Geopic of the week: Ironstone concretions

I’ve posted a little about concretions before, but on a visit to the Capitol building in DC this week our tour guide pointed out small iron concretions weathering from the pillars which he said have been mistaken for British bullets that are lodged in the sandstone columns. 

Concretions, if you recall, are mineral masses that grow in rocks by precipitation, usually from mineral-rich groundwater.  They are often visually interesting and probably the most misunderstood rock or mineral on earth.  Like billowing cumulus clouds, they take on fantastic shapes that  lead to colorful interpretations from the uninformed observer. 

Here is a collection of odd or interesting concretions gathered from various parts of Arkansas. 

This one in the above pictures is just begging to be called a fossilized egg.

These are a couple of disk-shaped concretions that grew between beds of shale.  If we were to cut them open they would look something like these next ones

The iron minerals in the above examples have arranged themselves in near-concentric bands. 

This same concentric precipitation often leads to so-called “pipes” that are stuck in rocks like these here

Finally you have my personal favorite – Ancient Native American dentures!

 

Hope you enjoyed these and next time you find an interesting concretion you’ll know what you’re looking at.