Complex sentence: The loch was about 3 miles in circumference and from six to twenty feet deep which, when drained, was found to contain two small islands or lake dwellings of the pre-historic inhabitants.
Split into simpler sentences: The loch was about 3 miles in circumference and from six to twenty feet deep, but was subsequently drained. It contained two small islands which were found to be lake dwellings of the pre-historic inhabitants.