The rings are formed by eroding a hole into a series of flat layers.

However, one of the most fundamentamental and fascinating issues is how those flat layers were originally deposited. Several theories exist, flavoured by the viewpoint or bias of the author:

* Annual or seasonal layers on the bed of a lake or ocean of water.

* Annual or seasonal layers of dust deposited from the atmosphere

* Annual layers of ice/dust deposited by polar processes

* Rythmic layers deposited by a series of volcanic ash-falls

* Much longer-term layering caused by climatic cycles. Again, either dust, ice, water, or volcanism can be implicated

* Even more exotic scenarios such as my own ephemeral and unstable atmosphere model for the Noachian, of which you will probably hear more in the next couple of years.