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Coal and Clay Mining in Jefferson County

Coal and clay have been mined in the area of present Jefferson County since well before the turn of the century. According to Eldridge (in Emmons and others, 1896) coal was being mined at Golden as early as 1861. Hayden, in 1869, described the Tertiary rock section and noted that, "The clay is much used for fire-brick and potters ware" (Hayden, 1869, p.128) and went on to mention a pottery works at Golden.
Clay mining in the county has usually been done on a fairly small scale at any one place and at any one time. However, mining has been continuous for many decades so that the aggregate amount of clay extracted has been considerable and a number of initially small workings now extend several thousand feet along the outcrop.
Occurrence of Clay

The minable clays in the South Platte Formation are usually found in the upper part of the unit and range in thickness from three feet to 15'. The clay beds are interbedded with fine to medium grained sandstones, and some individual beds are continuous over distances of several thousand feet. The clays of the South Platte Formation have been particularly desirable because of their excellent refractory properties, that is the ability to resist high temperatures once the clay is fused. Clays with this property are not continuous throughout the length of the South Platte outcrop but are limited to the area extending from Coal Creek to just south of the I-70 highway cut and to a small area just north of the South Platte River.

The clays of the Laramie Formation have been extensively mined for use in brick and tile manufacture. The best clays are found in the lower part of the formation in beds ranging from a few feet to as much as 20' in thickness. They are interbedded with fine grained sandstones, coal, carbonaceous shales, and clayey sandstones. The proximity of the coal and clay beds in the lower part of the Laramie is such that some clay mines have been superimposed on coal mines of earlier vintage.

Mining Methods

The coal and clay mines of Jefferson County have been worked in a variety of ways. Since the amount and aerial extent of surface subsidence are dependent, in part, on the way the underlying mine was worked it is well to examine each of the methods used. The selection of a mining method at any given locality, whether for clay or for coal, was largely dependent on the dip of the beds to be mined and on the technology available at the time.

Open Pit Mines

Before World War II nearly all coal and clay mines in Jefferson County were worked underground by pick and shovel and were developed with a room and pillar pattern. After the war the increased cost of labor tended to make most such operations uneconomic, while the development of better and faster mechanical equipment, such as bulldozers, power shovels, loaders and trucks, made open pit mining more attractive. In the last ten to 15 years most clay mining in the county has been done by surface methods and a number of mines which were once worked from underground have been converted to open pits. Some of the larger pits are now nearly a half mile long, 500' wide and 50' to 60' deep.

All of the Laramie clay beds have nearly vertical dips and are bounded, top and bottom by either sandstones or sandy shales. When the clays are mined by open pit, the sandy shales and clayey sands are also extracted (for use as filler) and the sandstones are left behind. The resulting landscape is one of vertical walls of sandstone 20' to 30' wide, 50' to 60' high and several hundred feet long separated by moatlike mined out areas.

All of the geologic information contained on this page was taken directly from Coal and Clay Mine Hazard Study and Estimated Unmined Coal Resources, Jefferson County, Colorado by Amuedo and Ivey, 1978.
Extracted from ((, Feb. 1999.