ORIGIN, MODIFICATION, AND AGE. 237 



occur caves, natural bridges, and underground water-courses. On Cave 

 creek, 4 miles west of Evansville, is the Big Gypsum cave in the Medi- 

 cine Lodge gypsum, about 200 feet in length and 15 feet high at the 

 east entrance. The western portion is low and covered with water, so as 

 to be almost impassable. The section near the cave shows 30 feet of 

 Medicine Lodge gypsum covered by 15 feet of red shales, then 15 feet of 

 the Shimer gypsum. The natural bridges found in this region represent 

 remnants of old caves or underground water channels whose roofs have 

 for the most part fallen in, leaving the bridge as a clue to the former 

 history. > 



Origin, Modification, and Age of the Deposits. 



northern area. 



The gypsum deposits of northern Kansas rest on a nearly level lime- 

 stone floor over a large area. Although this limestone is an evidence of 

 life, no traces of organic forms are found in the gypsum series above 

 it, for life would be destroyed in a basin in which large quantities of 

 gypsum were being deposited. Thus there is evidence of an abrupt 

 change in physical conditions. Through some disturbance an arm of 

 the sea was cut off, at least temporarily, and the evaporating water de- 

 posited gypsum. The dip of the floor is westward in the direction of the 

 open sea of that time. The fact that the Blue Rapids gypsum contains 

 clay seams irregularly distributed through it indicates that this gulf was 

 a drainage basin for small streams. The foregoing conditions continued 

 until at least 8 or 9 feet of gypsum were deposited, and then the old 

 connection with the sea was either restored or the erosive and transport- 

 ing powers of the streams were increased, so that the series of sandy 

 shales buried the gypsum. It is not possible now to determine definitely 

 whether evaporation continued long enough for the deposition of salt, 

 but probably not, as there is no trace of salt detected in this region. 



Much of the gypsum was subsequently removed by solution, for the 

 upper surface is very irregular. This was accomplished before the con- 

 solidation of the shales, for they lie horizontal and are not folded to 

 conform to the irregularities. This irregularity of the upper surface is 

 shown in all the mines, but perhaps best in the Winter's mine, where 

 the gypsum for 200 feet in the hill lies in long, rounded masses, with the 

 long direction parallel to the hill slope. 



The limestones with the Permian fossils and flint nodules, 50 and 100 

 feet above the gypsum and separated from it by buff sandy shales, in- 

 dicate an increased subsidence of the area and a change in physical con- 

 ditions favorable to life. Near the close of the Lower Permian or the 



