Whitefish Dunes Geology

The Niagara Connection

The rocks around Door County tell a story of what was once here. The exposed dolostone give clues of a shallow warm sea once covered the area around 425 million years ago. This time period is known as the Silurian Sea.

Clues to the age of the rock are found in the fossils. These fossils were made when shifting sediment on the bottom of the sea covered the coral or shell. Once covered, water and dissolved minerals slowly replace the original material. As the rock around the fossil wears away, it is revealed to you. Look for fossil sea shells and coral reefs in exposed rock throughout the Door Peninsula.

Dolostone forms what is known as the Niagara Cuesta. Shaped like a giant bowl this band of rock goes from Wisconsin around the north end of Lake Michigan and Huron to Niagara Falls. The Door Peninsula and Niagara Falls lie opposite each other on the rim. Over millions of years the once-flat sea bottom deformed into the bowl. Only edges of the cuesta are seen above ground. The remainder lies buried below several layers of rock. The Niagara Escarpment is the cliff part on one side of the cuesta. Rocky bluffs of Potawatomi, Peninsula and Rock Island State Parks are examples of this edge.

Sandy Hitchhikers

Push a magnet into the sand and discover clues to its origins. Magnetite, an iron mineral found in the Lake Superior basin, will cling to the magnet. These sand grains are debris that glaciers eroded from the bedrock of Canada and dumped into Lake Michigan.

Depositing the Sand

Over thousands of years, accumulating sand has closed off Clark Bay to create Whitefish Dunes and Clark Lake.

The formation of the dunes and the closing of Clark Bay took a long time. About 5,000 years ago, Cave Point defined the bay. When water levels rose, the point was submerged, but remained as a sand bar. This slowed down water currents, causing the sand that water carried to deposit southwest of the point. When the water began to recede, the first bits of what would become the dunes peeked above the lake level. Since, they have continued to grow, eventually closing the bay and forming what we know today as Whitefish Dunes.