Glacial striations on basalt ‘tiles’.We recently did…

Glacial striations on basalt ‘tiles’.

We recently did a post on the basalt columns of the Devil’s Postpile national monument in California (see http://tinyurl.com/lznpc9o), and some readers commented that the top looked like tiles. Long after the lava lake cooled, the entire rock formation was eroded and neatly planed off by ice sheets during the Pleistocene glaciations that ended around 10,000 years ago. The lines running from top to bottom reveal the general direction of ice movement, and are caused by rocks and pebbles embedded in the glacier’s bottom scraping across the basalt surface as it eroded it to its current tile like appearance. If you run your fingers down the striae, you can feel which way the ice went, as they are smooth in the direction of travel, and rough against it.

Loz

Image credit: Eric from SF/Wikimedia Commons