122 Kindle — Unconformity at the Base of the 



dome of older rocks in a broadly crescent-shaped belt having a 

 zigz&g line of outcrop more than 250 miles in length. Sepa- 

 rated from this area by a narrow belt of Carboniferous rocks 

 is the valley of the Cumberland River in southern Kentucky. 

 The black shale extends up this valley from Tennessee about 

 50 miles. It is probable that this formation extends without 

 interruption across the entire state beneath the younger rocks 

 outside the above described areas in which strata older than 

 the Chattanooga shale are the surface rocks. 



description of unconformity. — A large number of outcrops 

 showing the contact of the Chattanooga shale and older rocks, 

 which are widely distributed along the lines of exposure of the 

 shale as outlined above, have been studied by the writer. In 

 nearly every district visited physical evidences of an erosion 

 interval have been observed. These evidences may be referred 

 to three classes, namely : (1) irregular of mammillary surface 

 of the subjacent limestone, (2) beds of residuary clay beneath 

 the shale, and (3) deeply excavated trenches and shale-tilled 

 cavities of solution in the underlying limestone. The first and 

 second named phenomena have a wide distribution, but the 

 third, as might be expected, is found rather rarely. In local- 

 ties where the limestone base of the black shale has been 

 recently denuded of the shale, one very frequently finds the 

 surface of the limestone hollowed and pitted in the peculiar, 

 irregular, and often angular manner similar to that which may 

 be seen at nearly any limestone quarry where the rock has been 

 stripped of its residuary clay. A good example of this hum- 

 mocky surface on the Devonian limestone where the shale has 

 been recently removed occurs southeast of Crab Orchard, Ken- 

 tucky, 1-J miles on the Gum Sulphur road. Near the school- 

 house at this locality the highly uneven hummocky surface of 

 the limestone can be traced directly under a 10-inch bed of 

 reddish brown clay which separates the black shale from the 

 weather-worn limestone below. Here one sees in the vertical 

 cross-section of the limestone beneath the shale the angular 

 ridges, truncated cones, and mammillary protuberances covered 

 with a clay which differs from the usual residuary limestone 

 clay only in being of a duller color. The correspondence in 

 appearance between the surface of this irregularly worn lime- 

 stone and that usually seen in limestones recently denuded of 

 their clay covering is complete and unmistakable. Numerous 

 examples of subaerial erosion of this kind have been observed 

 around the fringe of Chattanooga shale which circles the older 

 rocks south of the Ohio. On the west side of the Cincinnati 

 geanticline, south of the Ohio river L5 miles, evidences of the 

 pre-Chattanooga erosion interval are apparent near JBrooks 

 station. In the bed of Brooks Run, between the railroad and 



