Sea Becomes Stone

Sea Becomes Stone. . You can see deep time in the overlook walls. Platteville limestone formed about 455 million years ago when a sea covered what we now call "Minnesota." As sea creatures died, their shells fell to the ocean floor. Building up over time, they compressed into stone. The Platteville limestone layer is one of many that have formed over the past 500 to 453 million years., Works Progress Administration (WPA) workers quarried Platteville limestone and used it to build the overlook., Decorah Shale , 454–453 million years BP , seen in limited areas along the bluff, Platteville Limestone , 455–454 million years BP , highly resistant to erosion , About 12,000 years ago, Glacial River Warren emptied Glacial Lake Agassiz. The rush of water eroded the landscape. It formed the bluff, exposed geologic layers, and revealed fossils. Look for fossils in the wall to your right. You might even see a few in the overlook wall., Glenwood Shale , 455 million years BP , a very thin layer, St. Peter Sandstone , 458–455 million years BP , commonly seen in bluffs