 To the south of Thirlmere is its feeder valley of Wythburndale, which rises eastward to its source below Greenup Edge.Above Wythburndale Ullscarf displays a near continuous line of crags, the principal faces being Castle Crag and Nab Crags.A series of low tops crown the edge above Nab Crags, one of them bearing a prominent stone structure visible from the valley below.