Historical Geology, Sediments and climate

Clashach Cove, Scotland. Analysis of the exposed yellow sandstone reveals that this was once a hot arid sandy desert. (Image: Gary Rogers, CC BY-SA 2.0)

In this article we shall look at how various kinds of sediment are characteristic of the climates in which they are formed, so that the lithified counterparts of these sediments can be used as an indicator of climatic conditions in the past.

The reader may find it useful to look back at the main articles on deserts, glaciers, paleosols, ooids, and coal to put the remarks below into their proper perspective.

Sediments and paleoclimates

There are a number of types of sediment which are symptomatic of the climatic conditions under which they formed.

• Glacial sedimentation and erosion. As was discussed in the article on glaciers, we can detect their former presence by such clues as striations, glacial till, and dropstones. Obviously the former presence of glaciers indicates a former cold climate.
• Similarly the presence of aeolian sand, with its distinctive large-scale cross-bedding, pinstripe laminae, etc, indicates an arid and usually a hot climate. The same may be said of features such as playa lakes, indicative of low rainfall and a high rate of evaporation.
• Redbeds are sedimentary rocks cemented together chiefly by iron oxides; these are characteristic of a dry climate.
• Ooids are formed only in warm, shallow, agitated water, and so are indicators of a warm climate.
• Coal requires a peat swamp, and therefore cannot form in an arid climate.
• The soil types known as laterites are produced in tropical conditions with seasonal alternation between a monsoon season and a dry season, and so the corresponding paleosols indicate such conditions where they are found in the geological record.

How do we know?

For information about how we know that glacial till is deposited by glaciers, or that coal is lithified peat, and so forth, the reader should refer to the main articles on those topics.

Given that knowledge, the inference from the sedimentary rocks to the climate in which they were formed is a fairly obvious one. It is difficult, after all, to suppose that in times gone past glaciers flowed in conditions of sweltering heat, and it is a downright contradiction in terms to think of a dry swamp. Similar remarks apply to the other sediment types listed above; unless the laws of physics or chemistry were significantly different in the past than they are in the present, it is hard to see how sediments in the past could be characteristic of different climates than the corresponding sediments in the present.

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