My husband and I have found several (100+) perfectly round rocks. They are only in one area that we've been able to find--it's almost as though there is a vein of them. Some of them are actually joined, making it look like a peanut, and some of them are knobby. From what I could find on the internet these strange rocks are classified as female (round, smooth) and male (knobby, oblong). Some of them that we've found have been cracked almost perfectly in half and the inside is either sand or nothing at all, forming a bowl. They range in size from about marble sized up to the size of maybe a golf ball and perhaps a big bigger. Can you tell me what these are? Like I said, we've never found them anywhere else except for one location.
Jane: What you have are concretions. These rocks, usually form when carbonate shell material is deposited in a sandstone. After burial and what we call digenesis, the carbonate shell material is dissolved and then begins to be deposited as radial cement growths, forming the spherical concretions you have found. The roundness is not due to any mechanical process like you would find in a stream or creek, but rather by purely chemical processes. As the new carbonate precipitates cementing the sand together it starts from a seed point, maybe another bit of carbonate fragment, and grows outward, forming the sphere. Then later after that process has stopped, the surrounding sandstone or siltstone weathers and the spheroids are released.
Below is a url to a pdf file on the formation of large spherical boulders common to North Central Arkansas. You have smaller examples formed by the same process. I don't know the location you round yours, but likely the smaller spheres were just arrested in development due to changes in the geologic regime and they never were able to grow to the size and appearance of those shown in the document at the link. Them being hollow only means that some changes in the depositional process and growth of the cementing carbonates occurred. You might test to see if the rocks fizz when placed in contact with HCl or sulfuric acid.