When rainwater seeps down cracks in limestone it reacts with the rock and gradually dissolves it away. Over time, the cracks become wider and deeper, forming depressions known as sinkholes or shake holes. In limestone country, streams sink into the cracks and flow underground, leaving dry stream beds which only run after heavy rain. The streams reappear at the surface as springs where limestone gives way to other rock types such as sandstone or mudstone. Buttertubs Pass between Swaledale and Wensleydale has a limestone landscape feature called the Buttertubs. These provide a visible example of vertical-sided sinkholes that have been cut into the limestone, leaving between them undercut and unstable rock pillars about 10m tall.