How and why is this formed?

How and why is this formed?

Observation - How and why is this formed? - UK and Ireland. Description: This was seen in a hill range called the Fannichs, in a stream that flows into Loch Glascarnoch, about 20 miles from Ullapool. Does anyone know how or why it was formed?

This was seen in a hill range called the Fannichs, in a stream that flows into Loch Glascarnoch, about 20 miles from Ullapool. Does anyone know how or why it was formed?