This was formerly a sandstone quarry.
Before that, as shown on the 1:10560 OS map from 1864, there was a small hill called Fairy Knowe at this location ("knowe" is equivalent to "knoll"), with a summit height of 574 feet; there was, as yet, no quarry here. The equivalent maps from 1899 to 1932 show the quarry eating away at the southern face of the Knowe, and expanding from there. I have used the title "Carman Quarry", because that is what appears on present-day mapping, but it should be noted that it this was a later name; it was originally called the Fairy Knowe Quarry.
The name Fairy Knowe may be an indication of what Joseph Irving, writing in 1879, described as "a once popular superstition which still lingers in the locality"; see NS3875 : Havoc Hole
for more details. In the same work, Irving also wrote that Carman Hill is "celebrated in the superstitious legends of the district". He does not give the details of these legends, but (as will be explained below) they are likely to have been connected with the Fairy Knowe, which, as my photograph shows, is located on the lower slopes of Carman Hill.
(in a Geograph article) for more on these traditions, and Link
for more on the quarry itself.
The colour of the underlying soil is best seen in the mound at the right-hand side of the photo. That mound is not the Knowe. The latter was much larger: it embraced all of the green area that appears in the foreground and middle distance of the photograph, and more besides. To the north, it extended to the line of trees that can be seen in the middle distance. Those trees stand on top of a crescent-shaped ridge that separates the green edge of the quarry from the brown hillside behind it. The ridge is a remnant of the Fairy Knowe; in fact, it is what remains of the northern edge of the Knowe: NS3679 : Northern edge of the Fairy Knowe
Some features of geological interest can still be seen in the old quarry: NS3678 : Traces of ancient mud cracks
To the left, the hill in the background is Carman Hill, and the "bump" visible to the right of its summit is the location of some boulders that lie within an ancient hill-fort: NS3779 : Ancient hill-fort on Carman Hill