This little black pebble, now sitting on my desk, traveled a lot today. After picking it up beneath a cliff face in southwestern Montana, I carried it in my pocket for a few hours and then drove some 20 miles to this college dorm where I’m staying. It’s the most this pebble has moved for millions of years.
Some time ago–this morning, a week, a year, 10 years, 100 years—the pebble weathered out of a much larger rock and fell to the ground. Its worn, rounded edges tell me that before it became part of that larger rock, it traveled down a stream bed –and its size tells me that its source probably wasn’t too far away. As is typical of stream gravel, its movement was irregular, marked by short bursts of movements during floods separated by longer periods of rest on a gravel bar or in the channel itself. Somewhere along the line, the pebble became buried by more sediment, probably because the land subsided or the river channel switched to another position. Eventually, the pebble and the rest of its surrounding sediment turned into rock.
All this took place some 125 million years ago early in the Cretaceous Period during uplift of an early version of the Rocky Mountains. As the mountains rose, they eroded, shedding material off their flanks and ultimately into streams, where they became worn and rounded, my pebble among them. Today, we can see the folded and faulted remnants of those mountains. They include a sequence of sedimentary rock that records the area’s geologic history dating back more than 500 million years. One of the layers in this sequence, called the Phosphoria Formation, provided the raw material for the pebble. Another layer, called the Kootenai Formation, contains the rock that held the pebble.
Using a handlens, I look closely. The pebble is incredibly fine-grained, with a texture more like a piece of caramel than most typical rocks. It’s chert, made of sub-microscopic silica from single-celled organisms called radiolaria. These creatures float in ocean waters and settle to the bottom when they die. The Phosphoria Formation formed during the Permian Period about 270 million years ago when more than a hundred feet of chert accumulated on the sea floor.
I think that’s what fascinates me the most about this pebble: its very existence speaks to a geologic history from ocean bottom to mountain range. Not only that, but it exists in two time periods: today and the Cretaceous. Throw it on a gravel bar and you couldn’t tell it apart from the modern pebbles.
Perhaps even more extraordinary is how non-extraordinary this pebble really is. Take any handful of loose sand that’s weathered from a nearby sandstone outcrop and look closely: those tiny sand grains are just like the pebble, only smaller. They’re sand grains now, just as they were when they became a part of the nearby rock. And like the pebble, they came from a still older rock –which itself has a history. And it’s all in transit.
To view and/or download more images of sedimentary rocks –or any kind of geology for that matter–please take a look at my website: geologypics.com