Interesting paper Tom. I don’t know whether the item “greyeagle” has is man made or not but there are possibilities for natural – even if it was used for something - since it for sure isn’t granite and it doesn’t look like hardstone. As observed, it isn’t an anchor stone on account of the lack of a groove, hole or other attachment feature. It’s also uncharacteristically large for such an item.
I think I have mentioned this before, but there is a big swathe of sandstone, mudstone and shale in California which is rich in calcite-cemented spherical concretions of various sizes – some of them quite large. There is a major exposure of these formations in Mendocino County, between Point Arena and Gualala where thousands of them have weathered out of the steep cliffs onto the beach. Appropriately enough, it’s known locally as “Bowling Ball Beach”, but you’ll find it on the map as Schooner Gulf Beach. Here’s a picture:
[Pic by brocken Inaglory - Creative Commons license]
They can be almost perfect spheres, but the very large ones tend to get trapped on the beach and are then subject to uneven erosion from wave action.
You can see the same concretions in roadcuts south of Ferndale in Humboldt County and elsewhere. The host formations represent deep ocean material formed under great pressure that were carried northwards by the advancing Pacific plate about 65 million years ago and subsequently became uplifted. They also form the current western wall of the San Joaquin Valley (and the Sacramento Valley) almost as far south as Bakersfield, so it wouldn’t be surprising if such concretions were also eroded out into the valley itself – particularly since it was flooded with seawater for around 60 million years. It didn’t become a freshwater lake until its outlets were blocked by sediments and it filled with glacial meltwater before gradually drying out.
The concretions are relatively hard (but brittle and prone to cracking) and normally yellowish when found in sandstone, or greyish when found in mudstone and shale.
