Rocks scratched by glaciers during the last ice age, leaving grooves called striations in the Precambrian (Proterozoic) rock surface, at Kosterhavet National Park on Mörholmen island in the Koster Islands in Sweden.
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/55118510@N03">Geological Survey of Sweden SGU</a>  (photographed by Thomas Eliasson)
CC BY 2.0
<a href="//commons.wikimedia.org/wiki/Flickr" class="mw-redirect" title="Flickr">Flickr</a>: <a rel="nofollow" class="external text" href="https://www.flickr.com/photos/55118510@N03/5128597660">Isräfflor, Mörholmen; Glacial striae</a>