Angular Dropstone

Another good example of a boulder-sized dropstone deposited in the glacial mud. This boulder is a piece of dolomitic limestone, which forms in warm shallow seas. A current theory suggests that the earth lurched from an "icehouse" to a "greenhouse" several times between 760 million and 550 million years ago. This might explain why limestone blocks with fossil algae called stromatolites are embedded in the glacial mud. (Photo by Rob Allen)