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.
Most of the former coal mines in the county were quite small and their underground workings were of limited extent. Between 1883, when the state first started keeping records, and 1950, when the last mine in the county shut down, 34 mines produced 6.7 million tons of coal. Of this total, 5.7 million tons were produced from the three mines at Leyden and one million tons from the other 31 mines.
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.
The coal which has been mined in Jefferson County is found in the lowermost part of the Laramie Formation. It occurs in beds which range from a few inches to as much as 15’ thick and is interbedded with sandstone and clay beds typical of the lower part of the Laramie. Individual coal beds appear to thicken and thin and no one coal bed is continuous over the entire study area. Because of insufficient data, it is not possible to correlate coal beds either between mines or between mines and drill holes.
A number of sedimentary units have been mined for clay in the county. Included in this group are the Lykins, Morrison, South Platte, Benton, Pierre and Laramie Formations. Most of the clay however has come from just two units, the South Platte and the Laramie.
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, Between these two areas the clays are not particularly refractory and no mining, other than prospecting, has been done. Nearly all of the easily mined South Platte clays between Coal Creek and I-70 have been exhausted and no mines are being worked there at this time. However, there is an active clay mine (1978) in the South Platte Formation in the southern part of the county 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 of the Laramie clays extends from the north edge of Rocky Flats southward through Golden to Mount Carbon. Between Mount Carbon and the South Platte River there has been no clay mining and, owing to rapid subdivision development, it is unlikely that clays in this section of the Laramie will ever be a resource.
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.
In this case a vertical shaft was sunk from the surface to the bed to be mined. Haulageways were driven horizontally to the limits of the property and rooms or drifts were opened up off the haulageways; pillars were left between the rooms for roof support. A system of cross-cuts was then made to connect the rooms. When the mine was fully developed by rooms and pillars, the latter were systematically pulled out, allowing the roof and overlying strata to collapse.
The only mines within the project area which have beds that are nearly horizontal are the Leydens No.1, No.2 and No.3. These mines were extensively developed by the room and pillar mining method. However, the mining was terminated before many of the pillars were pulled so that a large system of open, interconnected, underground chambers still remains. These open chambers, supported by pillars of unmined coal are used by Public Service Company for the storage of natural gas.
In Jefferson County it happens that all coal and clay mines in steeply dipping beds are overlain by resistant sandstones. These sandstones form the caprock of prominent ridges or hogbacks which have a northward to northwestward trend. Mines were developed by making an opening (adit) at the base of the ridge on its dip slope (east) side, A horizontal tunnel (crosscut) was then driven westward until it intersected the clay bed. At this point two other tunnels (drifts) were driven parallel to the surface trend of the clay bed, one on either side of the crosscut. When the drifts reached the property line, mining in a upward direction (stoping) was begun. Rooms (stopes) were opened by following the clay bed up-dip and pillars of clay were left between the stopes to support the roof overhead. The stopes were enlarged upward until the unweathered clays changed to weathered soils near the surface. The last few feet of soil and overburden were then allowed to collapse into the open stope, It is this subsidence of overburden which has left the light colored scars seen on the sandstone ridges north of Golden.
If a mine was to be opened in nearly vertical beds and if there was little topographic relief a shaft was sunk adjacent to, and parallel with, the coal and clay bearing units. At convenient depths, mining levels were established by driving crosscuts horizontally from the shaft over to the coal and clay seams. Drifts were opened on either side of the crosscut and driven along the trend of the seams until the property lines were reached, Stopes were then developed in the roof of the drift and were carried upward toward the next higher level.
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