Strange line in the sky. What was it? Here's an explanation

Sunday morning saw a layer of high strato-cumulus cloud at around 7000 feet above sea level over parts of Yorkshire and Derbyshire. If you are interested, its scientific name is strato-cumulus undulatus translucidus. Some of you spotted a strange line across that cloud. See photos below:

This strange line was caused by a persistent aircraft condensation trail way above the layer of strato-cumulus cloud, up at around 30 000 feet in fact, casting a shadow on the cloud sheet below.

The above image taken over Derbyshire shows more clearly what is going on. Here the cloud layer is thinner allowing a view of the condensation trail which is much, much higher than the cloud layer.