Help!

This is a request to help me identify something I saw.

I recently plew from Gatwick to Vancouver. While over Greenland, which was almost entirely covered in cloud, I saw a black and perfectly straight line in the cloud far beow. The line was parallel with the path of the aircraft and based on the planes speed and period of time I saw it I estimate it was about 75 miles long. I wondered if it was a shadow cast by the plane’s condensation trail but it was to the sout of the aircraft.

Any ideas on what this line was? Shadow from a condensation trail from another plane flying to the south of our perhaps? (I couldn’t see another aircraft)