My 100+ year old floors were recently refinished, and for the most part, they look great. But there are some long dark arcing marks across many boards in a few rooms that look like a mistake. The contractor says those are from ancient damage to the floor that has taken on its own natural "stain" over the years. I asked if he could have sanded deeper to get rid of them, but he said that the floors were already sanded as far as they could go. There are also large dark patches in the middle of most rooms where there used to be linoleum tiles, but he said that's normal too.

Does that all sound like a reasonable explanation? I can live with them if that's just the way it is, but I want to be sure that they aren't marks from the contractor himself. He's a very experienced refinishing contractor, especially with these old floors, but I'd like a second opinion. Thanks.

Photos to come next...

I couldn't get these to post in here, but here's a link to the photos of my floor if you don't mind checking it out...

https://postimg.org/gallery/n7ppytnq/

From the pictures it looks like you have a vertical grain fir or pine floor. These floors are soft and bruise from pressure on the surface that affects the absorption
of the finish. Sometimes it will show up only after the finish is applied.
Sanding deeper can get the marks out, but this also makes the flooring thinner.
With an old floor that has been sand more than twice the flooring get thin enough to stress the top part of the groove which can break away from the rest of the board causing a splinter. If the floor has been sanded thin you may not have any splintering until the floor gets used/ stressed more and some of the grain will break at the top of the groove. To help keep your floor serviceable I recommend, if you can access it from below, to fasten strips of blocking between the floor joists so the boards move in unison when stressed by traffic so the top edge of the groove will not break.
All it takes is a 2 inch wide rip of half-inch plywood screwed to the underside of the flooring between the joists, with a bead of sub-floor adhesive run down the middle of the strips. This will make a major difference to help preserve the serviceable life of the floor. Even just re-enforcing at the doorways and high traffic areas is a good investment. You most likely can not get the quality of wood that you have in your old house.

Thanks for the reply. From the photos, and from knowledge of the sanding/refinishing process, does it appear that the bruising might be from the contractor himself? Or is it more likely old damage from previous owners moving furniture, etc?

For more background info...

From the original appearance of the wood and layers of carpet and/or paint and/or linoleum that was removed from various rooms, I do not believe this floor has ever been sanded or refinished previously since it was first laid down around 1900.

This is damage from years ago.

Thanks for taking the time to share your insights.