Gravel

• 1. Introduction

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  Gravel is a loose aggregation of rock fragments, and almost half of all of it ends up locked inside concrete. The rest spreads beneath the roads we drive on, settling into road bases and surfaces with or without asphalt or other binders. One cubic metre of it weighs roughly 1,800 kilograms. A single cubic yard tips the scales at about 3,000 pounds. It seems like the most ordinary material on Earth, scattered across riverbeds and quarries without a second thought. Yet this humble mix of crumbled stones carries a precise vocabulary of its own. How do geologists tell granule from pebble, gravel from rubble, natural deposit from crushed stone? Why does porous gravel underground turn into a source of drinkable water? And what does a windblown ripple of gravel in Argentina have to do with the planet Mars? The answers begin with a question of measurement.

• 2. The Problem Of Size

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  Two millimetres is where many geologists draw the first line, defining gravel simply as loose rounded rock particles over that diameter, often without specifying any upper limit at all. The trouble is that no single definition holds everywhere. The technical meaning shifts by region and by area of application. The Udden-Wentworth scale, widely used by geologists in the US, splits the material into granular gravel and pebble gravel. ISO 14688, used in soil engineering, grades it instead as fine, medium, and coarse across ranges of 2 millimetres, 6.3 millimetres, 20 millimetres, and 63 millimetres. Shape matters as much as size in some definitions. Gravel is sometimes distinguished from rubble, which sits in the same size range but stays angular rather than rounded. Even national institutions disagree on the boundaries. The U.S. Department of Agriculture and the Soil Science Society of America set their own range, the German Atterburg scale sets another, and the U.S. Army Corps of Engineers defines gravel as particles under 3 inches that are retained by a number 4 mesh, a screen with a spacing of 4.76 millimetres. Colloquially, people stretch the word even further, using it for a mixture of stone, sand, and possibly some clay.

• 3. From Bedrock To Riverbed

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  Quartz is the most common mineral found in gravel, and it earns that place by being hard, chemically inert, and free of the cleavage planes along which softer rock splits apart. Most gravel begins as bedrock that disintegrates as it weathers, breaking into fragments that each usually contain multiple mineral grains. The reason is structural. Few rocks have mineral grains coarser than about 8 millimetres, with exceptions like quartz veins, pegmatites, deep intrusions, and high-grade metamorphic rock. Rivers do the shaping. As water carries the fragments along, often within a few tens of kilometres of their source outcrops, the particles are rapidly rounded. Where the water slows or spreads, the gravel comes to rest. It settles as blankets or bars in stream channels, builds alluvial fans, gathers in near-shore marine settings fed by streams or coastal erosion, and piles up in the deltas of swift-flowing streams. The upper Mississippi embayment holds extensive chert gravels thought to have travelled less than 100 miles from the periphery of the embayment.

• 4. A Catalogue Of Kinds

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  Bank gravel is naturally deposited gravel mixed with sand or clay, found in and beside rivers and streams, and known too as bank run or river run. The varieties multiply from there, each named for where it lies or what it carries. Bench gravel rests on the side of a valley above the present stream bottom, marking where the stream once flowed at a higher level, a term used most often in Alaska and the Yukon Territory. Pay gravel earns the most colourful nickname, pay dirt, for gravel rich in gold and other precious metals recovered through gold panning. Some types are defined by use rather than origin. Pea gravel, also called pea shingle, is clean stone about the size of garden peas, used for concrete surfaces, walkways, driveways, and as a substrate in home aquariums. Crushed stone, distinguished from gravel by the U.S. Geological Survey, is rock crushed and graded by screens then blended with fines, made from granite, limestone, dolomite, and other rocks, and known by names like crusher run, DGA, QP, and shoulder stone. Other types map onto the landscape itself. Piedmont gravel is carried down from high places by mountain streams onto flatter ground, plateau gravel caps a plateau above the usual stream-terrace level, and lag gravel is a coarse surface left behind once finer particles are stripped away. Shingle, the coarse and well-rounded sediment of beaches and rivers, rounds out the list with its smooth pebbles, cobbles, and occasional small boulders.

• 5. The Commercial Bedrock

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  In 2020, sand and gravel together accounted for 23 percent of all industrial mineral production in the U.S., worth about 12.6 billion dollars. The scale of extraction is hard to picture. Some 960 million tons of construction sand and gravel were produced that year, dwarfing the 68 million tons of industrial sand and gravel, which is mostly sand rather than gravel. The material is a major basic raw material in construction, and most of it goes into concrete as aggregate. What remains is spread as road base, mixed into blacktop, packed in as construction fill, and put to a myriad of smaller uses. Supply rarely runs short, since gravel is widely and plentifully distributed as river deposits, river flood plains, and glacial deposits. That abundance shapes the economics. Environmental considerations and quality decide whether alternatives become more economical, and crushed stone is already displacing natural gravel in the eastern United States, with recycled gravel growing more important too.

• 6. Water, Plants, And Stone

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  Natural gravel can carry water through itself at rates sometimes reaching above 1 centimetre per second, a high hydraulic conductivity that turns porous deposits into important aquifers. The same openness that lets water flow underground works against life at the surface. Where gravelly soil dominates, plant life tends to be sparse, because gravel retains moisture poorly and holds few of the mineral nutrients that finer soils carry. Over deep time, the loose stone hardens into the rock record. Sediments containing over 30 percent gravel that become lithified are termed conglomerate, found in sedimentary rock of all ages yet usually as a minor component making up less than 1 percent of it. The largest accumulations gather where the rivers once spread widest. Alluvial fans likely hold the greatest stores of gravel in the geologic record, preserved in formations like the conglomerates of the Triassic basins of eastern North America and the New Red Sandstone of south Devon.