The Concord Fire Department spent Wednesday morning fighting a stubborn brush fire burning on a steep bank between the Merrimack River and the rail line just south of Sewalls Falls Road.

“We think it had been smoldering for a couple of days,” said Capt. Jeff Stone, battalion chief at the scene. No buildings were involved and no injuries reported from the blaze, which covered about four acres of very steep riverbank.

Stone said there is no obvious cause to the fire. Although it extends to the edge of high-voltage power lines it does not appear that electric sparking was a cause and there are no homeless encampments nearby where people might have lit a fire that accidentally spread. Because fire travels uphill much more readily than downhill, Stone said it is unlikely to have started next to the CSX line and spread down to the river.

The department established its command center alongside the Concord Monitor building on Sewalls Falls Road and used off-road equipment to get to the fire. Stone said the rail company shut down traffic on the line, which is little used these days, and brought in a truck that can ride on rails to help shuttle equipment there.