P&Q University Lesson 3- Geology

By |  June 10, 2015
Sources of aggregate, such as sand and gravel and rock for crushed stone, were formed by geologic processes.

Sources of aggregate, such as sand and gravel and rock for crushed stone, were formed by geologic processes.

Humans have used rocks for millions of years, and these rocks are geologically classified according to characteristics such as mineral and chemical composition, permeability, texture of the constituent particles and particle size. These physical properties are the end result of the processes that formed the rocks, and these events produce three general classes of rock: igneous, sedimentary and metamorphic. Crushed stone, sand and gravel are the main types of natural aggregate used in North America. Aggregate is used in nearly all residential, commercial and industrial building construction and in most public-works projects such as roads and highways, bridges, railroad beds, dams, airports, water and sewer systems, and tunnels.

The widespread use of aggregate results not only from its general availability but also from economic considerations. Aggregate of good quality commonly is available near the site of use at relatively low cost. This aggregate can essentially be obtained and used with a minimum of processing.

However, even though crushable stone, sand and gravel resources are widely distributed throughout North America, availability is not universal. Some areas are devoid of sand and gravel, and some potential sources of crushed stone may be lacking or covered by overburden that is too thick to allow economical surface mining.

In some areas, moreover, aggregate does not meet the physical-property requirements for certain uses, or it contains mineral constituents that react adversely when used in cement concrete. Furthermore, citizens commonly prefer that aggregate not be mined near their homes.

Crushed stone, sand and gravel were formed by geologic processes. Volcanoes, glaciers, wind, rivers and seas formed the shape and character of rock materials over millions of years. The gravel used today may have been deposited thousands of years ago – just yesterday geologically. Hard, dense limestone may have been deposited as a limy ooze hundreds of millions of years ago. When an aggregate supply is required, geological investigations can determine the location, distribution and nature of potential aggregate in an area.

Sand-and-gravel deposits are products of erosion of bedrock and surface materials and the subsequent transport, abrasion and deposition of the particles. The principal geologic agent that affects the distribution of deposits of sand and gravel is water. Consequently, gravel is widely distributed and abundant in glaciated areas, in alluvial basins, and in, adjacent to or near rivers and streams. Windblown deposits generally are fine grained and rarely used for aggregate.

Sand and gravel deposited by rivers or streams is widely distributed throughout North America as stream-channel or terrace deposits. In hilly or mountainous areas, bedrock is chemically and physically weathered and is progressively broken into smaller and smaller particles. Chemically less resistant minerals are dissolved or altered into clay minerals; the more resistant minerals remain as rock fragments.

Depending on the composition and structure of the bedrock and on the climate, land cover and topography, the remaining soils may range in thickness from almost nothing to many tens of feet, and may range in composition from nearly all clay, through mixtures of clay, silt, sand and gravel, to nearly all sand and gravel, to rubble.

Gravity and sheetwash move some of this material downslope, where it forms a deposit called colluvium. Eventually, the colluvium is moved into valleys of relatively high gradient streams. In the stream channels, rock fragments are subjected to abrasion, rounding and sorting. The stream-transported material is deposited in channels and on floodplains and consists of sand and gravel in some areas and silt and clay in others.

Erosion can alter an already established floodplain. If the river or stream incises its channel, the older channel and floodplain deposits are preserved as terraces. Repeated downcutting can result in the formation of a series of terraces or terrace remnants.


Many of the extensive sand-and-gravel deposits in the northern parts and higher elevations of the United States are products of either continental or alpine glaciation. As a glacier advances over the land surface, it erodes the surficial materials and underlying bedrock, depositing till, which is a nonsorted or poorly sorted mixture of clay-size to boulder-size particles.

As the ice melts, rock particles that had been crushed and abraded by the ice are transported by meltwater. As these particles are carried along, they are further abraded and sorted. Angular fragments are rounded, and weak particles are broken into smaller pieces. Finer materials are carried away and deposited in lakes and ponds (glaciolacustrine deposits), while the coarser sand and gravel is deposited in and along the stream channels (glaciofluvial deposits).

Glacial erosion and deposition are complex dynamic processes. Hourly, daily and seasonal temperature changes and longer-term climatic changes affect the rate of melting and the volume of meltwater. The particle size of glaciofluvial deposits, therefore, varies greatly and the deposits accumulate in diverse topographic settings.


In the arid and semi-arid Western United States, many valley basins contain thick deposits of unconsolidated alluvium. Alluvium (detrital material consisting of clay, silt, sand or gravel) is eroded in the adjacent mountains and is transported to the basins during infrequent but torrential floods (typical of desert environments) down steep gradient streams.

On reaching the basins, the water spreads out of the channel and infiltrates into the ground. The sudden change in gradient and reduced transporting capacity causes deposition of sediment, producing alluvial fans. Generally, the coarsest detrital material is deposited adjacent to the mountains. Deposition becomes progressively finer in the alluvial valleys toward the center of the basins. In time, the fans formed in adjacent valleys coalesce to form continuous thick deposits of alluvium.


Bedrock is classified in three main groups based on its origin: sedimentary, igneous and metamorphic. The sedimentary rocks, limestone and dolomite, make up about 71 percent of current crushed-stone production. Igneous rocks, generally referred to in the aggregate industry as “granite” or “traprock” (basalt or diabase), compose 14 percent and 8 percent of crushed stone, respectively. Metamorphic rocks, such as gneiss, marble or quartzite also are used and, together with other miscellaneous stone, account for the remaining 7 percent.

Sedimentary rocks were formed by consolidation of loose sediment by chemical, biochemical or mechanical processes or by direct chemical precipitation. Chemically or biochemically deposited sedimentary rocks, such as hard, dense limestones and dolomites (calcium or calcium-magnesium carbonates), generally are good sources of crushed stone. Some limestone and dolomite, however, is too soft, absorptive or friable to yield high-quality aggregate.
Chert and flint are silicate rocks that were precipitated in water by organisms such as sponges. These rocks may be crushed for aggregate, but they may cause adverse reactions such as cracking and scaling in cement concrete.

Clastic (mechanically deposited) sedimentary rocks are classified according to the sizes of individual particles. Rock that consists mostly of pebbles and larger size fragments is conglomerate; rock that consists mostly of sand-sized particles is sandstone; and rock that consists primarily of silt- or clay-sized particles is siltstone or shale, respectively. Of these rocks, hard, dense sandstone is the only type that generally is a source of crushed stone. In areas where no other material is available, it is a major source of aggregate, but it constitutes less than 3 percent of total U.S. aggregate production.

Igneous rocks solidify from a molten or partly molten state and are classified further by their origin. Intrusive igneous rocks solidify at depth within the earth and have coarse mineral crystals, owing to the slow cooling associated with deep burial. Light-colored intrusive igneous rocks commonly are referred to as “granite” in the aggregate industry. Extrusive igneous rocks solidify at or near the earth’s surface and generally are composed of small or microscopic crystals that were formed by rapid cooling.

Such rocks frequently are referred to as “traprock” in the aggregate industry. Igneous rocks that are hard, tough and dense commonly are excellent sources of crushed stone. However, some are very friable and others are very porous. Some siliceous igneous rocks react deleteriously when used as aggregate in cement concrete.

Metamorphic rocks form when existing rocks are subjected to heat and pressure within the earth. Common metamorphic rocks are slate, schist, gneiss, marble and quartzite. Of these, only gneiss, marble and quartzite commonly are used as aggregate.


In areas where natural aggregate is not economically available, does not meet physical or chemical specifications, or is preempted by land use or regulations, various other materials may be used as aggregate to avoid excessive transportation costs.

Natural materials such as shells, “clinker” (bedrock that was baked or melted by underground coal fires), and caliche (calcium salts) have all been substituted for crushed stone or sand and gravel. Aggregate can be manufactured from clay and shale expanded by firing. In addition, certain types of waste have even been used, including blast furnace slag, steel slag, ash, coal refuse, mine tailings, waste glass and shredded rubber tires.

Recycling cement concrete and asphalt (bituminous mix) currently is a viable alternative for natural aggregate, and can be both economically and environmentally beneficial. In some states, recycling is either encouraged or required.

Depending on the material being processed, aggregate can be a variety of colors.

Depending on the material being processed, aggregate
can be a variety of colors.


Natural aggregate occurs where nature placed it, not where people need it. To concisely describe aggregate availability and quality in the United States, this chapter divides the country into 12 regions. In general, each region has similar occurrences of aggregate within its boundaries but is fairly distinct from other regions.

In any region, quality problems can limit the use of aggregate, and conflicting land use, regulations, and other social and economic factors can restrict aggregate development. It should also be noted that these regional divisions are extremely simplified, and are intended only for general reference.


■ Topography
Tall, massive mountains alternating with relatively narrow, steep-sided valleys. Larger valleys may have higher terraces.

■ Crushed Stone Resources
Mountains are underlain by igneous and metamorphic rocks, flanked by consolidated sedimentary rocks including some limestones. Potential sources of good-quality crushed stone are generally available.

■ Sand and Gravel Resources
Stream-channel or terrace deposits and limited glaciofluvial deposits are present. Quality problems tend to be localized.


■ Topography
Alternating basins or valleys and mountain ranges.

■ Crushed Stone Resources
The mountain ranges are commonly underlain by igneous, metamorphic and consolidated sedimentary rocks. Potential sources of good quality crushed stone are generally available.

■ Sand and Gravel Resources
Large alluvial basins commonly contain extensive deposits of poorly sorted sand and gravel deposits. In addition, terraces and beaches found on mountainsides consist of well-sorted sand and gravel. Some areas are deficient in sand and gravel. Quality problems tend to be localized.


■ Topography
Gently sloping plateau underlain by a thick sequence of extensive lava flows separated by soil zones and interbedded sediments.

Crushed Stone Resources
The northern part of this region commonly is underlain by basalt that is generally a suitable potential source of crushed stone. The southern part is underlain by basalt and other igneous rocks, some of which have physical or chemical properties that do not yield good aggregate.

■ Sand and Gravel Resources
Stream-channel, terrace and glaciofluvial deposits are well distributed throughout the northern parts of the region. In the rest of the region, sand and gravel are rather limited, commonly being restricted to river and terrace deposits. Many gravel deposits have chemical and physical properties that do not yield good aggregate.


■ Topography
High plateaus with deeply incised canyons, mountains, deserts and badlands. Erosion has produced extensive, prominent cliffs.

Crushed Stone Resources
Generally underlain by poorly consolidated to well-consolidated sandstone, shale and limestone, with the sandstone and shale being most prevalent and most extensive. In places, the rock units contain significant amounts of gypsum or halite. The soft sandstone and shale tend to afford poor quality crushed stone.

■ Sand and Gravel Resources
Stream-channel, terrace and glaciofluvial deposits occur along parts of the valleys of major streams, especially adjacent to the mountain ranges in the northern and eastern parts of the region. In the remainder of the region sand and gravel is generally limited to alluvium and terraces. Gravel commonly is derived from the erosion of soft sandstone or shale, and tends to be of poor quality.


■ Topography
Large, flat, gently eastward-sloping plain. Significant topographic features include sand dunes and wide valleys of braided streams that flow from the Rocky Mountains eastward across the plain.

■ Crushed Stone Resources
Bedrock consists of soft semi-consolidated sediments, almost all of which is unsuitable for use as crushed stone.

■ Sand and Gravel Resources
Stream-channel and terrace deposits of the major rivers and their tributaries. Deposits generally are progressively finer downstream from the mountains and commonly are deficient in coarse sizes. Gravels frequently have chemical or physical properties that do not yield good aggregate.


■ Topography
Topographically complex region including lowlands and plains as well as hilly and mountainous areas.

■ Crushed Stone Resources
Most of the northern part of the region and the part of the region flanking the Rocky Mountains is underlain by sedimentary rocks consisting mostly of sandstone, shale, and conglomerate. With the exception of scattered limestone, these rocks generally are poor sources of crushed stone. Most of the south-central region is underlain by limestone and dolomite. These rocks commonly yield suitable sources of crushed stone, although the presence of chert and other deleterious minerals may restrict certain uses.

■ Sand and Gravel Resources
The land surface in most of the region is overlain with a residual soil formed by the chemical and mechanical breakdown of the bedrock. The residual soils in the western part of the region are overlain with substantial thicknesses of windblown deposits. Stream-channel and terrace deposits are the source of sand and gravel throughout the region. The quality of sand and gravel is variable and depends on the type of parent material.


■ Topography
Flat plains, rolling hills and low rounded mountains.

■ Crushed Stone Resources
Bedrock consists of consolidated sedimentary rocks including sandstone, shale, limestone and dolomite. These rocks are most prevalent in the central part of the area and are less prevalent in the east. The western parts of the region generally lack limestone or dolomite and generally have inadequate sources of crushed stone. Throughout the region, even where suitable bedrock exists at depth, the thickness of overburden may be too great to extract the rock economically.

■ Sand and Gravel Resources
The major sources of sand and gravel aggregate are glaciofluvial, stream-channel and terrace deposits. Sand and gravel deposits are generally abundant throughout the region. In the northern part of the region, much of the aggregate is buried by fine-grained deposits. Aggregate is almost completely absent in the Illinois Coal Basin. Aggregate from the Missouri River tends to be contaminated with deleterious material from the underlying soft sandstone and shale bedrock. In the Great Lakes area and scattered throughout the region, aggregate may contain minerals with chemical properties undesirable for certain uses.


■ Topography
Rolling hills and low mountains.

Crushed Stone Resources
Bedrock consists primarily of granite, syenite, anorthosite, and other intrusive igneous rocks and metamorphosed sedimentary rocks consisting of gneiss, schist, quartzite, slate and marble. Most igneous rocks make suitable sources of crushed stone. The characteristics of metamorphic rocks vary widely, with the schists and slates commonly lacking the strength and shape characteristics for desirable crushed stone.

Sand and Gravel Resources
Most of the valleys and other low areas commonly contain stream-channel, terrace and glaciofluvial sand and gravel. In several areas, the unconsolidated deposits consist of clay and silt deposited in lakes that formed during the melting of the ice sheets. The major sources of good quality sand and gravel aggregate are glaciofluvial, stream-channel and terrace deposits.


■ Topography
The topography of this region is varied and consists of low, rounded hills, long rolling ridges and mountains, and contains the highest peaks east of the Mississippi River.

■ Crushed Stone Resources
Bedrock consists of igneous and metamorphosed sedimentary and igneous rocks including granite, gneiss, schist, quartzite, slate, marble and phyllite. Granite and gneiss are commonly used as crushed stone, whereas schist, slate and phyllite are commonly avoided. Potential sources of good-quality crushed stone are generally available.

Sand and Gravel Resources
The mountains are bordered by low-gradient streams flowing in relatively narrow valleys. The land surface is underlain by saprolite, which is clay-rich, unconsolidated material developed in place primarily from the chemical weathering of the underlying bedrock. The valleys are underlain by relatively thin, moderately sorted alluvium. Stream-channel and terrace deposits are the source of sand and gravel throughout the region. Quality problems tend to be localized.


■ Topography
The topography ranges from extensive, flat, coastal swamps and marshes occurring near sea level, to rolling uplands occurring near the inner margin of the region.

■ Crushed Stone Resources
Consolidated bedrock generally is inaccessible in the Coastal Plain region, except near the inner margin, and in southeastern Florida, which is underlain by semi-consolidated limestones that are of marginal use as aggregate.

■ Sand and Gravel Resources
The region is underlain with extensive deposits of semi-consolidated sand, silt, clay and gravel, with sand being the predominant surficial material. Most gravel deposits occur near the inner edge of the Coastal Plain and occur as stream-channel and terrace deposits. These deposits are limited in occurrence and are the primary source of natural aggregate in the region. Isolated deposits of beach and terrace gravels are scattered throughout the area. The quality of the gravels depends on the parent material. The gravels in the Atlantic coastal area tend to be high in quartz and other silica minerals, whereas those in the Gulf coastal area are calcareous. Coarse materials are so limited in this region that shells are commonly substituted for gravel. Sand and gravel occur in parts of the Mississippi embayment as isolated terraces. Sand and gravel also occur at depth under the alluvial clays of the Mississippi River floodplain.


■ Topography
A complex area consisting of coastal plains, mountain ranges and intermontane plateaus.

■ Crushed Stone Resources
Underlain by a diverse assemblage of rocks. The principal mountain ranges have cores of igneous and metamorphic rocks. These are overlain and flanked by sedimentary and igneous rocks. The sedimentary rocks include carbonate rocks, sandstone and shale.

■ Sand and Gravel Resources
Approximately half of the region, including the mountain ranges and adjacent parts of the lowlands, was covered by glaciers. Glaciofluvial deposits in these areas commonly contain sand and gravel. In the intermontane plateaus, sand and gravel commonly is mined from stream channels, terrace deposits and placer-mine tailings. The Arctic Coastal Plain consists of silt, sand and gravel, with the northeastern part having moderate to high potential for sand and gravel. Much of the aggregate is frozen to some degree, and excavation requires drilling and blasting.


■ Topography
Islands formed by lava that issued from one or more volcanic eruption centers. The islands have a hilly appearance resulting from erosion that has carved valleys into the volcanoes and built relatively narrow plains along parts of the coastal areas.

Crushed Stone Resources
Each of the Hawaiian Islands is underlain by lava flows. Andesite and basalt flows are commonly used as a source of crushed stone. In addition, clinker from the tops of lava flows and cinders from cinder cones are also used as stone aggregate.

■ Sand and Gravel Resources
In some areas, alluvium of older and modern terraces and alluvial fans contain poorly sorted sand and gravel of variable quality. In coastal areas, a thin layer of coral and shell fragments, volcanic debris and clay form discontinuous beach deposits, which may be used as aggregate if they meet the required specifications.



Contributors to this chapter include the following, in alphabetical order:
National Stone, Sand & Gravel Association
U.S. Geological Survey


Lesson 3 Quiz

1. What process formed sources of aggregate, such as sand, gravel and rock for crushed stone?

2. What are the three main types of natural aggregates used in North America?

3. Where in the United States can one find thick deposits of unconsolidated alluvium, detrital material of clay, silt, sand or gravel?

4. What is the term for non-sorted or poorly sorted mixtures of clay-size to boulder-size particles?

5. What three types of rock make up 71 percent of crushed-stone production?

6. What percentage do igneous rocks make up the material used for crushed stone?

7. What are the most prevalent crushed stone resources in the Colorado Plateau and Wyoming Basin region?

8. True or False: Coral, shell fragments, volcanic debris and clay found in coastal areas of Hawaii can be used as aggregate?


Click here for the quiz answers.


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