Tag Archives: leisegang

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. 

empty egg closeupempty egg shellempty egg

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

two disks 2two disks

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

horn of plenty 2horn of plenty1spiral diskTaco1

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

wicked smile 2wicked smile

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.

Geopic of the week: Prim boulders


Prim boulder is a name given to curious, round, often nearly spherical sandstone boulders that are common in the area around the town of Prim in Cleburne county, Arkansas.  Though the town of Prim boasts a noteworthy abundance of these unusual specimens, they actually have been found within a 100 mile area from Newton to White county.  The one pictured here is still attached to the outcrop of sandstone:  Many are found laying around at the surface, the rock from which they came having long ago weathered away. 

Geologists believe these formed by precipitation of calcite and iron minerals from ground water.  The calcite and iron minerals precipitate in concentric bands and make that part of the sandstone more resistant to weathering so that the boulders remain after the rest of the sandstone has eroded away. 

If you are interested in seeing Prim boulders for yourself there are many on display around the community of Prim within easy sight from highway 263.