Search the Community

These come from limestone quarries in central Iowa, near Springville, which appear as crushed gravel for roadbeds. My sister picks these up on her walks and wonders if these are the result of volcanism, or fossils, or what? My friend and fellow fieldtripper Kent Budge gives the following explanation, but suggested that we post here to get the insights of real experts. “These are chert nodules that are formed by microscopic plankton called radiolarians, which are little microorganisms that pull silica out of sea water to make their shells, less than 1/16”. They can pull a lot of silica out of sea water and settle onto the sea floor when they die. Once they have settled onto the sea floor, they will dissolve again, unless there is an event that covers them up. If they get buried so that they are preserved, the radiolarians will dissolve slowly, but the silica takes as a more stable form of silica, and precipitates, forming nodules. A lot of chert, if you slice it up and look at it under a microscope, you can see the little skeletons. Sometimes you can see diatoms which also have silica shells, or glassy sponges. The reason these are formed into these neat clam-like or finger-like shapes is because they form in bedding planes on layers in the rock. There is some kind of nucleus, that they start from, which might be an organic fossil. It might also be from narrow channels in a bed that the chert pipes up from. “ Your thoughts?