Hi, Ken,
It looks to me like a fossil-bearing limestone. From what I can see in the picture, the wavy lines look like fossil clam shells, and all of those circular bits look like a kind of a fossil called a crinoid. My guess (which is all it is, since it's only a photo), is that the light part of the rock, the matrix that binds the fossils together, is a calcium carbonate. The shells probably stick out a bit giving it the rough texture because they would be made of aragonite, slightly harder than the calcite matrix. The round shape is probably just from weathering, maybe being rolled around in a strong surf, or maybe dumped off a glacial river. The brownish black patina looks like a mixture of iron and manganese hydroxides, which is a common groundwater precipitate and gives many rocks a kind of varnish. Try and see if you can scratch different places with a penny.

From RockDr - October 22, 2009 at 20:55:07

Message: 68456