A 'little' ice, gravel, and water - Terminal Moraines
Terminal Moraines are large gravel banks stretching across a glaciated glen.
Terminal Moraines are formed by a glacier bulldozing it way down a glen, scraping up and pushing a pile of gravel before it. Terminal moraines mark the furthest advance of a glacier, and by implication the point from which they began their retreat.
In the right conditions, a loch will form behind it as the glacier melts, the loch may grow slowly filling the glen until the surface of the loch reaches the lowest part of the moraine, and begins spilling over. If the size of the loch, and the size of the moraine are compatible - the moraine will continue to hold back the loch as the overspill becomes a burn. If the size of the loch, and the size of the moraine are incompatible - the overspill will cause a catastrophic breach, draining the loch in a geologic-instant.
Among the most obvious terminal moraines in the upland of Mar is Derry Dam (prehistoric) in Gleann Doire. On the approach from the south it appears very much like a huge man-made earthwork dam. It would have held back a loch about 2-miles long before the almost enevitable catastrophic breach still evident at its eastern end, through which Uisge an Doire continues to flow.