The theme of the recent seminar I attended with the Yellowstone Association Institute was geomorphology, the study of landforms – why they look the way they do today and the processes that caused them. For three days we drove up and down and walked all over Lamar Valley in our attempt to understand these processes that have occurred over the past 2.5 million years of Earth history during the Quaternary Period. This more recent slice of geologic time includes the last period of global glaciation along with the advance and retreat of large Northern Hemisphere ice sheets.
|Soda Butte Creek cutting into toe of landslide|
However captivating geomorphology might be, nevertheless a constant refrain ran through my head for the entire three days: I wanted see some really old metamorphic rocks on this trip! The Beartooth Mountains of northern Wyoming and southern Montana are Archean terrane, at least 2.5 billion years old, and I knew they were out here somewhere.
OK so maybe geomorphology does not exactly lend itself to the study of 2.5 billion-year-old rocks. Nevertheless, I would be on the lookout. Perhaps a passing glacier had exposed some nice gneiss or shimmering schist as it scoured its way across the prehistoric landscape.
|Bus stop for geomorphology in Lamar Valley|
In Lamar Valley we were in a high relief area of the Park, outside of the caldera. Some of this relief is due to old, hard resistant metamorphic rocks (there they are!) having been uplifted during mountain–building events while other relief is most likely due to tectonically active faults. The Yellowstone “hot spot” does add additional relief to the landscape from relatively recent uplift and subsidence due to heat emanating from the ginormous magma chamber lurking beneath the surface.
|Mountains of Absaroka Volcanics|
The youngest sedimentary rocks seen here in Lamar Valley are the Madison Limestone of the Mississippian Period, about 320 million years old – any younger sedimentary rocks have been eroded away. Additionally, because 50–million-year-old Absaroka Volcanic rocks are sitting directly on top of this Madison Limestone, there is an “unconformity” or gap of around 250 million years here in the geologic record.
|Cliffs of Madison Limestone at Pebble Creek|
The Absaroka volcanism of 50 million years ago occurred as the Beartooth Mountains to the north and east were being uplifted. The cliffs surrounding Lamar Valley (such as Specimen Ridge) are these Absaroka Volcanics – they appear as stratified layers because much of the volcanics occurred as mudflows (similar to Mt. St. Helen’s mudflows).
Here and there we found boulders of mudflow breccia and fragments of 50 million year old fossilized trees in the alluvial fan deposits coming off the mountains.
|Absaroka Volcanics seen from Pebble Creek|
|Mudflow breccia from Absaroka Volcanics|
|Fossilized tree section|
In a stream bank on Soda Butte Creek we were amazed to see a thin lens of light gray ash deposited from the explosion of Mt. Mazama (Crater Lake, OR) around 6800 years ago. Click on the images to see if you can find the ash lens – in the third image a red arrow points it out.