Sediments and sedimentary rocks make up only a small fraction, about the 8%, of the volume of the Earth’s crust: a ‘peel’ of debris of various origin accumulating on the surface. These rocks are also very fragile as their sedimentary record is easily destroyed by all those processes linked to plate tectonics that continuously smash, deform, melt, intrude, and metamorphose the sediments that form on our planet. Nevertheless, it is thank to the study of sedimentary rocks that we discovered how deep time is, that humans are only seconds of billions of years of the Earth’s history.

‘No vestige of a beginning – no prospect of an end’ (James Hutton, 1788)

The fundamental discoveries of how climate, environments, and even continents incessantly mutated and life evolved are thank to the study of sedimentary rocks! Layer after layer, sedimentary sequences are like a book, often fragmented or difficult to read with crumpled or half-burned pages, but that recorded millions of years of evolution of a planet in constant change. Clastic rocks can tell us stories of environments, like ancient floodplains or deltas, that do not exist anymore, but also of ancient mountains that have been eroded and reduced to a pile of sand. Carbonate rocks preserve fossils, many of them of extinct organisms, that tell us about how climate and life evolved in the past. Other sedimentary rocks tell us stories of desiccation of entire seas or lakes (evaporites), deep marine environments where only some organism with silica shells could deposit (siliceous rocks), or even one of the largest extinction events ever occurred that produced the iron that we exploit today (ironstones like banded iron formations).

Our society itself relies on sedimentary rocks, since fossil fuels exist only because the remains organisms were buried in sedimentary rocks and other sedimentary rocks had the right properties to store hydrocarbons at depth for millions of years. This is not the only example. Even sand, perhaps one of the most abundant resources on our planet, is rapidly disappearing in many places due to human activities.

In this section, I will show you the main structures, fossils, and types of sedimentary rocks that can be observed at outcrop and microscale and how learning to recognize them allows us to read through the pages of the Earth’s past. If you like what I am doing, follow me on my socials or support me at the price of a coffee!