These sorts of sample turn up at university geology departments on a regular basis by people hoping they are meteorites. Frequently they are proven to be bog-iron. http://en.wikipedia.org/wiki/Bog_iron The "crater" could also be explained by a glacial feature called a kettle or pothole. They can occur in clusters, but can also be solitary. http://en.wikipedia.org/wiki/Kettle_(landform) My work ISP is blocking picturepush but if this feature is in an area previously glaciated (ie most of north america north of say Kentucky), then this is a possibility.