Geology

The rocks around Door County tell a story of what was once here. The exposed dolostone gives clues of a shallow warm sea that 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 seashells and coral reefs in exposed rock throughout the Door County 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 County 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.

How the dunes were created

The beach at Whitefish Dunes State Park appears calm and unchanging. But in reality, the beach is always a work in progress. It is constantly being constructed and reshaped by strong forces of nature.

The wind is the major sculptor of the beach. When it blows across the smooth surface of Lake Michigan, there’s little to hinder its momentum. It hits the shore with a fury, picking up grains of sand and pushing them inland. As the wind velocity slows over land, sand drops to the earth and in time, a pile is formed.

Gradually, the pile grows into a tall sand dune. Slowly wind blows sand grains up the gently sloping upwind face of the dunes. Once cresting the top, the wind moves down the steeply sloping backside. This creates momentum for the wind to pick up sand grains and continue inland. The wind speed slows once again as it heads inland. Sand drops to earth beginning another sand dune and the process starts over.

The younger dunes on the shore block the wind and allow plants to establish on the older dunes. These plants live and die over hundreds of years, creating real soil, yet the soil remains sandy. Over time the grasses and shrubs make way for the forest.

If you’ve ever felt the sting of sand pelting your skin on a windy day at the beach, it’s because you’ve been caught in the middle of this sculpting process.

Sand moves in many ways

Sand on the beach ranges in size from pebbles to very fine sand grains. Wind-formed dunes consist of medium to fine sand grains. As the wind blows, the majority of the sand is moved by a process called saltation. This is a bouncing of the grains across the beach. Coarse sand is too large to be moved by this process and is moved short distances by "surface creep," rolling the grains along the surface. Fine silt particles are carried in suspension by the wind for long distances.

Sand stabilizers

Between the edge of the water and the beginning of the dunes, dry sand is constantly in motion, dancing, shifting and floating. This makes the beach a treacherous place for plants to take root. One kind of plant that does live here is marram grass. It spreads its tuberous roots just under the surface of the sand and forms an underground web that helps hold the sand in place.

Though marram grass stabilizes the sand for its own survival, an inadvertent side effect is that it makes it possible for other kinds of vegetation to begin to take hold. Other plants take advantage of the increased stability of the sand surface and start to colonize areas the marram grass helped make safe.