Cliff-top dunes, also known as perched dunes, are dune

Dune

In physical geography, a dune is a hill of sand built by wind. Dunes occur in different forms and sizes, formed by interaction with the wind. Most kinds of dunes are longer on the windward side where the sand is pushed up the dune and have a shorter "slip face" in the lee of the wind...

s that occur on the tops of cliff

Cliff

In geography and geology, a cliff is a significant vertical, or near vertical, rock exposure. Cliffs are formed as erosion landforms due to the processes of erosion and weathering that produce them. Cliffs are common on coasts, in mountainous areas, escarpments and along rivers. Cliffs are usually...

s. They are uncommon in most parts of the world, because they only develop under unusual geomorphological conditions. Processes by which they may be formed include:

• a dune advances up a pre-existing slope, which is then eroded to form a cliff;
• a dune forms during a period of high sea level, then sea level drops, exposing a cliff face.

They are most often seen as coastal landforms, but can also occur along lakes and rivers, and in deserts. Places where they are relatively common include the west coat of Jutland

Jutland

Jutland , historically also called Cimbria, is the name of the peninsula that juts out in Northern Europe toward the rest of Scandinavia, forming the mainland part of Denmark. It has the North Sea to its west, Kattegat and Skagerrak to its north, the Baltic Sea to its east, and the Danish–German...

, Denmark

Denmark

Denmark is a Scandinavian country in Northern Europe. The countries of Denmark and Greenland, as well as the Faroe Islands, constitute the Kingdom of Denmark . It is the southernmost of the Nordic countries, southwest of Sweden and south of Norway, and bordered to the south by Germany. Denmark...

. They can also be found in certain places along the shores of the Great Lakes

Great Lakes

The Great Lakes are a collection of freshwater lakes located in northeastern North America, on the Canada – United States border. Consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario, they form the largest group of freshwater lakes on Earth by total surface, coming in second by volume...

 in Michigan

Michigan

Michigan is a U.S. state located in the Great Lakes Region of the United States of America. The name Michigan is the French form of the Ojibwa word mishigamaa, meaning "large water" or "large lake"....

, including the Grand Sable Dunes in Pictured Rocks National Lakeshore

Pictured Rocks National Lakeshore

Pictured Rocks National Lakeshore is a U.S. National Lakeshore on the shore of Lake Superior in the Upper Peninsula of Michigan, United States. It extends for 42 miles along the shore and covers...

 and in the Sleeping Bear Dunes National Lakeshore

Sleeping Bear Dunes National Lakeshore

Sleeping Bear Dunes National Lakeshore is a United States National Lakeshore located along the northwest coast of the Lower Peninsula of Michigan in Leelanau County and Benzie County....

, most dramatically on South Manitou Island

South Manitou Island

South Manitou Island is located in Lake Michigan, approximately west of Leland, Michigan. It is part of Leelanau County and the Sleeping Bear Dunes National Lakeshore. The uninhabited island is in land area and can be accessed by a ferry service from Leland...

.