Figure 1.  Map of the Project Demonstration area with close-up of the South Platte River.

Location, Origin, and Mining

The above map on the left shows the drainage basin of the South Platte River and its tributaries in the project demonstration area from Denver to Ft. Collins and Greeley. Named streams are in valleys with known or probable commercial deposits of gravel. All other streams lack commercial gravel deposits, although they may contain local occurrences. Commercial gravel is concentrated in streams that drain glaciated valleys. During the ice age, glaciers eroded large quantities of rock from the mountains. Meltwater from glacial ice flooded stream valleys and transported large volumes of gravel down to the plains. Since the last glaciation, about 10,000 years ago, modern streams have reworked some of the gravel, but little new gravel has been brought from the mountains.

The above map on the right shows the area north of Denver where the USGS has conducted detailed studies of commercial gravel deposits. The South Platte River north of Denver is the last major commercial gravel resource in the Denver metropolitan area. Most of the deposits upstream in the South Platte, Clear Creek, and Bear Creek have been mined or precluded from mining by urban development. North of Denver, gravel mining has steadily moved downstream since the early 1970’s, and now may be approaching the downstream limit of commercial viability.

When the deposits north of Denver are exhausted or preempted by other land use, aggregate for the Denver area will by necessity come from stone quarries in the mountains or from gravel deposits in valleys to the north, such as the St. Vrain River.

  Figure 2.  Cross-section showing gravel at the South Platte River Valley at the Howe Pit. 

The Third Dimension

The section above shows sand, gravel, and other deposits of the South Platte valley in the vicinity of the Howe pit. The valley here actually consists of three valleys. The largest (and oldest) valley was filled by the high terrace deposits (called "Broadway") east of the Howe pit. Gravel in the old valley is covered by a thick deposit of windblown dust. Next in size and age is a remnant of a younger terrace (called "Piney Creek"), dating from the last glaciation. The young terrace forms a small step at Brighton Road, between the level of the high terrace and the modern valley surface.

The modern valley of the South Platte was formed during the last 10,000 years. The valley surface consists of the floodplain and very low terraces. The modern valley contains the most valuable gravel deposits, and most gravel mining takes place on it.

The modern valley is underlain by three distinct gravel layers, each about 5-10 feet in thickness. The layers differ in coarseness and color and they can be traced throughout this segment of the South Platte valley. Wood in the upper layer dates from less than 300 years ago. The coarse basal layer is crucial to gravel mining because it supplies much of the coarse gravel. The basal layer is thought to be a remnant of gravel deposited by glacial meltwater.

Beneath the terrace deposits is bedrock. In the Platte River valley north of Denver, bedrock is clay of the Denver Formation. The bedrock clay forms an impermeable seal at the bottom of the gravel aquifer, confining ground water flow to the gravel of the river valley. After mining gravel, the pit walls can be lined with clay to create a water-tight reservoir. The reservoir is separated from the gravel aquifer by clay walls and can be used to store water for municipal use.

Mapping Gravel Deposits

The below map and the sections show the distribution of gravel layers in the South Platte River valley north of Denver. These maps and sections were prepared originally at a detailed scale and will be available in electronic format as one of the products of the project.

The map on the left shows the distribution of major terraces, underlain by gravel of different ages and quality. The oldest terrace (red) forms erosional remnants on hills above the valley; gravel deposits in the oldest terraces are small and aggregate quality is low. Gravel beneath the high terrace (brown) is abundant but commonly deeply buried under windblown dust. Gravel beneath the lowest terraces and floodplain (shown in yellow) is abundant, yields high-quality aggregate, and lies near the surface.

The sections on the right shows thickness, overburden, and lateral continuity of gravel and other deposits in the South Platte valley. Dark brown layers represent coarse gravel; light brown represents sandy gravel. Overburden is shown by black (soil) and light blue-gray (windblown dust).

  Figure 3.  Maps of the gravel deposits and sections of the South Platte River.
Click on the above figure to see it at a higher resolution.

Sampling Gravel Particle Size

Particle size of gravel is important to determining the commercial value of a deposit. Coarse gravel yields a higher proportion of aggregate suitable for concrete construction and asphalt paving.

Figure 4.  Photograph showing a commonly used method to sample gravel.

Figure 5.  Photograph of a sieved sample.

The photo on the left shows a commonly used method to sample gravel. A vertical trench is first cut in the gravel face. Gravel is then scraped from the trench with a hoe or shovel and collected at the bottom in a bucket or on a tarpaulin. Depending on the coarseness of the gravel, about 30-100 lbs are collected for a sample. Care must be taken to sample evenly across the entire trench. Our studies have shown that much of the variation in estimation of particle size comes from collecting the sample.

After a sample is collected, it is sieved. Sieves like the ones in the photo on the right are used to catch the proportion of gravel larger than 1 ½, ¾, 3/8, and 3/16 inches. Material that passes through the 3/16 inch sieve is coarse sand or finer. Each size fraction is weighed and expressed as a percentage of the weight of the whole sample. Results are plotted in a variety of ways, the simplest being the bar chart example shown in the lower right. More elaborate plots and calculations are used to determine statistical measures, such as average particle size, etc. Statistical measures are commonly used to compare samples.

Figure 6.  Particle Size/Weight Percentage Bar Chart.

The bar chart displays the particle size, in weight percent, for each size fraction of gravel in the upper gravel layer in the vicinity of the Howe pit. The chart is a composite of nine samples from three pits and represents the results of sieving about 400 lbs of gravel, but it looks the same as a single analysis. Such composite sampling can be used to characterize a large deposit of gravel, or any part of a deposit, such as the upper layer near the Howe pit. In commercial exploration and planning the development of a gravel pit, the actual yield of aggregate products, classified by particle size, can be determined by processing part of a deposit.

Other Indications of Aggregate Quality

A variety of field and laboratory tests are used to assess the quality and suitability of gravel for use as aggregate. Shown below are four charts that show three measures of quality that can be applied at a deposit site. The examples shown are for gravel in the South Platte valley, in the vicinity of the Howe pit. Commonly, to permit comparison, particles of a specified size fraction, such as pebbles measuring ¾ to 1 ½ inches, are selected for study. This size is in the range commonly used for construction.

Figure 7.  Four charts that show three measures of aggregate quality.
Click on the above figure to see the charts
at a higher resolution.

Lithology, or rock type, is an important indicator of durability and of potential problems such as chemical reactivity with Portland cement. In the example shown, the lithology of South Platte gravel is shown by a pie diagram. Gneiss, granite, and pegmatite - all derived from mountains - make up about 80 pct of the rocks in the gravel and account for the durability of the gravel. Among the minor rocks present, only some of the volcanic porphyries may contain minerals that could cause a chemical reaction with cement. Our study shows that potentially reactive rocks are probably not abundant enough to cause concern.

Particle shape is an indicator of strength. Particles are classified by axial ratios (A = long, B = intermediate, C = short) into equidimensional, disc-shaped, rod-shaped, and blade-shaped. The principal concern is that the proportion of thin, flaky particles (blade-shaped particles with axial ratios below 0.5) not be abundant. Such particles in concrete or asphalt could be potential sites of weakness. Our study shows that, for South Platte gravel, weak particles are not abundant.

Particle shape can also be expressed as a single number, termed "sphericity". The formula and a bar chart showing sphericity of South Platte gravel is shown in the example. The single number finds use in statistical comparisons.

Particle roundness is the degree to which particles lack angular corners. Roundness is determined by visual comparison with standards and is classified by letter (A, angular, through E, well rounded). Possibly, highly rounded particles may not adhere to cement or asphalt matrix, or may shift under load in loose aggregate applications, such as road base, but not much is known about the effect of rounding on aggregate quality. Although our studies show that South Platte gravel is mostly rounded, we know of no problems associated with its use.

In the South Platte valley north of Denver, particle lithology and shape are the same for all three gravel layers. Only minor variation was noted with distance downstream. Rounding decreases downstream.

U.S. Department of the Interior, U.S. Geological Survey URL: http://rockyweb.cr.usgs.gov/frontrange/virtour/howepit3.htm Contact: Carol Mladinich  mailto:csmladinich@usgs.gov Updated: 05/16/2001