Needle ice is formed when the ground is above freezing but the air is below freezing. Water in the soil rises to the surface and freezes, forming tiny needles of ice that often push the top layer of soil two or three centimetres into the air.
Here we see an undisturbed patch of needle ice on the path that climbs up the south-western ridge of Yewbarrow. Needle ice can be rather misleading for the walker, making the path look clear and unfrozen as the layer of ice is covered by soil and even small pebbles.