The potash deposits of North Dakota and Montana are extensions of the rich deposits now being mined in Saskatchewan. These deposits underlie 28,500 km 2 (11,000 sq miles) in northwestern North Dakota and 7,800 km 2 (3,000 sq miles) in northeastern Montana. The Middle Devonian potash deposits occur in beds within the Prairie Formation, an evaporitic sequence composed primarily of halite. The Prairie Formation evaporites were deposited within the Devonian Elk Point Basin, which extended from northwestern Alberta to the northwestern North Dakota-northeastern Montana area. On the U.S. side of the border, the Prairie Formation salt reaches a maximum thickness of 168 m (550 ft) in Burke County, North Dakota, whereas in Saskatchewan it exceeds 213 m (700 ft). North Dakota's potash ranges from 1,707 m (5,600 ft) deep near its eastern limit in northwestern Bottineau County to over 3,660 m (12,000 ft) in eastern McKenzie County. In Montana, the deposits range from 2,530 m (8,300 ft) in Daniels County to 3,500 m (11,500 ft) in Richland County. Three potash zones can be identified in Saskatchewan and North Dakota; two, in Montana. In North Dakota, the two major beds, the Esterhazy and Belle Plaine, reach a combined gross thickness of 25 m (83 ft) and a net thickness of 17 m (55 ft). In Montana they reach a combined thickness of 19 m (61 ft) gross and 13 m (43 ft) net.