A River Stone
Tens of millions of years ago the pink granite of the Sawtooth Mountains was formed. It was lifted, eroded, broken, and shaped by forces that make mountains. Its rosy hue and granite spires have seen great change and been created through change. This rock’s journey continues.
Long ago snow began to fall high on the mountains. It accumulates on steep rocky slopes in the cracks and ledges where it cannot be blown off by the winter winds. Sun warms the rock and melts the snow. Water trickles down mountain faces and into the cracks. As night comes, and the temperature drops, the trickle freezes into glassy ice. The water expands as it freezes in the cracks, slowly moving rocks, moving mountains. Sunny days and freezing nights pass through the seasons and inches the big granite to the edge of the ledge. In early spring a series of warm sunny days sends a steady stream of water into the gap between the mountain and granite boulder. But as with weather in the mountains, spring turns chilly again and ice builds up until the expanding water crystals push the granite block the last few millimeters and it tumbles down the mountain’s face.
Cracking and booming, its fall echoes through the valleys in the dead of night. Chipping and shattering, it comes to a stop among the debris at the bottom of the cliffs. Plumes of rock dust glow white in the moonlight. Two large rocks are all that are recognizable from what once rested high above.
The two granite rocks settle into their new location on the mountain as yearly temperatures drop. The rocks become buried under layer upon layer of snow through the winter. As spring comes and summer passes by, the snow condenses, becomes icy, but does not melt. Decades and centuries pass and snow accumulates in the mountains piling higher and higher, weighing more and more until it begins to slide down the mountain under its own weight. A glacier has formed.
Deep underneath the icy glacier one of the two rocks is wrapped in an ice grip. It becomes one with the glacier and is carried along down the mountain side, a few feet a year. Sometimes moving faster, other times catching and holding still.
The rock is ground against others, scraped across bedrock, and chipped by ice. It leaves behind small chunks, grains of sand, and individual crystals of quartz and feldspar. As it moves along, inch by inch, it begins to shrink, eroded by forces greater and older than the rock itself.
Now the rock is far from where it started high on the mountain’s face. It is deep in the valley that is being carved deeper every year. One day the glacier releases its grip and the rock is left behind to be ground down by other rocks passing over it. It is now a quarter of the size of the big granite block and has been chipped and worn. One small piece, small enough to hold in your hand, is broken off and rolled, crushed, and dragged along by the mighty glacier. Ten years later the fragment gets caught up in the melt water flowing under the glacier and continues its journey by river.
As part of the river, the rock has eroded to a cobble on the river bottom. The sharp edges are rounded and smoothed by the constant coursing of water, sand, and pebbles. While now much less dramatic this river cobble is still traveling and being shaped by its environment.
The river water is always cold, fed by mountain springs and melting snow. As the climate warmed the glacier retreated and all that remains is a patch of snow, shaded from the most direct sun by the mountain itself. This is the snow that feeds the river.
Our river cobble tumbles downstream in the rushing high water of spring. It is rounded and smooth, a smooth river stone to hold in the palm of your hand. Polished by time, it is flecked pink, white, and black. Wet and glistening in the sun, this small pebble started high on the mountainside. Perhaps it had never seen light at the center of a granite block but now it lays under the wavering light in a river pool far below the mountain. And tomorrow, maybe a caddis fly larva will decide to latch on and build its home out of bits of sand before it hatches and the round river rock’s story continues.
Hannah Fake is the Lead Naturalist for SIHA. She loves hiking among the boulders that have tumbled down the mountainside and imagining what the area would have looked like with glaciers.