Correspondence. 189 
Some 200 feet above the salt beds the gypsum reaches its culmina- 
tion. At this horizon there are thick ledges of massive white gypsum 
which lie level and extend for long distances across the country. 
Ordinarily there are two of these ledges, each from 10 to 30 feet thick 
separated by 10 to 20 feet of red clay shale. Professor Cragin has 
given this massive gypsiun the name "Cave Creek gypsum," from a 
creek in Comanche county, Kansas. The lower (and usually thick- 
er) gypsum ledge he calls the Medicine Lodge gypsum, and the up- 
per ledge the Shimer gypsum. The intervening clays are designated 
as the Jenkins clays.* It frequently happens, however, that this clay 
entirely disappears, in which case the single gypsum ledge is as much 
as 50 feet thick. On the other hand in certain sections there are 
three ledges of gypsum, a thinner ledge coming in below the Med- 
icine Lodge. 
The gypsum hills are hills of erosion. The soft clays below the 
massive ledges are readily acted upon by water. The ledges them- 
selves are relatively much harder and consequently resist erosion. 
The slope of the country is to the east, while the ledges lie com- 
paratively level. This causes the greater erosion to the east, and 
as the underlying clays are removed the gypsum ledges remain as a 
cap forming the escarpment of the hills, which rise like a wall from 
the level plains to the east. The slopes below this escarpment are 
often as much as 200 or 300 feet high, and consist of blood red clays, 
sometimes grass-covered but more often barren of vegetation. These 
slopes are often covered with selenite crystals, which on a clear day 
reflect myriads of light-points and have given to one part of the hills 
the characteristic name "glass mountains." 
Throughout the region of the gypsum hills there are numerous 
pronounced erosion forms. Deep and narrow canyons, flat-topped 
mesas, pinnacles and turrets, mansard mounds and buttes, and nat- 
ural bridges and caves may be found in most localities. These forms 
ara due not only to the solubility of the gypsum ledges but also 
to the fact that the underlying clays are so easily eroded. The 
gypsum caves are of particular interest. They are of all sizes from 
mere cracks in the rock to immense caverns a mile or more in 
length with sometimes dozens of chambers leading off in all direc- 
tions. In southern Kansas and northern Oklahoma many of these 
caves are known locally as "bat caves" from the fact that they are 
the home of multitudes of bats. These animals reinain inside during 
the day and in the evening pour out of the mouth of the cave in a 
continuous stream, sometimes for an hour at a time. 
The gypsum hills extend in a general northeast and southwest 
direction for a distance of nearly 500 miles from southern Kansas 
to west-central Texas. The northern limit so far as I have been able 
to observe is in northwestern Barber county, Kansas, on the north 
bank of the Medicine river, near Sun City. From this point the hills 
trend southeast and approach within six miles of Medicine Lodge, 
• Colorado College Studies, vol. vi, March, 1896, pp. 27-39. 
