The Conococheague Formation
During a small window of nice weather earlier this week I decided to drive around and observe some of the various limestone formations exposed within the Shenandoah Valley, while at the same time getting to see the Virginia State Arboretum for the first time (because who doesn’t like arboretums?). My first stop was at Mulberry Run just north of Strasburg to view an exposure of the Conococheague Formation along Route 55. This is an early Cambrian deposit of limestone that formed roughly 520 million years ago.
This outcrop has a lot of interesting features. It has interbedded dolostone (light gray) and limestone (medium gray).
The limestone contains spherical grains called ooids. Ooids form in shallow warm tropical seas like the Bahamas under the influence of wave action. Here is a close up of these ooids looking through my hand lens:
This is truly a window into a different world! The Appalachian mountains have not yet come into existence and the south east portion of our continent is engulfed in a shallow sea. Within this sea are mats of cyanobacteria that form large dome shaped colonies called stromatolites. Here is one of the stromatolites you can find at Mulberry Run:
Here’s a closer look: 
Below is a picture that I borrowed from sharkbay.org of living stromatolites in Western Australia:
