Oakland resident Andrew Alden usually spends his time covering earth science for About.com, but his other gig is blogging on Oakland's geologic treasures at Oakland Geology. Among his recent posts is a 15-million-year-old, clearly layered chert (compact rock of microcrystalline quartz) on Grizzly Peak Boulevard in the North Oakland hills. Says Alden: [It] formed in quiet offshore waters not far from here. That is, there were no hills and no Bay here. Mount Tam, in the background, didn't exist. The whole Coast Range didn't exist, maybe a few island chains. The Sierra Nevada was a low, ancient range of hills. Since then the crust has been shredded and pressed, tilting these chert beds upright and even over." Probably not what motorists zipping by there would notice.