Short answers, pulled from the story.
A single grain of gravel measures between two millimeters and sixty-four millimeters across. This specific range separates it from sand below and cobbles above in the Udden-Wentworth scale used by geologists in the United States.
The German Atterburg scale defines gravel as particles starting at four millimeters instead of two. The U.S. Army Corps of Engineers uses a number 4 mesh screen with spacing of 4.75 millimeters to filter them out.
In 2020, sand and gravel together accounted for twenty-three percent of all industrial mineral production in the United States. That year saw a total value of approximately twelve point six billion dollars generated by this sector.
Some deposits like those in the upper Mississippi embayment originated less than ten million years ago near the periphery of the basin. Natural processes create these aggregates through erosion along coastlines and deposition in alluvial fans.
Plant life remains generally sparse in locales where gravelly soil dominates the landscape because the inferior ability of gravels to retain moisture limits root growth for many species. Mineral nutrients exist in smaller amounts within finer soils that accompany these rocky deposits.