Detailed view of the Alabama river rock used to surface the NASA <a href="https://en.wikipedia.org/wiki/Crawlerway" class="extiw" title="w:Crawlerway">Crawlerway</a>. Loose rocks provided a softer and more robust surface for the giant crawler-transporters to move on than solid asphalt.
NASA
<a rel="nofollow" class="external free" href="http://environmental.ksc.nasa.gov/projects/crawlerWay.htm">http://environmental.ksc.nasa.gov/projects/crawlerWay.htm</a> (<a rel="nofollow" class="external text" href="http://environmental.ksc.nasa.gov/assets/graphics/crawlerway14_tn.jpg">image source</a>)