An Excerpt from Jim Conrad's
Naturalist Newsletter of May 29, 2005

issued from California's Sierra Nevada Foothills

SLATE

slateFred and Diana have a friend who paints miniatures on small slabs of slate but she lives in a slateless part of Utah. Therefore, a few days ago we made a trip up into the Sugar Pine zone, to Slate Mountain. We drove only about five miles but the trip was nearly entirely uphill and along unpaved national-forest roads so Slate Mountain looked, smelled and felt a lot different from here.

Now, if you take a layer of clay such as that found in a mudflat or on an ocean floor at the mouth of a big river, and for millions of years submit that clay to very high pressure and maybe even add some heat, the resulting product will be the rock type known as shale. Then if for more millions of years you add more pressure and/or expose the shale to very high temperatures, the shale will metamorphose to the rock type known as slate. Therefore, the slate in Slate Mountain started out as loose clay or mud and, remembering last week's story of how this part of California rose from the Pacific's floor and fused with the future North American continent, probably that clay was once mud on the Pacific Ocean's floor. Some of Slate Mountain's slate is shown above.

(This mud --> shale --> slate story isn't unusual. Sand lithifies to sandstone, and sandstone metamorphoses into quartzite. Similarly, calcium- rich goo on an ocean's floor hardens into limestone, and limestone metamorphoses into marble. Deposits of rounded pebbles harden into conglomerate.)

As no sharp dividing line separates clay from sand - - it's all a matter of particle size with the smaller particles being clay and the larger sand -- there's no precise boundary between shale and sandstone, and slate and quartzite. There's a lot of shaley sandstone, and sandy shale.

If you metamorphose slate more than usual it becomes phyllite, which can be greenish, grayish or reddish instead of "slate colored." I think some of what we collected that day on Slate Mountain could be called phyllite because often its fracture plane was wavy instead of flat and much of it was grayish.