The inimitable and humorless British geologist Arthur Holmes wrote “Micas all have perfect cleavage...” He continues in a departure from this alluring image, “because their tetrahedral sheets and bonding atoms are all arranged in parallel layers.”

It is the reflective, sparkly qualities of mica that give Chestnut Hill our characteristic soils. It might be easy to overlook, except that these sparkly flecks in the rocks, the soils, and the sands are such a defining aspect of our local area, right at our feet. Almost every turn of the soil here reveals more delightful remnants of that perfect cleavage and millions more tiny flecks shining and sparkling in the sun as nowhere else in Philadelphia.

Rain, running in small rivulets, leaves alluvial sands with mica flecks that dazzle like a spangly Taylor Swift outfit. Like the pink coral sands of Bermuda and the dark volcanic sands of Hawaii, our silvery mica sands characterize Chestnut Hill.

The geology has a fascinating story. Our mica comes from rocks that have been crushed, pummeled, transported, and twisted 500 million years ago at depths of many miles before being uplifted, moved thousands of miles, and then eroded away. Our Appalachian Mountains were taller than the Rockies back in their heyday 500 million years ago.

The thought amazes as we live so happily among the luxurious Wissahickon flora and fauna at the lower levels of those once colossal mountains.

Beginning 1 million years ago, our uplifted, twisted, and metamorphosed bit of Wissahickon Park in the Piedmont Province has been cut continually by the creek, dropping 100 feet to the Schuylkill River going toward the Coastal Plain, which starts near the Philadelphia Museum of Art and goes to the shoreline, all of it moved from far away by the power of various continental tectonic plates.

Mica’s shimmering and sparkling qualities are why it is widely used in cosmetics. Milled to a powder and mixed into skin lotions, mica makes the bewitchingly glowing and brightening quality of some powders and creams.

My late geology professor at Penn, Dr. Robert Giegengack, once wanted to help us understand eons, epochs, ages, and the millions of years here and there. He took a roll of toilet paper, unspooled it a bit, and rolled it back up, equating its 1,000 sheets to what we know as the geological time way back before amoebas and humans evolved from the ooze. Then, Giegengack took a very sharp pair of tailor’s scissors and said, “Here is the time man has been on earth relative to all the time we know rolled up here.” He cut the fuzz off the end of the last piece of toilet paper. Just the fuzz! That is our place in history.

Mica flecks and mica sands get tracked inside everyday, as we can see by putting a flashlight on the floor to see their sparkles. In our Wissahickon neighborhoods, mica is everywhere and has been for many sheets of paper before. It remains to sparkle, fascinate, and bewitch us all.

George Cadwalader

Chestnut Hill