Ripple marks on the bed of the Lake Mukrz in Tleń, Poland.
<a href="//commons.wikimedia.org/wiki/User:W%C5%82odzimierz_Wysocki" title="User:Włodzimierz Wysocki">Włodzimierz Wysocki</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span> (own work by uploader)