So what do sidewalks have to do with geology? Well, to begin with, they're made in large part of rocks; the other major component, cement, is man-made, and is therefore not technically rock. But perhaps more interestingly and importantly, gravel is used in very large quantities and is very massive, and thus is prohibitively expensive to transport very far. In consequence, nearly every community of more than a few people has its own source of gravel. That gravel in turn represents at least a few aspects of the local or regional geology.
In the case above, you can tell from the well-rounded pebbles that the gravel has undergone
stream water transport. (Wave or current transport would result in the same rounding.) I have no doubt that this is from Willamette River fluvial sediments. Most of the Willamette gravels are derived from the west flank of the Cascade Range. There is some contribution from the Coast Range, but most of the Coast Range is underlain by grungy sedimentary rock that does not survive transport very far. If you look at the gravel in the Mary's river, which drains a substantial chunk of the Coast Range west of here, and has its confluence with the Willamette in the southern part of Corvallis, it is made up of almost entirely basalt. Additionally, the Coast Range component of the Willamette drainage is smaller and gets much less total precipitation than the Cascade component. So the above "random sample" of the rocks in my local drainage are not really random at all. It's dominated by Cascade volcanic rocks, and is further segregated in that rocks less resistant to weathering and erosion, those that don't survive transport, are either un- or under-represented.
Ranking from most abundant to least abundant we have
- Basalt- can be difficult to tell if it has been altered in this context. I look for a lack of polish.
- Altered volcanics- a broad and not entirely useful category, these are most often infused with some sort of microcrystalline quartz, such as chalcedony, or its non-fibrous analogues. The parent material is still the dominant component, but they have been hardened by the addtion of silica.
- Intermediate and Felsic volcanics- don't seem to survive transport too well unless they have been "altered," as above. Additionally, the Cascades are quite dominated by basalt; intermediate and felsic volcanics are simply not as abundant to start with.
- Jasper, agate and various other micro- and cryptocrystalline varities of quartz. These are fragments that crystallized in void spaces, so there is no "parent rock." Frequently in vivid colors, and take a nice polish from countless soles grinding on them.
- Petrified wood (technically for most fossil wood in the western Cascades, "permineralized wood," i.e. original woody material is still present, but has been completely infused and entombed in microcrystalline quartz)- probably belongs grouped with #2 or #4, but it's cool enough to warrent it's own bullet, IMHO.
So where do you live, geologically? Show me your sidewalk! I'm not going to tag anyone, unless no one plays...