What is a "perched lake"

One of the top sites on Fraser Island is Lake Mackenzie. The lake starts from a large depression in the sand. Leaves and other debris fall into the depression and over a very long period of time form an impervious layer at the lake bottom, like a pool liner. The lake then just fills up with rainwater. There is not other water source; no springs, no creeks etc. The water is crystal clear and the bottom is beautiful sand, just like the beach.