One of the most obvious features of the Yorkshire Dales landscape is the numerous limestone drystone walls that snake across the land. These are often very old, built of stones without any mortar. They can look as though they are just a pile of stones thrown together, but in fact there is a lot of skill involved in building drystone walls. Some of them have holes for livestock to move through - small bolt holes for rabbits, larger 'cripple holes' for sheep. These walls criss-cross the valley floor in Upper Wharfedale and some extend right up the valley sides. Around Kettlewell, there is evidence of an Iron Age (approx 499 BC - 71 AD) co-axial field system - long field boundaries that run from the valley bottom right up to the high limestone plateau.... though most of these walls are more like 150 - 200 years old.