Rocks Remember by mark mcfaddan, geologist

You can watch "Pend Oreille Country" online at https://www.pbs.org/video/pend-oreille-country-ierduv/

To a farm kid growing up in the 60’s on the western Snake River plain, north Idaho was a mystic place. Enormous lakes surrounded by seas of evergreen forest on steep mountain slopes were on all the maps and in the brochures and books, but did they actually exist? Many southern Idaho residents were pretty sure that Riggins was considered northern Idaho, and Moscow was even further north, probably on tundra. Somewhere beyond Coeur d’Alene was likely the abrupt edge of the world. Or Canada.

The green, mountainous lake country of north Idaho seems like a different world in comparison to the southern part of the state.

An eventual migration to the University of Idaho generated many revelations, but the panhandle proper still floated in the mists of imagination somewhere under the northern lights. Introductory geology classes brought some of the generalities about northern Idaho into focus: small oceans of granitic magma had melted their way up through even more ancient rocks that seemed to confound some veteran geologists, and ice age glaciers had recently plowed southward over the landscape. To our amazement, we learned that we inhabited a rather brief intermission within the current ice age, and that this frigid chapter was far from over.

Evidence of geologically recent sculpting of the landscape is everywhere in Pend Oreille country. This glacial polish on the 1.5 billion year old bedrock is only a little over 10,000 years old, indicating the span of history in the rocks.

All was pretty nebulous until that first upper division geology class took a flock of eager young students up to meet the rocks around Lake Pend Oreille face to face. Lesson #1 was that we were now in North Idaho, not to be confused with the general northern half of the state. This turned out to be serious business in the panhandle, terminology not to be taken lightly. We were greeted by a landscape freshly exhumed from the ice, one that had been sculpted, crushed, and worn down by the sheer force of continental glaciation.

Students of the University of Idaho/Washington State University geology Field Camp learn to read the rocks along the northern end of Lake Pend Oreille, Summer 2018.

Most of the bedrock had weathered the assault fairly well; it was old and tough and had already survived previous ice ages, the rise of the Rockies, and the stretching of the entire western part of North America. The enormous stack of layer after layer of impossibly old sedimentary rock was miles thick; it could be found from northeastern Washington to Glacier National Park, and from British Columbia to central Idaho. The 1.5 billion year old Belt Supergroup was just as intimidating as its name.

Layers of sandy sediment are now solid rock, and have been tilted by folding and uplift related to the rise of the Rocky Mountains. They document local geologic history as the stone pages of Earth’s diary.

Our wise and seemingly omniscient professor was familiar with the Belt rocks, and he did a wonderful thing for all of us on that glorious September day in the early 1980’s. He dropped us off in the deep end of practical education for a long walk along the northern end of Lake Pend Oreille and had us learn to read the rocks. Our job was to recount the story that we could decipher from thousands of feet of layers, actual pages of Earth’s diary. We were terrified, but it was a nice day for it.

Mudcracks forming in a drying puddle, dimpled by rain from the previous night. Clues like these are commonly preserved in sedimentary rocks, recording the depositional environment.

The rocks were tight-lipped at first, and it took a great deal of patient coaching to learn which features were significant and which were trivial. Gradually, their arcane language began to make a little sense, and they loosened up to tell of an enormous lake or inland sea that had covered the northern half of Idaho, all of western Montana, and adjacent parts of Canada. The story was written in elements like cracks frozen in stone from vast mudflats, just as modern mud in puddles dries in the sun and shrinks and curls. Ripples from long-vanished moving water were now solid rock, evidence of sheets of sand swept across a nearly flat landscape.

1.5 billion year old muds of the Belt Supergroup sediments dried and cracked at the surface, and are preserved here because sand was washed in to fill and hold open the cracks.

The pages in the book of Earth history ranged from millimeters to meters in thickness, each one recording a storm, a time in the sun, or a rise in the ancient lake. The pages were preserved from a time long before plants colonized the land, long before burrowing organisms existed to churn and erase the evidence in the layers. Miles and miles of layers, and not a single track, trail, or burrow. It was a mind-boggling story, but if we turned to the shallow water edges of the modern lake, we could observe similar features forming in modern environments, most soon to be obliterated by the daily activity of modern life forms in the bottom muds.

Side view of layers in 1.5 billion year old Belt Supergroup rocks. Reddish layers of cracked mudrocks record drying in the Precambrian sun. Mudcracks have sharp edges and are filled by gray layers of sand swept in by storm events on a landscape before the advent of land plants.

Learning to read the small-scale features in outcrops and roadcuts was fascinating, but the story behind the large-scale landforms was nearly overwhelming. The most recent advance of the Purcell lobe of the continental ice sheet had put the finishing touches on the last 2 million years of glacial activity, and the evidence was fresh. After all, the ice had retreated last only about 10,000 years ago, a mere flicker in the vastness of geologic time. Valleys had been widened and steepened, lower hills had been sanded off to rounded nubs, and gravels of all sorts had been bulldozed in front of the last ice. Glacial Lake Missoula was old news to geologists, but the magnitude of its ice dam at Clark Fork and the number and frequency of outburst floods from its repeated collapse was still coming into focus. Montana had the recurring lake, and Washington had the channeled scablands, but the ice dams were an Idaho story, and this time language was a different dialect of geology – a big one.

The lower elevations of the Pend Oreille country have been profoundly sculpted by the repeated passage of the Purcell lobe of the Cordilleran Ice Sheet. Antelope Mountain near Clark Fork owes its shape to many episodes of glacial erosion.

Over the decades, I have gone back to the Pend Oreille country countless times to learn more of what the rocks and the landscape had to teach me. Often it was with bunches of college students from my classes, many who were visiting Lake Pend Oreille for the first time. Watching their enthusiasm and joy of discovery as they soaked in the scenery and put together the clues of its history was always priceless. Many times it was with the outstanding geologists of the Idaho Geological Survey as we put together geologic maps of the region.

Many thanks to Linda Schactler and Chris Smart of Central Washington University for the use of their animation re-creating the Missoula Flood, which swept through Idaho and created the scablands of Washington.

Our job then was to track the composition and structure of the mountains as we worked to understand and document the details of this gigantic four-dimensional geologic puzzle. The views were magnificent, and the cross country traverses were as epic as the comradery.

With the Outdoor Idaho crew near where the Purcell Trench glacial ice smoothed and polished the bedrock with repeated ice-dam movements.

Great regional geologists taught me even more dialects in the language of rocks, and friends and family reminded me to slow down and actually enjoy the panorama. I have never visited the rocks of the Pend Oreille country without learning something amazing from this special corner of Idaho. And I always will.

“Geologists have a saying – rocks remember.” - Neil Armstrong