Sand

Geologists adore the sedimentary rocks. These are the rocks that contain the Earth’s archives. These rocks outline areas of crustal subsidence and mountain uplifts. They reveal the distribution of ancient environments at the Earth’s surface. They record changes in climate and fluctuations in sea level. They preserve the record of life on our planet.

Of all the sediments, the sandy ones carry the most information about the widest variety of ancient environments: desert, piedmont, river valley, coastal plain, delta, beach, marine shelf and slope, in all their infinite variations.

Muddy sediments are much more abundant in the geological record, but since mud can accumulate anywhere there is slack water, onshore or off, shallow or deep, we depend on interbedded sand to help us understand their story.

Chemical concentrates, like limestone, dolomite, rock salt, gypsum, and coal are fascinating, and economically important, but they each form under very specific conditions. Their presence complements the story told by sand but cannot replace it.

On top of that, the layers of sandstone formed by sandy sediments are often the most eye-catching aspects of the landscape, forming resistant cliffs, rims, and benches when they are interbedded with more easily eroded mudstones and shale. And from an economic standpoint, the more porous sandy sediments are where much of our fresh water, oil, and natural gas is stored.

Santa Fe River carrying sand to the Rio Grande

Santa Fe River carrying sand to the Rio Grande

Continental Environments

Onshore, in continental settings, streams cut and and then fill channels with sand, carrying the finer mud on downstream, or depositing it over the banks during floods. Sediments deposited onshore by running water are called alluvium, and large amounts of sand can accumulate in the alluvial plains built up by streams. Where a stream enter a body of standing water, like the sea or a lake, much of its load of sediment is quickly dropped to form a delta, and sand can be deposited in large river mouth bars. As a delta builds outward, it extends the alluvial plain seaward, or fills the lake.

Coarse sand, mixed with pebbles and cobbles, frequently accumulates as alluvial fans at the margins of basins that flank mountain ranges, filling stacks of ill-defined and overlapping channels.

Fine sand can be remobilized by wind in arid climates, where it often collects in vast dune fields. These dune fields are a kind of large onshore sand bar. Since this sand is windblown, and not carried by running water, it is not considered an alluvial, but rather an aeolian sediment.

Permian red beds, with fossil dunes forming the steep cliff

Permian red beds, with fossil dunes forming the steep, poorly-bedded sandstone cliff

Marine Environments

Offshore, in marine settings, waves and currents rapidly redistribute the sand brought in by streams into beaches, barrier islands, and other kinds of shallow marine bars. Estuaries and lagoons also trap sand in their shallow waters. These environments are collectively called marginal marine or shallow marine environments.

Further offshore, fine sand can  be remobilized by undersea currents flowing in submarine channels cut into the continental shelves, and carried deep on to the ocean floor, where it accumulates as channel fill and thin overbank turbidites in submarine fans.

Shallow marine sand bars

Shallow marine sand bars forming ledges of sandstone

So you can see, sandy sediments truly are the great archivists of the sedimentary record. Learning to read the stories written in shifting sand will greatly enrich your appreciation of the natural world.