Unusual cloud formation

I don’t know what this cloud formation is called. I’ve been sky-watching for years, but never seen anything quite like this.
It looks like cirrus or alto cumulus but there is a strange ripple effect  which puts spikes in the sky.
It was the precursor to a line of thundery showers coming in from the North. The picture is taken taking facing NNE. Click the image to view a bit larger.