Friday fold: cherts at Mount Diablo, California My former student James O’Brien recently moved out to the left coast, and posted some photos earlier this week of a hike he took at Mount Diablo State Park in the mountains east of the San Francisco Bay area of California. There are some classic-looking Franciscan cherts exposed there, as these folded examples show: #ScienzeGeologichePage
Fault slickensides, Mecca Hills, Riverside County, California A minor right-lateral fault exposed along Hwy 195 about 7 km (4 mi) northeast of the San Andreas fault. Even though this is a minor fault, the fault exposure is quite large and impressively well preserved.
Maybe weathering is the key: So the story recorded here could therefore be: lava erupts, lava cools to basalt, basalt weathers from the top down, producing clay and rust-rich saprolite, new lava erupts, cooks weathered basalt beneath, baking it orange.