All About Dunes
How are Dunes Formed
As recently as 16,000 years ago, Michigan was covered with glacial ice thousands of feet thick. These glaciers contained a mix of boulders, cobbles, sand, and clay. Over time, the glaciers moved, scraping the surface of the land. The scraping action loosened rocks and broke them into smaller pieces. Then, as the glaciers melted, great amounts of rocky debris were washed into rivers and lakes. Wave action caused the debris to erode (break down) more until it became what we call glacial drift, which is a source of sand in most of Michigan’s dunes.
Formation of dunes is an ongoing process. Waves and currents move these tiny rocks inland, creating beaches along the Great Lakes shorelines. Winds blowing shoreward pick up the tiny grains of sand and move them along the ground. A slight obstruction, such as a clump of grass, a stone, or a piece of driftwood, breaks the force of the wind and causes the heavier sand grains to fall and settle against the obstacle, forming a slight mount or ridge. As wind moves more sand up to the top of the mound, the pile becomes so steep that it collapses under its own weight and sand avalanches down the downwind side (backslope) of the pile. This is called the slip face. As the action repeats over and over, the obstruction is eventually completely buried by an ever-growing mound and a dune slowly forms.
As a dune grows, the repeating cycle of sand inching up the windward side to the dune crest and then sliding down the slip face, causes the dune to move, migrating in the direction the wind blows. The fact that dunes migrate is fascinating because it makes them seem alive. But this movement can eventually bury any land, vegetation, and even structures on the downwind side of the migrating dune.
Dune Zones and Plant Succession
Dunes go through various stages of development as they gradually migrate away from the shoreline, stabilize, and eventually evolve into woodlands. This process is known as succession. Each state of succession creates different micro-environments or zones that are formed by changes in temperature, moisture, light intensity, and varieties of plants and animals. Vegetation plays a crucial role in stabilizing shifting dunes because it acts as a barrier to stop blowing sand and helps to anchor the sand. It also adds nutrients to the sand, which eventually allows it to transition into soil.
Beach Zones are where water meets land. There are not many creatures that live on the beach because the weather conditions are constantly changing and there is little shelter from the wind, waves, heat, rain, and cold. Most of those that do, survive by burrowing under the sand to get away from the harsh elements.
Most of the wildlife seen here are usually just visitors that live elsewhere. Among these visitors are scavengers ranging in size from small flies and beetles to shore birds such as herring gulls and even bald eagles. Remains of fish, birds, and insects wash ashore and provide food. Mammals that venture down to the beach include raccoon, skunk, and fox. They will wait until dark, after the hot sand has cooled, to search the shoreline for something to eat.
Even though Kitchel Lindqust Hartger does not actually encompass any Lake Michigan shoreline, this zone, which lies just to the west of the Preserve, plays an integral role in the formation of its dunes.
The Foredune Zone (or Embryo Dune) is the first dune ridge behind the beach. Although foredunes are above the beach wave action most of the time, they are sometimes subject to rough conditions during storms.
Like the beach, life in the foredune is a daily struggle against shifting sand, scarcity of nutrients, rapid water drainage, high evaporation rates, and storms. Out of reach of waves, a few hardy plants, such as marram grass, are able to survive. These plants are known as pioneer species because they are one of the first plants to become established. By stabilizing the sand with their extensive root systems, they increase the sand’s capacity to retain water and nutrients and make it possible for other plants to gradually take hold. A foredune stabilized by these plants can host a wide variety of wildflowers and shrubs. These dunes will remain stable as long as the vegetation is undisturbed.
Unlike plants, animals can escape the extreme temperatures and harsh conditions. Most birds migrate seasonally or retreat daily to the cover of heavy vegetation. Many animals are nocturnal and are most active during the cooler nighttime hours. Like the plants, animals that live in the grass-covered foredune have special adaptations that help them survive the extreme temperatures of summer and winter. Sometimes, surface temperatures of the open dunes can reach 120 degrees Fahrenheit.
There are three parallel ridges of foredunes in the Dune Preserve and within its vicinity. The houses between North Shore Drive and Lake Michigan sit on the foredune. The second, slightly higher foredune ridge runs next to the road, along the Preserve’s western boundary. The third, even taller row of foredunes is slightly further inland and lies between the Hartger and the Lindquist Trails.
The Trough Zone (or Interdunal Wetland) is sheltered by the foredunes and is where winds are slowed and sent swirling, sometimes scooping out low areas for interdunal ponds. These shallow pools can vanish and reappear depending on summer droughts and fluctuating water levels in Lake Michigan.
The moisture and shelter found in these areas allow for more plants and animals to find the habitat they need. It is an important breeding ground for insects and amphibians. Frogs burrow in the mud, and insects, like water striders, may skate across the water’s surface. Heron and other shorebirds feed on the rich aquatic life. Rushes and sedges grow well in the shallows.
Kitchel Lindquist Hartger has multiple interdunal wetlands that can be found along the trails. You can find some in the open dunes along the Hartger and vandenBerg Trails and even within the forests along the Lindquist Trail. Depending on conditions, they can be either wet or dry.
The Backdune Zone (Dune Forest) were once foredunes along the shore of Lake Michigan when water levels were much higher hundreds of years ago. As the lake receded and newer dunes formed closer to the shoreline, the winds slowed on these older dunes, allowing more and more plant life to become established. As leaves and other plant and animal matter decayed, rich topsoil formed which eventually allowed for larger trees like oaks, pines, and maples to grow and stabilize these dunes. The heavily forested habitat in this zone acts as a shelter from the harsh changing weather environments of the beach.
The uniqueness of the dunes forest stem from the fact that they develop steep, barren sand slopes that are a short distance from the open dunes. The contrast between cool, shaded dune forests and the extreme temperatures and intense sunlight of the open dunes can be striking. Though thin and slow to accumulate, the topsoil of the dune forests supports a variety of spring wildflowers and woodland plants. Backdune forests are an important shelter and food source for migratory birds and mammals.
The best example of a backdune forest at the dunes preserve is Kitchel Dune, the large forested dune visible from the outdoor classroom building. This dune is the oldest forest on the preserve and can be considered a climax successional community (old growth forest). The forest here also provides excellent habitat for many wildlife species.