BILLINGS, Mont. (AP) — One of the largest coal-fired power plants in the western U.S. will close two of its four units in coming days as the Montana facility edges toward an eventual total shutdown.

Colstrip Units 1 and 2 — built in the 1970s when massive strip mines were being developed across Montana and Wyoming — will close by Jan. 5 or as soon as they run out of coal to burn, Talen Energy spokeswoman Taryne Williams said Thursday.

The plant employs about 300 people and is the main driver of the economy for the surrounding town of Colstrip, which has about 2,300 people. But it’s been unable to compete with surging investments into renewable energy and cheap natural gas, as the coal plant’s operating costs have risen with the need for better pollution controls.

Some employees for now will be re-assigned to decommissioning work that will last through mid-2020, Williams said. She said there are “no hard and fast numbers or timelines” as the company considers how many workers will be needed for the remaining two units.

The closure of Units 1 and 2 was long anticipated as demand for U.S. coal collapsed in recent years, and came despite vows by elected officials in Montana to find ways to keep it open.

The two closing units are operated by Pennsylvania-based Talen, which co-owns them with Puget Sound Energy of Washington state.

Sen. Duane Ankney, who represents Colstrip in the Montana Legislature, said the impending closure was a “prime example” of how out-of-state interests were hurting the coal industry to the detriment of Montana.