At GZ, look west at the Bitterroot Mountains, then look a bit to the south to see the Bass Creek Crags. They are the mostly flat topped granite spires, "Gunsights" formations that you will see.
Questions to be answered:
1. Tell me how many rocky outcrops do you see? Estimate how tall you believe they are.
2. Looking at the Bass Creek Crags, can you tell which way the glacier moved? Explain your answer.
3. From the lesson below, when was the last Ice Age that sculpted the crags?
4. MANDATORY: Post a picture of yourself, your signature item or GPS with the Bitterroot Mountains in the background. The crags shouldn't be in the picture.
Message me your answers, don't post them in your log.
GZ takes you to a good place to park and view the Bass Creek Crags safely. You can see them while driving down the Bitterroot Valley as they are a favorite mountain view of mine.
The Bass Creek Crags are steep, granite spires formed from ancient , resistant igneous rock shaped over eons by glacial erosion that carved deep gashes between the harder rock, leaving behind these dramatic, flat-topped formations similar to how granite intrusions became prominent land forms after being sculpted by ice.
Key Formation Factors:
1. Igneous Origin: The crags are composed of granite, a hard igneous rock formed from slowly cooled magma deep underground, making it very resistant to weathering.
2. Glacial Sculpting: During past ice ages, glaciers moved through the area, eroding softer surrounding rock and carving deep valleys (like Bass Creek), leaving the tougher granite formations standing as sharp, steep spires.
3. Differential Erosion : The deep "gashes" or colours separating the spires show how ice preferentially carved softer zones, creating the unique "Gunsights" appearance.
In essence, the Bass Creek Crags are remnants of a massive, hard granite intrusion that was exhumed and sculpted by glacier forced, leaving behind these distinctive, rugged features.
Lets take a deeper look at what happens when the glaciers sculpted the crags.
During the last Ice Age (around 20,000 years ago), massive glaciers flowed down the Bass Creek valley. Rocks embedded in the base of the glacier acting like sandpaper, grinding, polishing, and carving deep grooves (striations) into the bedrock. The glaciers eroded softer rock more easily, leaving behind the harder, more resistant rock formations (the crags) standing out. At the narrow notch below Bass Lake, you can still see these grooves.
The power of glacial ice was enough to cut deep grooved on the order of 50-80 cm (2-3 feet) across into the granite gneiss. In the photo, the ice flow direction was from right to left (west to east). The rock was also scratched by rocks embedded in the ice and polished by finer sediment in the ice.
Crag and Tail Formation Process
The "crag and tail" formation occurs when a glacier moves over an area of mixed rock strenght. The process involves:
- Crag Formation: A large, highly resistant rock formation ( such as the hard granite at Bass Creek) obstructs the flow of a moving glacier. The softer, surrounding rock material is eroded and ground away by the immense force of the ice, leaving the resistant rock "crag" protruding from the landscape.
Tail Formation: The crag acts as a protective barrier in its wake ( the leeward side, or "down-ice" direction), shielding the softer material or loose glacial till behind it from the full erosion so force of the ice. This protected, less- eroded or deposited material formed a gradually tapering ramp or ridge, know as the "tail," that slopes down from the crag.
The presence of these features provide evidence for the direction of former ice flow in a region. The Bass Creek Crags, in particular, give an indication of how rugged the Bitterroot Mountains are, with deep gashes in the granite separating the spires formed by these powerful natural processes.
