A large block of rock stands on the oncoming glacier. The moving ice plucks off or erodes weak rock fragments from the upper side of the rock. As the ice moves round and over the resistant rock it carries the eroded materials to the leeward side. The leeward side does not experience erosion. Eroded materials are deposited on the leeward side of the rock. With time the moving ice smoothens the side of the oncoming ice while deposited materials increase on the leeward side. The resistant rock is the crag while the materials deposited on the leeward form the tail.
OR
They are formed when there is a resistant block on the course of a glacier. The side of the resistant rock is eroded forming a crag. The opposite side where deposition takes place is called the tail.
maurice.mutuku answered the question on June 8, 2017 at 13:17
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