Category Archives: Rocks

Sand tracks

“In every outthrust headland, in every curving beach, in every grain of sand there is a story of the earth.” Rachel Carson, writing in Holiday magazine, 1958.

From the curving beach of St Catherines Island, another blog post in the grapheme series (parts I, II, and III). Sand scribblings. For the full coastal Georgia effect, view in a steam room with the heat turned up to one hundred degrees. Click on any image for the slideshow:

From the edge of the boreal

I’ve just returned from a short trip to northwest Ontario where I listened to some trees and rocks (a purpose that did little to impress when recounted to immigration officials).

KakabekaViewAmong the boreal delights were ravens, a species that according to the Objibwe, the First Nations people of the region, brought the world into being and gave the two mainstays of life: water and fish. In the presence of these highly social, intelligent, garrulous birds it is obvious why the Objibwe regard ravens with such respect. Awesome creatures.


Raven fly-by. Balsam fir and serviceberry in foreground.

A fledgling raven sat in the tree above my tent, calling to its sib and two parents. The following recording, made amid a haze of mosquitoes, captures some of the birds’ vocalizations. The loud, insistent squawk is the youngster. The Objibwe name for raven is gaagaagi and young ravens are known as gaagaagiins, names that capture the talkative nature of these birds.

And as we listen, insects gather to gather atoms for the regional taxation system. Naked mammals are in the highest tax bracket. Note the backcurved sheath, exposing the penetrating stylet. Her hind legs are twitching in delight.

mossiekakabekaUnder the ravens, insects and balsam firs: old, old rocks.

chertBIFThese cherts (from the Gunflint formation) are 1.88 billion years old and contain the oldest known fossils of any lifeform in North America. Until some Australian and African finds beat the record, they were the oldest known fossils from anywhere: life’s first recorded mark upon the Universe. J. W. Schopf’s 2000 PNAS paper has some great photos of these microscopic cells, our (great)^1,880,000,000-grandparents.

In lieu of interpretative signage at this site of Universal importance, we have ♥KIMI blazed on a fir tree. What is amazing to me is not that someone would put their mark on a tree, but that Kimi or her friend came walking in the boreal forest prepared with a can of pink spraypaint just in case. Gotta love Homo sapiens’ complicated inner world, all jumping out of our nerve cells: those microfossils gave rise to some interesting phenomena.


The lichens grow on, poking fresh new growth from under their pigmented parts. If we knew the growth rate of the lichens, we could date this new Gunflint stratum quite accurately.


The Newtownian Apple, the Darwinian Galapagos, and the Archimedes’ Bath of Geology.

The city of Edinburgh is overlooked by the cooled remnants of ancient volcanoes. The most famous of these fiery landmarks is Arthur’s Seat, the highest point in the city and a popular place from which take in the view (and the wind) after a vigorous climb. The dome of Arthur’s Seat was formed from the inner “pipe” of a volcano, the central conduit through which molten rock rose. The rest of the volcano has, over about three hundred and fifty million years, eroded away.

Arthur's Seat, viewed from one of many well-worn access trails. Who "Arthur" was is unclear.

Arthur’s Seat, viewed from one of many well-worn access trails. The origin of the name, including the identify of “Arthur,” is unknown.

My guides: Photographic evidence that locals and their half-tamed wolves sometimes join visiting American tourists at the peak.

The Grindley mountain guides, Julia, Kathy, and Hannah: Photographic evidence that locals and their half-tamed Scottish wolves sometimes join visiting tourists at the peak.

Next to Arthur’s Seat, facing Edinburgh castle (itself perched on volcanic rock, one of the vents from Arthur), an escarpment curves around, presenting an impressive cliff line to the city. These are the Salisbury Crags: cliffs that contain some of the most famous rocks in the world.

Salisbury Crags.

Salisbury Crags, viewed from the southeast.

It was on the Salisbury Crags that James Hutton, the Scot whose work founded the modern science of geology, saw evidence to refute the “Neptunian” view that rocks had been recently deposited as sediments in a catastrophic flood. Hutton’s views inspired Charles Lyell who in turn strongly influenced Darwin. So the Salisbury Crags are also foundation stones for modern biology.

In the crags, Hutton saw flows of magma crunching into sedimentary rock (magma is molten rock; lava is the above-ground manifestation of magma). The sedimentary rocks were formed in an ocean, but the magma was not; it came from a volcano then flowed into (and distorted) the other rocks. Here, then, was evidence that the world’s geology could not be explained by catastrophist, diluvian processes. Instead, these rocks (and unconformities from elsewhere in Scotland) showed Hutton that the world was very old and that ongoing interaction between processes like vulcanism, sedimentation, and erosion could explain how the physical features of the world came to be.

In Theory of the Earth (1795) Hutton writes (with my explanatory additions in square brackets):

the whin-stone [rock formed from magma/lava] is interjected in form of strata, having various degrees of regularity, and being of different thickness. On the south side of Edinburgh, I have seen, in little more than the space of a mile from east to west, nine or ten masses of whin-stone interjected among the [sedimentary] strata. These masses of whin-stone are from three or four to an hundred feet thick, running parallel in planes inclined to the horizon, and forming with it an angle of about twenty or thirty degrees, as may be seen at all times in the hill of Salisbury Craggs.

…these masses, which have flowed by means of heat among the strata of the globe…[are] subterraneous lavas, as they may be termed.

This view is confirmed by present-day geologists. The crags are a sill (a sheet of igneous rock intruded between sedimentary layers) that formed when magma flowed out into the oceans, pushing into sedimentary layers.

For good measure, here is the most famous passage from this book. Hutton emphasizes a uniformitarian worldview: nature operates according to a system of rules and these rules are consistent across time and space. He makes an analogy to the law-like motion of the planets (as would Darwin, many decades later, in the concluding paragraphs of The Origin) and ascribes the operation of the physical world to these natural laws, not to higher causes. We now know that his famous “no vestige of a beginning, no prospect of an end” is not true, at least as far as the Earth is concerned. But the rest of his vision still stands.

We have now got to the end of our reasoning; we have no data further to conclude immediately from that which actually is: But we have got enough; we have the satisfaction to find, that in nature there is wisdom, system, and consistency. For having, in the natural history of this earth, seen a succession of worlds, we may from this conclude that there is a system in nature; in like manner as, from seeing revolutions of the planets, it is concluded, that there is a system by which they are intended to continue those revolutions. But if the succession of worlds is established in the system of nature, it is in vain to look for any thing higher in the origin of the earth. The result, therefore, of this physical inquiry is, that we find no vestige of a beginning,—no prospect of an end.

Dolerite (cooled lava flow) on top. Sedimentary rock below.

Hutton’s famous rocks. Dolerite (cooled lava flow) on top. Slab of pushed up sedimentary rock below.

I'm pointing to the famous point of contact between the two rock types. Ably assisted by Rocky (yes, a geological dog) and Kathy.

I’m pointing to the point of contact between the two rock types. I’m ably assisted by Rocky (a geological dog) and Kathy who sports her new snow leopard headgear.

Edinburgh’s growth and the industrial revolution created a market for the crags’ cooled magma, dolerite. The hard rock was used for paving streets, including some as far away as London. Quarrying activities on the crags removed large quantities of rock, but Hutton convinced the operators to leave one small rock with a particularly good example of a hematite seam (formed by hydrothermal venting into the rock). So Hutton not only founded modern geology, but enacted the first conservation program for significant geological formations. The rock stands to this day.

Some of Edinburgh's streets are lined and paved with dolerite to this day. This example (inscribed outside the Scottish Parliament) may be the world's only example of a gutter with mineralogical engraving.

Some of Edinburgh’s streets are lined and paved with dolerite. This example (outside the Scottish Parliament) may be the world’s only example of a street gutter with mineralogical engraving.


Hutton’s Rock, being closely observed by Edinburgh’s next generation. The rock marks the original front wall of the cliff. The quarry pushed this wall back to the location seen behind. Arthur’s Seat rises in the background.

Hematite seam running through the middle of Hutton's Rock. It is worn smooth by the many tourists who clamber over the rock each day.

Hematite seam running through the middle of Hutton’s Rock. It is worn smooth by the many tourists who clamber over the rock each day.

Remarkably, no signage notes (or protects) these treasures of our shared intellectual heritage. Tourists walk past ignorant of the fact that they are passing the Newtownian Apple, the Darwinian Galapagos, and the Archimedes’ Bath of Geology. Bouldering hands have chalked up nearby rock faces. Hammers have chipped at some exposed areas, including a portion of Hutton’s Rock. In hindsight, I feel bad for sitting on the famous rock, adding my little bit of buttock-polishing to the well-rubbed patina. The whole area is officially a designated Site of Special Scientific Interest, but this has not resulted in any on-site signage. Surely a place that tore apart a befuddled worldview and launched one or more new scientific disciplines deserves more than a “Beware” sign?


Danger — Enlightenment may strike and spread across Europe. It happened once; it could happen again. So think with care.

The crags also host some biological gems. Peregrine falcons patrol the cliff faces. Fulmars nest on precarious rock ledges: these are birds of the wild North Atlantic, yet here they are at the center of a major European city.

This peregrine flew past, sending pigeons swirling away.

This peregrine flew past, sending pigeons swirling away.

Northern Fulmar, getting its nesting area ready for spring. The bird is lodged way up a cliff. Any mammal skilled enough to climb up there will be rewarded with a face-full of vomited fish oil. Welcome to Edinburgh, laddie.

Northern Fulmar, getting its nesting area ready for spring. The bird is lodged way up a cliff. Any mammal skilled enough to climb up there will be rewarded with a face-full of vomited fish oil. Welcome to Edinburgh, laddies and lassies. Fulmars are related to albatrosses, soaring birds of the wind-blown oceans.

The view from Hutton's Rock: Edinburgh.

The view from Hutton’s Rock: Edinburgh.

[Thanks to Chris Clinkscales for comments that clarified the magma/lava terminology in this post and for insights into geological mechanisms — see comments section below.]

Fifty Shades of Grey: Woodland Edition

Sitting in the woods with my class last week, I was struck by how grays had come to dominate. The light environment is transformed. Of course, a “fifty shades” wisecrack had to work its way into my impromptu lesson on the visual aesthetics of the forest. The witticism turned into a small project for my walks of the last week: pay attention and find these shades. So here they are, fifty photographs of variations on the theme.

Gray is the most egalitarian of hues. Indeed, its essence is that is not a single color. Instead, gray gives us a muted echo of all the light spectrum, a moody version of white. Contrast this with the bias of other pigments — reds, blues, yellows — that reflect just a tiny slice of the light available to them.

Gray is an unassuming mirror of the world and a quiet companion for its more assertive kin. It absorbs metaphors with ease, having combined light and dark: ash, silver, lead, pepper. A suitable tone, then, for winter reflections.

Happy Solstice, fellow ramblers.

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Limestone foray

I rambled halfway down the mountain to visit some caves this morning. The Cumberland Plateau is a layer-cake of rock: the top layer is sandstone, the bottom is limestone. Sandstone does not dissolve in water, but limestone does. Over millions of years, this dissolution of rock by water has gouged holes in the lower reaches of the mountainside, creating an impressive network of caves.

From the outside, looking in.

From the inside, looking out.

A phoebe nest on the inner wall of the cave, about fifty feet back from the entrance. These nestlings were raised in near darkness. Their first flight out into the world must have been a revelation.

"Solomon's Temple" cave entrance. No Ark of the Covenant here, just limestone-adapted spleenwort ferns.

An underground stream surfaces briefly at the base of the cliffs. Inside the cave, this stream roars away at the bottom of large pits. I was by myself, so I did not scramble into the cave to see them again -- rule number one of caving is 'don't go inside dangerous caves alone'.

Multiple layers of eroding and fractured rock: another reason for caution.

My favorite spot around these caves. This is a vertical tube, about twelve feet wide, running straight down from a hole on the forest floor, emerging far below in a pile of debris on the side of the cliff face.

Just below the cliffs and caves, I found the first wildflower of the the year, “pepper-and-salt” or “harbinger-of-spring”, Erigenia bulbosa. It seems a little early to be harbinging the spring — we’ve got plenty of winter to go yet — but it is good to be reminded that spring will come. These little flowers belong to the carrot family and their tuberous root is supposedly edible. I’d rather leave them rooted and feed on the hope they offer.

Five ridges

From Morgan’s Steep, Sewanee, early this morning. The ridges are the rocky wake left by the slow eastward erosion of the Cumberland Plateau. The air was thick with water vapor rising from the wet woods, blurring the focus on everything in the distance.

A little jaunt in the early morning…

…down to Bridal Veil Falls, below Morgan’s Steep. Unlike yesterday when the air was warm and the spring peepers were calling, a cold front has pushed some real November chill into the woods. The frogs were silent, but a Winter Wren was singing its heart out. Surely this species is our most vigorous songster, heard only in the winter months (this video and recording from Lang Elliott and Bob McGuire is remarkable — these are not easy birds to approach).

The stream at Bridal Veils comes out of the sandstone scree, hits the limestone layer, then plunges into a pit.

Junebug was fascinated by the pit, but didn’t take the leap. There are some huge toads, snails, and slugs down there. Even in summer, the air at the bottom is moist and cool.

The big waterfall is not the only force sculpting the rock here. Limestone dissolves readily and the rocks have been shaped by years of trickles and oozes.

Even a tiny drip over the lip of a rock has cut a U into its path.