The field trip was to the nearby Yuba Goldfields, an area of the Yuba River that has been massively altered by the Gold Rush and its aftermath. First the debris tossed into the river from hydraulic mining upstream raised the riverbed by as much as 84 feet, causing horrible flooding and polluting the land with toxic chemicals used in the hydraulic mining. (This debris raised the riverbed over the entire length of the river and all the other rivers it flows into; the Yuba River flows into the Feather River, which flows into the Sacramento River. Flooding caused by the debris in Sacramento forced the I Street bridge in Sacramento to be raised twenty feet, and much of the debris washed into the San Francisco Bay, raising the bottom of that as well.)
Much of the debris in the lower Yuba River was then dredged back out of the river, both to reduce the flooding and to search through the debris for more gold. In the 10,000-acre area called the Yuba Goldfields, the top 150 feet of soil was turned completely upside down, filtered to remove all rocks from it, and dumped back on the ground - the soil first, and the rocks on top of it. This took place over a span of many decades, primarily from about 1906 to 1957. Currently, that same debris is being sold as gravel for concrete and landscaping purposes; it is estimated to be worth about $15 billion (yes, billion) for that purpose alone. About 3,000 acres of the land is public-owned, but a private company called Western Aggregate owns much of the other 7,000 acres, plus some mining rights to some of the public land. In 1987, that company actually blocked off a public road to prevent the public from accessing the public land, and started having people arrested for trespassing on the public land - including some people who actually lived in the area that the blocked-off public road was the only access to - until a lawsuit in 2000 forced them to re-open the public road and allow the public access to the land. However, the road is a gravel road full of potholes and is not maintained by the county government, so it is still not really accessible to people without 4-wheel-drive vehicles.
Anyway, we made several stops during our trip to the goldfields. First we stopped on a major road some distance away, to view them along the horizon.
This is what we saw. Those hills along the horizon are formed from piles of dirt and gravel dumped by the dredging machines; there are ponds in between them.
I also photographed the native harvest brodiaea (Brodiaea elegans) growing by the side of the road where we had stopped. It was growing on the edge of Beale Air Force Base, but we did not see any growing in the goldfields themselves.
Our next stop was by the side of the road a bit closer to the goldfields. Here the professor stopped to show us the pillow basalt she was standing in front of: lava that had solidified in the characteristic billowing shape that indicates it erupted under the ocean and was rapidly cooled by ocean water before it even had time to flatten out. Lava that erupts on land flattens out before it cools.
Here are some closeups of the pillow basalt.
And here is Susan posing with a less pillowy section of the basalt in the same area.
Our next stop was at the Yuba River, where the professor instructed us to collect rocks and think about how they were formed and why they were distributed the way that they were (larger rocks nearer to the river, smaller ones farther away because the river was able to carry them farther).
This is the land where we had stopped.
This is our view of the opposite bank of the river.
Here is Susan looking at rocks.
I, of course, was also looking at plants. Not that there were very many to look at, on our side of the river, where the land had been turned upside down so thoroughly. But I liked this willow, which was growing directly on the bank of the river.
It was a geology field trip, of course, not a botany field trip. So I mostly took pictures of rocks. There were certainly some unusual ones.
The half-dark, half-light rock is not half wet or half shadowed. Half of it is just darker than the other half.
After a while, we all gathered near the cars again, carrying bags of the rocks we had collected.
The professor divided us into groups to discuss what we had observed.
For people without 4-wheel-drive vehicles, or people who weren't interested in continuing further, that was the last stop. But those of us who were interested and had 4-wheel-drive vehicles continued down the gravel road full of potholes. We saw a dredged canal with some of Western Aggregate's machinery in it.
And another dredged channel, directly across the road from the previous one.
And the remains of an old company-owned gold-dredging town called Hammonton, which ceased to exist in 1957 when there was no longer enough gold left in the debris to keep the town going.
Then we went home. We're really, really hoping that we can manage to take some more field trip classes from this professor at her new junior college in the San Francisco Bay Area, but it will be kind of a long drive for us and it's not yet clear whether she'll even be able to teach such classes at that school, so . . . wish us luck!