These earthworks are the waste material from medieval iron pits, deposited around the openings of shallow shafts that led down to the ore bed. The shafts have now collapsed but, in the hollows left by subsidence, trees and bushes have found protection. There is no evidence that iron was worked here after the sixteenth century.
Zoom in and you can make out the remnants of ridge-and-furrow ploughing around the four bottom-left pits, but the 1953 aerial photo on page 257 of Beresford's "Medieval England - An Aerial Survey", second edition 1979, clearly shows that the ploughing predates the pits. Since the working of the pits began in the late twelfth century, all of this site was arable farmland over 800 years ago.

Photo Copyright Chris Allen