Rectangular <a href="https://en.wikipedia.org/wiki/Joint_(geology)" class="extiw" title="w:Joint (geology)">joints</a> in siltstone and black shale within the <a href="https://en.wikipedia.org/wiki/Utica_Shale" class="extiw" title="w:Utica Shale">Utica Shale</a> (<a href="https://en.wikipedia.org/wiki/Ordovician" class="extiw" title="w:Ordovician">Ordovician</a>) near Fort Plain, New York.
<a href="//commons.wikimedia.org/wiki/User:Rygel,_M.C." title="User:Rygel, M.C.">Michael C. Rygel</a>
CC BY-SA 3.0
<span class="int-own-work" lang="en">Own work</span>