fekBL. I SOUTH DAKOTA. 299 
rest of the central region of crystalline rocks, and is ifiost characteristically exhibited 
|i the great lines of cliffs in the infacing escarpment surrounding thai region. Mr. 
aggar has suggested that there be applied to it the Dakota Indian name for the 
ilack Hills — Pahasapa. The formation consists of a thick deposit of massive gray 
mestone, usually outcropping in precipitous cliffs with many picturesque irregulari- 
es of form, or with wide, flat surfaces. Caverns are of frequent occurrence, some 
I them being of large size. One, having several miles of galleries, is known as Wind 
ave, from the strong current of air which usually issues from its mouth. It is sitn- 
ted 8 miles north of the Hot Springs and attracts thousands of visitors. Crystal 
lave, in the northern Black Hills, is also a very interesting cavern, with many large 
eposits of dog-tooth spar on its walls. 
The most extensive exposures of the Pahasapa limestone are in the great plateau 
'est of Custer. Here the formation begins in a line of high cliffs surmounting 
opes of crystalline schists and the relatively thin sheets of Englewood limestone 
rid Deadwood sandstone. A view of one of these cliffs is showm in tig. 274. In 
ennington County the plateau has a width of 10 miles of continuous limestone out- 
rop, constituting the most elevated area in the Black Hills excepting the small 
immit of Harney Peak. To the west the limestone passes beneath the sandstone 
f the Minnelusa formation, but it is exposed again in the arch of the steep anticline 
ear the Wyoming-South Dakota line. Hell Canyon cuts deeply into the Pahasapa 
mestone, as does also the wider canyon known as Pleasant Valley. East of the 
rystalline rock area the limestone stands out on many conspicuous knobs, or lies on 
ne eastern slopes of ridges due to the Deadwood quartzite, but it does not attain 
le high altitude which it has farther west. With decreased thickness, the more 
apid dip to the east soon carries the formation below the surface in that direction, 
ut it constitutes the walls of many of the canyons of the streams from Beaver Creek 
orthward. French Creek has extensive cliffs of the limestone, and Spring Creek 
as cut a long, deep canyon through it. 
The thickness of the Pahasapa limestone in the central and southern Black Hills 
aries from about 500 feet at the northwest to 225 feet on the east and southeast. 
II along the eastern side of the hills it appears to have the latter thickness, with 
ight local variations. It does not present any noteworthy lithologic subdivisions, 
ut its upper part is often siliceous and flinty and stained red to a greater or less 
xtent from the overlying red beds of the Minnelusa formation. At its top there is 
sually a red shaly bed of slight thickness, containing oval concretions of hard silica 
om 6 inches to 2 feet in diameter in greater part. Fossils occur sparingly through- 
ut the formation, including Spirifer rocky montanus, Seminula dawsoni (Athyris sub- 
lita), Productus, and Zaphrentis, a fauna which indicates lower Carboniferous age. 
MINNEKAHTA LIMESTONE. 
This formation, known in previous geological reports as the purple limestone, is a 
>rominent member of the Black Hills series. It is thin, averaging less than 50 feet 
n thickness, but it is hard and flexible and covers moderately extensive areas of the 
uter slopes of the Minnelusa formation. Southwest of Hot Springs it constitutes a 
)rominent anticlinal ridge, which extends south to Cascade Spring. It is proposed 
o designate this formation the Minnekahta limestone, because a distinctive geo- 
raphic name is required, and the region near the hot springs, originally known as 
he "Minnekahta" by the Indians, is a typical locality. The springs rise through 
irevices in the formation just west of the town of Hot Springs, and the exposures in 
he vicinity show all the characteristic features which the formation presents. The 
)rominence of the Minnekahta limestone outcrops is due largely to the fact that the 
)verlying formation is soft, red shale, which has been deeply eroded, leaving the 
mderlying limestone bare on slopes up which the red shale originally extended. 
