INTRODUCTION
This paper is the result of a Bureau of Mines program to characterize roof conditions near outcrop in drift coal mines of eastern Kentucky (Sames and Moebs, 1989). Joints occur throughout the shallow 'overburden of eastern Kentucky, separating the strata into blocks or wedges. Except for bedding planes, they are the most important structural feature in the characterization of shallow rock mass in the region.
Unweathered, intraformational joints were common in the study area. No easily recognized regional joint systems were identified. However, highly weathered joints that are vertically continuous through various rock types, but spatially limited to areas near outcrop, were present in almost every drift mine examined. The orientation of these weathered joints was not consistent with the regional tectonic pattern, putting their origin and significance in question. Three underground sites were examined and four surface sites with large, unweathered outcrop exposures, were selected for detailed observations to resolve the geologic character of these
joints.
GEOLOGIC SETTING
This study was centered in MSHA District 6 in eastern Kentucky (Figure 1). District 6 is in the Cumberland Plateau, an area of sharp ridges, V-shaped valleys, and high topographic relief, commonly of 400-600 ft.
The plateau is underlain by rocks of the Lower and Middle Pennsylvanian Series consisting predominantly of sandstone and shale, with smaller amounts of claystone and coal.
Structural dips of the rock strata seldom exceed 1°. Topsoil is thin and weathering extends to varying depths, generally only a few feet on hillsides but much deeper along joints.
The heavily wooded hillside slopes are generally very steep, but vary over rock strata of contrasting strengths and resistances to weathering. Slopes on soft claystone and shale range from 9-27 pct (5-15°), while slopes on more resistant silty shale and sandstone, common to the study area, range from 38-57 pct (21-30ø). Thick sequences of resistant sandstone tend to form very steep slopes or cliffs.
