The light and dark pattern is caused largely by the presence or absence of frost. The layers are best exposed on downward facing slopes, which are generally without frost and hence are dark. Although sequences of layers can commonly be traced unbroken for considerable distances, breaks in the sequence do occur. This pair of pictures includes an example of an angular unconformity where one set of deposits truncates another at an angle. The image is about 90 km across.
North is to the upper left.