The Lore and Legend of a Cambrian Rock

I suppose practically everyone has a favorite rock, though it is hard to get people to talk about something so personal. One imagines most people’s favorites would be gems and crystals since they are likely to be prettier than the more utilitarian rocks. However, a list of state rocks, minerals and gems vaulted into public prominence by solemn acts of legislature reveals surprisingly many ordinary stones: coal (three states), granite (four states), marble (three states), limestone (two states) and even various forms of sandstone (four states). One wonders what lobbies might have greased such legislative choices: a secretive Society for the Promotion of Common Objects perhaps? A branch of the Masons?

 http://en.wikipedia.org/wiki/List_of_U.S._state_minerals,_rocks,_stones_and_gemstones

At the risk of saying too much about myself, I confess that my favorite rock is Erwin Formation/Antietam Formation quartzite, the southern branch of which is named after a site in Tennessee, and the northern branch of
which is named after a site in Maryland.

These rocks could also be referred to as members of the Chilhowee Group, a collection of sedimentary and metamorphic (or metasedimentary) rocks laid down in the long-gone Iapetus Ocean during the late Precambrian-early Cambrian era. As to when this was exactly, it was quite a ways back on the calendar. These rocks were already fairly old when the Boston Americans beat the Pittsburgh Pirates in the first World Series. For a long time, they were literally prehistoric. Eventually some histories were written, but if you go back far enough this quartzite wasn’t even rock yet. It was sand.

These beds of fused quartz sand, tending toward white but striated with reddish, purplish or brownish zones of mineralization, contain long straight tubes called Skolithos, presumed to be the fossilized burrows of marine worms. Skolithos is a highly characteristic early Cambrian trace fossil, that is, a trace of an animal rather than the animal itself. Whatever left the Skolithos, it was soft-bodied and did not get preserved.

The typical position of the major ichnofacies in marine and continental environments: Cr - Cruziana; G - Glossifungites; N - Nereites; Ps - Psilonichnus; Sc - Scoyenia; Sk - Skolithos; Te - Teredolites; Tr - Tripanites; Z - Zoophycos
(after Benton & Harper, 1997)

From: http://www.es.ucl.ac.uk/tf/ichno.htm

As this chart shows, the Skolithos sand beds (marked Sk) seem to have been offshore shelves of the old beaches that once lined the Iapetus Ocean, wave-sorted into homogeneous layers of shifting sand like the Outer Banks of the Carolinas today. As this ocean system closed up, the shifting would have slowed down and the type of sediment flow would have changed. Eventually subduction, pressure, and heat would have transformed the terminal state of the most stable sediment beds into rock. Later re-exposure, re-burial or re-heating would have
altered the rock type again---in the case of the Erwin, from sand to sandstone to quartzite, preserving the worm tubes throughout. The resulting rocks don’t exactly represent the entire topography of the Iapetus coast in a direct way. The Erwin/Antietam quartzite was formed from the most stable sediment beds that survived the subsequent alterations to that topography.

Cambrian quartzite is well exposed on the west side of the Blue Ridge across Central Virginia, in a lens over a thousand feet thick in some places. Its place in the Chilhowee Group helps tell the story of the Iapetus Ocean, which started as a volcanic rift valley of which the Catoctin basalt flows are the relict. For more, see below.

http://www.nps.gov/shen/naturescience/chilhowee.htm

The group started as river sediments and then as lake and lagoon deposits until the ocean was fully open, at which point white sandbanks along the shores of a deep sea provided a habitat for some of the earlier life on this
planet. Tropical limestone reefs and lagoons followed before the Iapetus was pushed back together and closed. It all sounds very exotic but the most amazing part is that it wasn’t a different world.

But I digress. This rock makes fabulous cliffs and excellent talus slides, and it also makes a great cobble.

The broken fragments of the Erwin/Antietam Formation slither downhill and then march down creek beds in ceaseless phalanxes like the brooms of the Sorcerer’s Apprentice, wearing themselves into characteristic cobbles
that can be ordered from the garden center and used for walkways.

To me the greatest mystery in the love of Ignatz Mice for Krazy Kat is the substitution of a manufactured brick for a decent cobble. Thank you, George Herriman, for your rich insights into the emotional lives of animals, plants, and minerals as seen below in a panel from the immortal comic strip.

But they can coexist… I hope.

As an added bonus, here is the newest inhabitant of our rockwork.

We have devised a habitat that could hardly be improved on, for an insectivorous lover of sun-baked rocks and porches.

It's a female, possibly "with eggs," and like many an animal deciding where to live, she has "voted with her feet," as it were.