Sometimes you can’t see the forest for the trees. Take, for example, the sedimentary rocks: a complex family of rocks that reflects the taking-apart, the rotting, the sorting, the dispersion, and the reconstituting of bedrock that operates so vibrantly on the lively surface of our planet. As one of my students wrote, after puzzling sadly over a tray of sedimentary rock samples in his lab, “the sedimentary rocks vary wildly from one another”.
And it’s true: the most recalcitrant, homogenous, chemically self-satisfied lavas and schists become sands of silica pure enough to make glass, clays fine enough to fire into porcelain, salt you can eat, carbon you can burn, lime you can cast into concrete, and pigments with which you can color your paints. Not to mention some less civilized – we prefer to say ‘immature” – members practically indistinguishable from the rocks that broke down to make them.
Working out a scheme to bring some order to this chemical fiesta is challenging. But in a fundamental way, the sedimentary rocks fall into two camps: those that were transported into place and those that formed in place.
Bedrock breaks down into rubble, sand, clay, and a variety of dissolved minerals here at the Earth’s surface, often mixed with organic material where soils form. This weathered material can be mobilized by slumping and sliding, running water, or wind, and transported elsewhere, exposing fresh bedrock to weathering. The mobilized material is called sediment, and the visible component made up of broken fragments of bedrock, resistant grains of quartz, and particles of clay is called clastic or detrital sediment.
The vast majority of this detrital sediment is transported by streams of running water, where it is efficiently sorted into various size fractions, and finally dropped, possibly to be remobilized by wind, waves, or turbidity flows. From this activity comes all the mudstone, shale, sandstone, conglomerate, and breccia that makes up the bulk of the sedimentary record. What all these detrital sediments have in common is the fact that they were transported into place.
The invisible component of dissolved minerals mixes freely and anonymously in lakes and lagoons, swamps and seas, where a variety of chemical and biological processes operate to precipitate out salts and silica of remarkable purity. Carbon, carried by invisible carbon dioxide, plays its part, trapped in vegetation, which can be preserved as coal, or bound up in the seas with calcium and magnesium to form limestone and dolomite. Depending upon the role played by inorganic vs organic processes, these sediments are called the chemical or organic sediment. But what they all have in common is the fact that they were formed in place. As geologist Noel James famously quipped, “carbonates are born, not made.” The same could be said about all the chemical and organic sediments.
Asking yourself whether the layers of sedimentary rock you’re seeing were transported there, or simply formed where they lie, should trigger a chain of questions that will have you thinking like a geologist in no time!