Learn more about the geology of Indiana Dunes National Park.
Much more recently, over the past two million years, Earth’s climate has alternated between cold, icy periods of time called glacial periods, and warmer interglacial periods, more similar to the modern day. During the glacial periods, ice sheets advanced into what is now the Great Lakes region. River valleys and areas of weaker bedrock acted as paths of least resistance for the advancing glaciers, and it was in these areas of weakness that the glaciers dug out basins. Arc-shaped mounds of sediment called moraines deposited at the ends of glacial lobes also helped define the drainage basin that would become the Great Lakes. Lakes much like the ones we see today likely formed and disappeared multiple times as the climate alternated between glacial and interglacial periods.
The most recent glacial period reached its peak about 20,000 years ago, and Lake Michigan formed as a result of the glacial retreat. Beaches and sand dunes formed along the shores of the lake over 10,000 years ago, as sediments carried by rivers and weathered from the shore by wave action were deposited along the coast. Remnants of these early phases of dune-building, now forested over, are found inland of the modern coastline (purple on the map below). The modern dunes and beaches (blue) began forming about 5,900 years ago.
A sand dune is more than just a big pile of sand – it is an active, moving landform that grows and changes over time. Dunes need three things in order to grow: a supply of sand, winds strong enough to move the sand grains, and a clear area where the sand can accumulate. The beaches of Lake Michigan meet all these criteria. Wave action eating away at rock and sand along the coast, particularly in the north where there are tall bluffs, produces sediment that is then transported southward. Eventually, the waves deposit the sand onto beaches. This supply of sand is then blown further inland, across a wide beach relatively clear of vegetation.
The growth of the dunes is critically tied to one notable plant which thrives in this sandy environment: beachgrass (Ammophila breviligulata). Beachgrass is common throughout the Indiana Dunes, where it is often seen in small piles of sand. However, the grass does not grow in these young dunes; rather, the dunes grow in the grass. The long blades of beachgrass act as traps that slow the wind and catch windblown sand. In an area covered in beachgrass, the resulting sand piles will eventually join together, forming a low ridge known as an incipient foredune. This is the first stage in building a sand dune. The image below shows small ridges of sand growing downwind of beachgrass clusters that form as a result of the grass slowing the wind due to friction.
The drag effect of the beachgrass and the firm grasp of its roots create a stable surface on top of the incipient foredune where many other kinds of plants, including trees, can grow. As the foredune continues to grow and develop a more diverse plant community, it becomes an established foredune.
If the vegetated surface of the established foredune is stable for a long enough period of time, its growth and movement will mostly cease, and a layer of soil rich in organic material will develop. But these are not the largest dunes seen in the park – how do we get from a forested, immobile foredune to moving sand dunes hundreds of feet high?
When foredunes are eroded, particularly when water levels in the lake are high, vegetation is cleared away to form patches of bare sand are called blowouts. Sand grains are blown up along the hollow of the blowout, eventually avalanching over the crest to form a steep downwind slope called a slipface. By that point, the blowout has developed into a mound with a concave upwind slope, a slipface, and ‘arms’ on either side where vegetation stabilizes the sand: a parabolic dune. Dunes of this type are the largest in the Indiana Dunes. From a satellite view, they appear as fingers or U-shaped lobes of sand pushing into the forests, their longest dimensions usually parallel with winds coming from the northwest.
Dunes in the park are still actively migrating downwind. They move as layer after layer of sand is blown from the front of the dune over to the slipface. The most active dune, Mount Baldy, can move up to 18 feet in a year, swallowing up entire trees as it advances, as seen in the image of Mount Baldy’s slipface below. Most sand movement occurs during storms in the fall and winter, when the wind is strongest. In the winter, ice mixed in with the sand holds the grains together, causing sand to build near the crest. The spring thaw then brings sand avalanches, and so most of the dune’s advance occurs in spring.
In addition to the large dunes, there are a number of smaller sandy features that reveal the wind’s impact on the land. For example, the sand is often shaped into ripples similar in appearance to those formed by moving water on the beach or in shallow rivers. The effect is especially visible where the sand is undisturbed by foot traffic. The way these features form relates to the way that sand grains move in the wind. The largest sand/gravel grains cannot be moved by the wind at all, while the smallest stay airborne for a long time – as is seen in desert dust storms. Between those sizes, sand moves by a process called saltation – a bouncing, rolling motion. On an uneven sandy surface, some slopes are angled toward the wind and are hit by saltating grains. This is where sand builds up, and where more grains are brought into saltation, impacting again some distance downwind in another area of sand buildup. If a slope faces away from the wind, saltating grains will fly right over it, and no sand will be deposited there. The result is that sand will accumulate at regular intervals, with the distance between accumulations increasing with faster winds. Thus, a sandy surface exposed to strong enough winds will be shaped into ripples perpendicular to the wind.
Wind can also erode the sand into interesting shapes, especially seen during storms, or in winter, when there is a combination of strong wind and more cohesive sand held together by rainwater, snow, or ice. Moving sand blasts the more cohesive sand, exposing layering within the dune and sometimes forming streamlined ridges. Other erosional processes, such as waves or human activity, can also expose layering within the dunes.
While the Indiana Dunes are mostly known for their beaches, the park also protects a number of wetlands. These ponds are formed by the movement of groundwater through glacial and coastal landforms. The hills and valleys created by the modern and ancient dunes, as well as the older glacial deposits, break up drainage in the region and affect groundwater flow. Groundwater recharged in the higher regions of the glacial moraines flows downward, eventually seeping out onto the surface to create ponds and wetlands in low areas between dune ridges. T he Indiana Dunes support a wide variety of plant and animal species, making the park one of the country’s notable biodiversity hotspots.
Human activities affect geologic processes at Indiana Dunes as well. Industrial development has drained wetlands and leveled dunes, damming of rivers and dredging of harbors has reduced the influx of sediment to the beaches, and climate change brings more storm winds and less protective ice in the winter (Kilibarda and Kilibarda, 2016). The effect of these changes is an increase in erosion and dune movement. The changes brought by human activity reveal the delicate balance the different geologic processes in the Indiana Dunes, and how easily that balance can be disturbed.