I collected this in a canyon in the Black hills of Eastern Wyoming. I don't usually think of fossils in that region, but there happened to be a limestone column with some brachiopods about a half mile from where this rock was found, suggesting a marine environment for some of the rocks in the area. I can't tell which geologic strata I was in, as things change quickly in the Black Hills http://pubs.usgs.gov/sim/2777/downloads/2777_sheet1.pdf , and there is material washing down the canyons all the time. This was in a dry creek bed.
approximately 20 cm across, 10 cm wide and 10 cm thick