, plus several small unincorporated places.
What ZIP codes are in Slope County?
58620 (Amidon) and 58643 (Marmarth) cover the entire county.
What is Slope County known for?
Its badlands scenery, cattle ranching and Marmarth’s well-documented dinosaur and fossil finds along the Little Missouri River.
More counties in this state: Towns and villages in North Dakota.