2002-03-04_REVISION - M1986015 (8)

Home Browse Search

MINING PLAN Nature of the Deposit to be Mined The gravel deposit on this site is moderately uniform in distribution and appears to follow a typical configuration. In general, the gravel is thicker near the river than it is away from the river. The overburden, as is also fairly common, follows a similar pattern - thicker away from the river than near the river. Nearly all of the gravel was deposited by the Arkansas River. Very little contribution occurred from adjacent lands. Most of that contribution was not in the form of gravel but fines eroded from the surrounding, gently sloping valley sides that are composed mostly of shale. The gravel deposit, however, does not follow this pattern consistently at all locations. Several drilling programs have been conducted on this site over the years and, when taken together, define the pattern fairly well. There were some inconsistencies in the values derived from holes drilled near each other, but that was probably due more to interpretative differences than actual differences. The cross-sections located at the end of this exhibit show a sampling of the variations seen across the site. The actual mining plan was prepared using more cross- sections than are presented here. On some of the planning cross-sections the gravel layer was fairly consistent across the length of the section and the overburden declined rather evenly toward the river. But on other sections the gravel layer varied widely as if deposited across hilly country. Because these were longitudinal sections, the "across hilly country" picture is an illusion. More probably, these variations represent different depths of erosion by the river as it meandered back and forth across former erosion surfaces. This pattern is often seen in fairly swift flowing rivers, like the Arkansas. Often the river depth on the outside of a meander bend is greater where cutting is more intense than near the inside of the bend where cutting is more gentle. Furthermore, the outside of the meander typically has a very steep channel slope while the channel slope on the inside of the meander is usually more gradual. This asymmetry is evident when adjacent cross-sections are linked laterally. Variations from place to place can also occur because meander changes are not usually smooth or gradual. Instead, meander changes are often induced by major floods. If one could view the changes as if in a time lapse movie, the changes would make the river channel appear to jump around. As a result of this "catastrophic" change pattern which often occurs in meandering rivers, gravel deposits can vary tremendously from place to place, both with respect to the quality of the material and the depth and quantity Pueblo East Pit Amendment Ex ibit D Page 2