This looks like a rather shallow trail or man made ditch. Did this start out as something quite deep and perhaps filled with water? Otherwise, I’m surprised natural erosion has not smoothed out and reclaimed this dug out fissure in the terrain. Every place else is as smooth as a golf course.
There are other pics that make the scale of the thing more obvious. It’s longer than Hadrian’s Wall by a few miles. There’s a ten mile long earthwork that antedates the Romans, but called the Roman Ridge, so they didn’t get the idea from the Romans. Speaking of Hadrian’s Wall, I’ll go out on a limb again and say that it was the northern boundary of Aurelius’ kingdom.
https://www.google.com/search?q=offa%27s+dyke&tbm=isch