Time is a flat circle

I've been sent two mysterious local circumferences that need explanation. The former is presumably the temporal shadow of the Hilly Fields bandstand, but what's the latter?

David asks: "I recently discovered that some of the shapes on Hilly Fields are the outlines of huts from WW2 but what is the rather large circle?"

Any ideas what this could be? Unearthed behind bramble and ivy on the Buckthorne Railway Cutting. @harbro @Eco_CP_Library @BrockleyCentral pic.twitter.com/KjgfYbyNmX

— Fourth Reserve (@FourthReserve) July 28, 2018