The loch contains six islands, one of which - known as the Ash Island - is evidently artificial. It has been formed, as a writer in the Statistical Account says, "by driving strong piles of wood into the moss or marl, on which were placed large frames of black oak." These were discovered in 1765, when the loch was drained for the purpose of procuring marl.
Tradition says that in early days it contained two large islands - one at the north end, which is now a peninsula, but still retaining the name of "The Isle," while the other, near to the south end, is called "The Fir Island," and appears to have been rendered famous in history as the spot where Edward I., on penetrating into Galloway in the year 1300, encamped, using the island as a place for shoeing his cavalry. To strengthen this supposition, we may state that near to this place many horse-shoes, of a form different to those now in use, have been found sunk deep in the mud [...]
The loch was formerly much larger than it is at present; and tradition narrates that there was a town which sunk, or was drowned, in its waters, and that there were two churches or chapels, one upon each of the large islands. The submersion of the town is in all likelihood a myth, although the truth of the story is believed by many of the old inhabitants; and we have heard that occasionally, during very dry seasons - that of 1826 being specially referred to - the roofs of houses have been discerned submerged in the loch. [...]
You can also read about the ancient Three Thorns of Carlingwark which grew near the loch. "From time immemorial they were used as a trysting-place by the lairds and yeomen throughout Galloway; and in history we find repeated mention of them made in connection with stirring events."