|Sparky and stromatolites, Medicine Bow Mountains, WY|
It’s a crisp but glorious Indian Summer day in the Holocene Epoch, and Sparky is standing next to Precambrian stromatolites ... the fossilized remains of a 1.7 billion-year old mat of cyanobacteria (aka blue-green algae) that grew in a warm shallow sea. This is what I love about geology, the opportunity to look far into the past and unravel a tiny bit of the surprising mysteries around us. Uncovered Earth said it so well when he wrote: “To appreciate a thing, you must first come to know it”.
For my knowledge and appreciation of the stromatolites of the Nash Formation, I thank one of the great legends of Wyoming geology, Dr. Samuel H. Knight. It was because of his “Precambrian stromatolites, bioherms and reefs in the lower half of the Nash Formation, Medicine Bow Mountains” (1968) that Sparky and I were able to hike along Precambrian reefs, for Doc Knight had drawn a detailed map of reef locations (click to view).
|Snowy Range, southeast face|
The stromatolitic Nash Formation also was severely deformed, but the rocks underwent only low-grade metamorphism, hence the wonderful preservation of stromatolites. In spite of multiple episodes of uplift and folding, the original orientation of outcrops often can be determined from the stromatolites themselves. In the next photos, the V shapes (as well as the black arrow and shoe) point to the ground at time of formation (the Vs are gaps between adjacent algal mounds).
The Nash Formation is crossed by sill-like masses of gabbro that had not been dated as of 1992.
|Gabbro intrusion in Nash Formation in foreground, with beautiful subalpine lake for scale.|