SADLER, Tex. -- Grass fires had half a dozen departments on alert Tuesday. Cross ties and burning creosote emitted black smoke that could be seen for miles. Stephanie Brletic reports from Southmayd at the American Cross Tie Walls on U.S. Highway 82.

The fires have been burning for hours, with six departments helping put out the flames. Firefighters tell us they will probably be out here for a while because the wooden cross ties are treated with creosote, an oily wood preservative.

Firefighters have to use Class A foam to fight the fire at American Cross Tie Walls, where one acre of cross ties stacked at least 5 feet high have burned to the ground.

The flames here are so intense that the firefighters are in “defensive mode,” meaning they are spraying the foam on the fire and using water to contain the fire.

Firefighters say they are still unsure of what caused these flames, but say it is not the first time this location has caught fire.

"It's not the first time that this has happened out here, so you know there will be hot spots for probably all night. We'll just see where we stand, every 30 minutes to an hour we'll do operational updates," Bill Macon of the Sherman Fire Department says.

The firefighters have been on the scene for around five hours, and say the fire is under control, but that it will need to be monitored throughout the night for hot spots.

No one has been injured, and no structures are in danger.