Valleys2.3 - Be able to recognise the appearance of the principal naked-eye lunar surface formations ...
2.4 - Understand the structure and origin of the principal naked-eye lunar surface formations, including:
e - valleys
Valleys are long depressions on the Moon's surface.
They are difficult to find with the naked eye. Although they can reach hundreds of kilometres long, they are usually only a few kilometres wide.
Valleys are unusual because other features such as craters or rays (ejecta from impacts) intersect them.
They are thought to have formed by ancient lava flows, collapsed lava tubes or geological faults.
They are sometimes labelled as rilles - cracks or clefts on the lunar surface notably near maria. They are geological faults caused by collapsed lava tubes. When lava flowed on the Moon, the top and sides of these tubes solidified first while lava flowed underneath. When this lava moved away or solidified, the top or roof of the tube collapsed, leaving these rilles.