I'm finding the lighting model a bit weird on this application, as features I've located on a LIDAR map generated for me are almost invisible on this one.
Anyway, it has reminded me to post that very image. I recently came across what appeared to be some significant excavations in woodland in Gleadless, Sheffield, strategically placed on high ground. They were relatively shallow (2-3m) and bowl-shaped - two large but separated features on the top of a small hill, and the others were a line of 6-7 along the ridge of a southwest-facing escarpment.
The two large ones are at least 10-12m across and may be smaller pits that were later connected, but it's very overgrown. The line of pits is more interesting - these are all elliptical in plan, almost touching each other, and descend down a gentle slope following the escarpment until it gets too steep. It may just be coincidence, but they also trace the line of the outcrop of a known sandstone unit, and are just metres from a main road and housing.
I asked my geologist friend to knock me up a LIDAR image of the area and there they were, clear as anything. My current thinking is they're ancient whitecoal pits as I can't think of an obvious mining use - the coal seams outcrop further east and I don't think there's ironstone here. They could have been dug for clay or maybe quarrying flaggy sandstone for roofing I guess, but the bowl shape makes it very impractical for that and they don't look filled-in. There's also no spoil heaps.
Below is the image with geology overlain, and also a crop of a hi-res scan of the line of holes:
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