Source: USGS, full map here.

More specifically, where Cathedral Peak granodiorite (pink; 86 million years old) meets Jurassic (light green)/Triassic (dark green) metavolcanic rock (150-250 million years).

We hit Sky Pilot Col at sunrise, left camp at 4 am. Had 3 passes to hit today and we’d be back in Twin Lake by late afternoon. On the other side of Sky Pilot Col is a zone of geologic convergence (*I am not a geologist). It actually made for somewhat pleasant talus walking. The shattered volcanic rock would fill in the gaps of the granite boulders. Instead of skiing down volcanic scree, or falling into crevices between granite boulders, we finally found perfect talus on the last day of Roper’s route.

Below is the view looking south from Shepherd Lake. Volcanic on the left, granite on the right. The surficial deposit just above the lake is dated to the Holocene (~12,000 years ago) or just after the last glacial period. That glacier took a little bit of metamorphic talus and a little bit of granodiorite talus, blended it all up, then dropped it here for us to walk down.

What is the difference between granite and granodiorite? Well… It’s very similar to granite, except it has more plagioclase than orthoclase feldspar… In other words: granite with black spots in it(?). Moonstone is mostly orthoclase.

andrew batjiaka