Need Help Identifying Rock

[mlkgeo]

Hi,

I've had this "rock" for about 2 years and it has been on a shelf reminding that I must find out what type of rock is this?

How does two seemingly different rocks get merged like that?

Thanks for any help!

Regards,

Mike

[jasong]

If I had to guess I'd say that rock wasn't merged, but grew more like a concretion on whatever the flatter rock is. 

There is also the potential it's some result of differential weathering/erosion where the rounded part is softer and the flat part is something harder like chert, and it spent time in a water course where the water rounded the softer part and the harder part was not as affected. But it looks more like a concretion to me.

[Geologyhound]

My first inclination is that the softer and lighter colored parent rock was fractured and the fracture was filled with the darker and harder material before this rock was separated from the original outcrop.  This can happen with sedimentary and igneous rocks by different methods.   As jasong mentioned, the rounding of the parent rock likely occurred by tumbling in a river or stream.

I’ve also seen this kind of shape when an asphaltic layer with some included iron fragments picks up mud and rusts it all together into an iron concretion.  But based on the picture, I think your rock is a weathered chunk of a filled fracture.