When I was about 15 years old, I came across a similar earthwork on some undeveloped land that my family owned. The land was known to have been used in different ways at different times: A fairgrounds, a Home Guard fort location, an encampment, a farm. The small earthwork that I found consisted of a very narrow and shallow swale (smaller than the one in your photos) running along the contour of a hill and roughly parallel to a power line right of way. I had always done my camping and hiking along the ridge on the OTHER side of the hollow from this shallow trench, so it was a real discovery for me. It was not in the correct location to be a remnant of the earthen fort, and it would have preceded contour plowing by many, many decades. As has been suggested here, it was probably a remnant of a mule road or a trail. These discoveries are interesting, and we very often must be in the right location, at just the right time of day, with just the right weather conditions and foliage conditions to see them. They can go unobserved for a very long time. Then, too, a lot of folks just generally aren't very observant, so they miss features of this sort.