In these two pics you can see more how the liner was finished off under the cap.
In the first pic, you can see the red brick on the left. This is what the flagstone was glued down onto. The liner was cut off even with the top of the redbrick. If you look real carefully you can see the white underlayment as well that is under the liner.
The second pic shows the end of a garden edger that was on the pond side. It of course just sat on the edge and held the liner in place, The flagstone cap was also glued to the top of the edger to help hold it in place. These edgers have broken down now after 17 years in the water. Interestingly, the edgers in the top pond are the worse. Those in the lower pond have actually held up and are 95% full intact. About 40-50% of the upper pond edgers have broken down and need replacement. This pond is being torn down in the spring and is being replaced by our new pond.
Craig