Shelf ice can refer to the ice

Ice

Ice is water frozen into the solid state. Usually ice is the phase known as ice Ih, which is the most abundant of the varying solid phases on the Earth's surface. It can appear transparent or opaque bluish-white color, depending on the presence of impurities or air inclusions...

 that forms when a portion of a lake

Lake

A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...

 surface freezes. It is often then washed upon the shore. This is common within 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...

.

Formation

Shelf ice forms from float ice. Float ice is like drift ice

Drift ice

Drift ice is ice that floats on the surface of the water in cold regions, as opposed to fast ice, which is attached to a shore. Usually drift ice is carried along by winds and sea currents, hence its name, "drift ice"....

, but seldom becomes large enough to support humans or large animals. The cold temperatures freeze small areas of the open lake, forming float ice. Similar ice will form along the shoreline when the lake is calm. Over time, the ice will build up from continued freezing on the open lake and from snow accumulation on the surface. The surface of the Great Lake seldom remains calm enough for the entire surface to freeze. Note: The smaller lakes, Lake St. Clair

Lake Saint Clair (North America)

Lake St. Clair is a fresh-water lake named after Clare of Assisi that lies between the Province of Ontario and the State of Michigan, and its midline also forms the boundary between Canada and the United States of America. Lake St. Clair includes the Anchor Bay along the Metro Detroit coastline...

, Lake Erie

Lake Erie

Lake Erie is the fourth largest lake of the five Great Lakes in North America, and the tenth largest globally. It is the southernmost, shallowest, and smallest by volume of the Great Lakes and therefore also has the shortest average water residence time. It is bounded on the north by the...

, and Lake Ontario

Lake Ontario

Lake Ontario is one of the five Great Lakes of North America. It is bounded on the north and southwest by the Canadian province of Ontario, and on the south by the American state of New York. Ontario, Canada's most populous province, was named for the lake. In the Wyandot language, ontarío means...

 will freeze over more often than the larger upper lakes. The wind pushes the float ice and creates waves which also move the ice. Over time, the ice will accumulate on the windward shores of the lake. Like sand, which is moved by the wind and the waves, the ice begins to accumulate along the east and southern shore of Lake Michigan

Lake Michigan

Lake Michigan is one of the five Great Lakes of North America and the only one located entirely within the United States. It is the second largest of the Great Lakes by volume and the third largest by surface area, after Lake Superior and Lake Huron...

. When pushed onto the beach, it remains and over time, the ice accumulates on top of the beach and the ice already pushed ashore.

On days with a strong off-shore wind, an ice sheet may reach out into the lakes are far as the eye can see. But across the lake, there is likely to be open water several miles wide.

Structure of shelf ice

Shelf ice is a floating mat of ice, but unlike a pond or a small lake that freezes over, the shelf is not a uniform sheet of ice. Created by the wind and waves, the shelf ice is a jumble of ice chunks, pushed onto each other. It is as if you took a pile of rubble and pushed up against a wall. The more you push, the narrow the pile becomes, and it rises in a ridge. But there is nothing stable in the pile. The individual pieces (ice in the case of shelf ice) are not initially connected; they only float upon the water surface and rest upon each other. Many become jammed together but throughout the structure, there are pockets of air. Since each piece of ice developed independently, each is of a different thickness, creating variations in strength, density, and depth.

Over time, the ‘shelf’ extends out from the shore in a long-lasting floating ledge. The ledge can become very thick depending on the size of the ice pieces and the size of the air pockets. The ice sheet can be stabilized by grounding on shore or lake bottom, through additional freezing from seaspray, precipitation and undersurface ice formation and through melt/refreezing cycles from solar heating and weather variations. Therefore, an ice shelf can vary from a large flat surface to a jumbled pile of blocks with parallel ridges.

When a ‘shelf’ extends out to a sand bar, a ridge will form that is higher than the rest of the shelf. If there is more than one sand bar along a beach front, the ice shelf will form a ridge along each sand bar.

Dangers

Because of the uneven surface and possible air holes throughout shelf ice, the ice may give way unexpectedly. One can plummet several feet through an air pocket into open water. Self rescue is difficult due to the friability of the ice and the rate at which the cold water will sap energy. Traveling in a group with the correct ice self-rescue tools lessens the danger.