Stratigraphy of the NPLD (possible True Colors; credits: Lunexit)

This image shows an example of layers in the Martian NPLD. These Deposits, part of the Planum Boreum dome, are composed mainly of water ice and small amounts of dust.

The layers within these Deposits are exposed by shallowly-sloping troughs that cut into them. This image is particularly interesting because it crosses complicated trough geometry, making the layers appear curved and exposing multiple stratigraphic levels.

Note that layers of different thicknesses are visible. Layer thickness is directly related to the accumulation rate of the layer; a higher accumulation rate will lead to a thicker layer.
However, a myriad of factors work together to influence accumulation rate, such as the amount of Sunlight reaching the Surface and the amount of water vapor existing in the contemporaneous Atmosphere.