This foredune area is characterized by a series of hills and swales. Mt. Tom, at 186 feet above the lake, is the highest of these ridge tops. Wind erosion has cut depressions, called blowouts, through these ridges. The three largest of these blowouts, Beach House, Furnessville, and Big Blowout, extend into the interdunes area of hills, pockets, and troughs. Big Blowout has uncovered an area of dead tree trunks known as the Tree Graveyard. This area was once a white pine forest before shifting sands buried it. Because sand is still unstable in the interdunes, vegetation here resembles that found on the foredunes.

The backdune area begins on the leeward slopes of active blowouts or on protected ridges. Tops and upper leeward slopes of the backdunes are forested with nearly pure stands of black oak, mixed with a few white oaks and stunted sassafras. Thick stands of blueberry, bracken fern, and greenbrier are found in the understory.

South of the dunes is a wetland area, composed of marsh, shrub shrub, and wetland forest. This large area is drained by Dunes Creek. Between the dunes and this wetland is a strip of sandy flats with greater organic matter. One sheltered cove on these flats has native white pines associated with oaks, tulip, white ash, and basswood.

The Indiana Dunes' location within the continent highly determines this cycle of climate. The air mass of the Gulf of Mexico is a major player in the climate. Southerly winds from the Gulf region transport warm, moisture laden air into the area. The warm moist air collides with continental polar air brought southward by the jet stream from central and western Canada. A third air mass source found in Indiana originates from the Pacific Ocean. Because of the orographic obstruction posed by the Rocky Mountains, however, this third source arrives less frequently in the state.

The effect of Lake Michigan on the climate of the Dunes is most important and this effect is most pronounced just inland from the Lake Michigan shore and diminishes rapidly with distance. Cold air passing over the warmer lake water induces precipitation in the lee of Lake Michigan in autumn and winter. As a result of this phenomenon, heavy winter precipitation, especially snowfall, can extend eastward from Gary inland to as far as Elkhart. Lake-related snowfall and cloudiness can extend to central Indiana in winter, driven by strong northwesterly winds. In the spring, daily maximum temperatures decrease northward in northern Indiana because of the cooling effect of the Lake. Average daily minimum temperatures in autumn are higher by the Dunes near the warmer lake surface than farther south.

A winter may be unusually cold or a summer cool if the influence of polar air is persistent. Similarly, a summer may be unusually warm or a winter mild if air of tropical origin predominates. The interaction between these two air masses of contrasting temperature, humidity, and density favors the development of low pressure centers that move generally eastward and frequently pass over or close to the state, resulting in abundant rainfall. These systems are least active in midsummer and during this season frequently pass north of the Dunes.

Purple loosestrife
Invades wetlands in northern Indiana, forming pure stands that choke out native vegetation. This eliminates food and cover for many wildlife species, which are dependent on a diverse mixture of native species to survive.

Bush honeysuckles
Grow so densely they shade out everything on the forest floor, often leaving nothing but bare dirt. This means a great reduction in the food and cover available for birds and other animals. Some species release chemicals into the soil to inhibit other plant growth, effectively poisoning the soil. Bush honeysuckles are found throughout the state, but are particularly invasive in central and northern Indiana.

Reed canary grass
Widely planted for forage and erosion control, has taken over large areas of both open and forested wetlands throughout Indiana. It forms monocultures by out-competing all the native wetland plant species. There may be native strains in the state; however, there is no reliable way to tell the native from the non-native strains.

Autumn olive
Often planted for wildlife food and cover in the past, can quickly take over open areas, eliminating all other species. Such monocultures actually reduce the variety and amount of wildlife food available. It is now found throughout Indiana.

Common reed
Grows in open wetland habitats and ditches primarily in northern Indiana. It can create pure, impenetrable stands, excluding all other wetland plants. Some populations are not invasive and may be native; however, there is no reliable method to tell the two apart.

Garlic mustard
Can grow in dense stands covering many acres of forest understory. Now found throughout Indiana, it is a particular threat to spring wildflowers, overtopping and shading them out. Compared to the diversity of plants it eliminates, it provides little food for wildlife

Oriental bittersweet
Occurs throughout Indiana and can overrun natural vegetation, forming nearly pure stands in forests. It can strangle shrubs and small trees, and weaken mature trees by girdling the trunk and weighting the crown. There is some evidence that it can hybridize with American bittersweet, thus threatening the genetic integrity of the native species.

Common and glossy buckthorns
occur in a wide variety of habitats in northern Indiana and spread quickly through natural areas by seed. They take over the understory and eliminate the diversity of native plants important to wildlife.

General Information

Vegetation within Indiana Dunes National lakeshore helps define habitat types such as rivers and streams, bogs, bottomland (floodplain), dune complex, foredune, interdune, and backdune complexes, marsh complex, mesophytic forest and prairies, panne, savanna complex and swap complex.