Rounded Rock Particles: (1) Natural soil. Most natural soils make a poor road surface. They are dusty when dry and muddy when wet.
(2) Natural gravel. This is found in stream beds or where streams or glaciers have deposited it in past ages. Gravel contains var>ing amounts of sand and rounded rock particles. Deposits other than those in streams generally contain some clay.
(3) Rock. For road building rock should be hard and tough rather than soft and brittle. It is almost al¬ways crushed for road use, but in a few countries expert workmen cut stone into paving blocks.
The particles in gravel are derived from many kinds of rocks. When rocks are picked up by moving water, they are gradually reduced in size by the breaking away of edges and corners. (The current's velocity must increase greatly to carry successively larger sizes.) The fragments become more or less rounded after long periods of rolling and skipping along stream bottoms.
The rounded fragments that make up gravel range from particles 2 millimeters (about 0.08 inch) in diameter to pebbles, cobbles, and boul¬ders. The spaces between the fragments may be more or less filled with grains of sand; gravel beaches, or shingle beaches, however, generally contain little sand.See Also Rock Falls:ROCK FALLS, city, Illinois, in Whiteside County, on the south bank of Rock River, across from Sterling, with which it forms one industrial and economic unit. It is situated 50 miles south¬west of Rockford at an altitude of 645 feet. The original plat for this site, called Rapids City, was entirely vacated in 1857 when the river was dammed and in 1867 the new town of Rock Falls was laid out on the present site. Like Sterling (at the end of an 800-foot bridge), this is a center for the manufacture of builders' hardware, bolts, nuts, rivets; garden and farm tools; ma¬chine tools, ambulances, and hearses; electric appliances and steel and steel products. There is a township public library, established in W--Rock Falls was incorporated in 1889 and is g< erned bv a mavor and aldermen. Pop. (% 10,261.
ROCK RIVER, river, Wisconsin and Illinois, rising in Fond du Lac County, in the eastern part of Wisconsin, about 30 miles west of Milwau¬kee. It flows south and southwest past Water-town, Janesville, and Beloit, into Illinois, where it passes Rockton, Rockford, Dixon, and Rock Falls before joining the Mississippi River just below Rock Island, a distance of about 285 miles.
On The Other Hand See Rounded Rock Fragments:GRAVEL is a sediment composed of rounded rock fragments that have been carried along and then deposited by streams. If the unconsolidated sedi¬ment is later solidified by a cementing material, it becomes the rock known as conglomerate. Gravel is used commercially for making concrete and as a loose paving material.
Four groups of detrital deposits may be dis¬tinguished :
Rudaceous deposits: These are composed mainly of fragments larger than 2 millimeters in average diameter. In screes (heaps of stones and rocky debris) and talus (rock debris that has gathered at the foot of a cliff) the fragments are angular and unconsolidated; in breccias they are cemented together. Gravels and shingle consist of more or less rounded, unconsolidated fragments, while conglomerates are the corresponding consolidated rocks— that is, they consist of gravels and larger stones firmly cemented together. Glacial till (glacial debris) or boulder clay is composed of angular or subangular frag¬ments, set in a poorly sorted, fine-grained matrix; the equivalent coherent rocks are known as tillites.
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