Small-scale climate cycles from Bridge Creek Limestone Member, Colorado. A: Schematic diagram of limestone-shale bedding rhythms with shales condensed to 30% original thickness on average. B: Analysis of burrowing in one such cycle, showing basis for graphs in A. ‘Ichnospecies’ = trace fossil species. (After Barron et al. 1985)
Alternating limestones and clays in the geological record usually reflect changes in climate. On the basis that the Earth is billions of years old, geologists attribute them to astronomical cycles that take 20,000 years or more to complete (known as ‘Milankovitch cycles’). The direct sedimentary evidence suggests we should be thinking of the annual cycle of summer and winter.
Here the limestone derived mostly from marine plankton, the shale from terrestrial uplands, more intensely eroded during the winter owing to heavier rainfall. The cycles are approximately 1 metre thick, with burrowing increasing from bottom to top.
For an in-depth discussion of these issues, see How old is the Earth?