Introduction

Crushed stone and sand and gravel are the main types of natural 
aggregate used in the United States.  Aggregate is used in nearly 
all residential, commercial, and industrial building construction, 
and in most public works projects including roads and highways, 
bridges, railroad and light rail beds, airports, water and sewer 
systems, and tunnels. Much of the infrastructure built during the 
1950s and 1960s has deteriorated to a point that requires 
extensive repair or replacement.  All this construction requires 
enormous amounts of aggregate.  In Colorado, for example, 
nearly 45 million tons of aggregate, or about 12 tons per person, 
were produced during 1994.

Aggregate Quality

Aggregate quality is determined by its physical and chemical 
properties, and is a combination of these two parameters.  The 
quality of the aggregate required for a specific use related 
directly to that use.  For example, to be used in portland cement 
concrete, aggregate should be both physically satisfactory, and 
chemically innocuous.  However, for use as road base, aggregate 
commonly only needs to be physically satisfactory; chemical 
properties commonly are irrelevant.

For this map, potential sources of aggregate are defined 
according to general physical and chemical quality.  Physical 
quality is defined as satisfactory, fair, or poor.  Satisfactory 
aggregate has physical properties that make it suitable for most 
purposes.  It contains clasts that generally are strong, hard, 
relatively free from fractures, and not chiplike; capillary 
absorption is very small or absent; and the surface texture is 
relatively rough.  Fair aggregate has physical properties that 
make it useful for many purposes, but it commonly can not be 
used where engineering specifications are strict such as in 
concrete or asphalt.  It contains clasts that generally are friable, 
moderately fractured, and flat or chiplike; capillary absorption is 
small to moderate; and the surface is relatively smooth and 
impermeable.  Poor aggregate has physical properties that 
greatly limit its use.  It contains clasts that generally are weak, 
highly fractured; friable; capillary absorption is moderate to 
high; and the surface is relatively smooth and impermeable.  In 
many circumstances, potential sources of crushed stone can be 
processed to improve its quality to meet special requirements.

Chemical quality is defined as either innocuous or deleterious. 
Innocuous aggregate contains no constituents that dissolve or 
react chemically to a significant extent that the atmosphere, 
water, or hydrating portland cement while enclosed in concrete 
or mortar under ordinary conditions.  Deleterious aggregate 
contains constituents in significant proportion which are known 
to react chemically under conditions ordinarily prevailing in 
portland cement concrete or mortar.  The reaction may produce 
significant volume change, interfere with the normal course of 
hydration of portland cement, or produce other harmful effects 
upon concrete.

How This Map Was Made

The key to mapping potential sources of aggregate is an 
understanding of the geology of the region.  This includes 
Quaternary geology for deposits of sand and gravel; 
stratigraphy, origin, and structural history of the region for 
crushed stone; and the subsequent weathering or alteration of 
both sand and gravel deposits or potential sources of bedrock for 
crushed stone.  The Geologic Map of Colorado (Tweto, 1979) 
serves as a primary source of information for determining the 
location of potential sources of aggregate.

This map was prepared by attributing the Digital Geologic Map 
of Colorado - USGS Open File Report 92-0507 (Green, 1992) 
with parameters reflecting the quality of unconsolidated 
materials and bedrock for use as aggregate (sand and gravel or 
crushed stone). The terminology and classification techniques 
used on this map are briefly described above, and are described 
in more detail in USGS Open File Report 95-582 (Langer and 
Knepper, 1995).


Use of this Map

Planners and other decision makers responsible for resource 
management should find this map useful in regional planning 
and land use management.  Because statutory regulations, 
technological capabilities, available funding, and land-use 
priorities vary from place to place and can be expected to 
change with time, this map is designed to provide a resource 
data base that will be useful over the years.  This map is in 
digital format and can be used with other digital data sets, 
according to the specific needs of a particular planning issue.  
As planning criteria change, the selection of pertinent resource 
characteristics can be adjusted to meet the changing needs.

This map was prepared at a scale of 1:500,000 (2 mm on the 
map equals 1 kilometer on the ground).  The details on the map, 
as well as the details of the map unit descriptions, are highly 
generalized.  Therefore this map is not intended to replace on-
site investigations.

Explanation

Potential sources of sand and gravel aggregate are identified 
according to their physical properties. Yellow and orange areas 
are where sand and gravel deposits are likely to be abundant.