Land bridges are formed when sea levels drop, exposing areas of the seabed that were previously submerged. These exposed areas can then act as a bridge between landmasses, allowing animals and humans to migrate between them.

Some famous examples of land bridges include:

* Bering Land Bridge: This land bridge connected Siberia and Alaska during the last glacial period, allowing humans to migrate from Asia to North America.

* Doggerland: This land bridge connected Britain to mainland Europe during the Mesolithic period, allowing for the movement of people and animals.

It's important to note that ice sheets are not land bridges themselves, but they can play a role in their formation. When ice sheets melt, they can cause sea levels to rise, which can create land bridges.