SINKS IN THE GYPSUM. 147 



examination are traced to seams of gypsum lying generally only a few 

 feet below the surface. The gypsum by percolation of water has been 

 decomposed, dissolved, and washed away, forming vertical holes in the seam 

 and channels of escape for the water to some lower level. These holes 

 by the action of rain and the drainage from limited areas of the surround- 

 ing clays become enlarged, and some of them have attained a diameter of 

 50 feet. The majority are, however, only from 2 to 5 feet in diameter, and 

 have all the characters of sinks in limestone countries. 



East of Sun Dance these sinks are numerous, and between Sun Dance 

 and Inyan Kara they are met every few hundred feet, dotting the surface 

 of the red clay. 



In the valley of the Belle Fourche, from the vicinity of Bear Lodge 

 to a point twenty miles northeast, the stream has cut its valley through the 

 upper Red Beds, here somewhat elevated by the fading extremity of the 

 uplift which produced the Hills. Above them the Jurassic strata are 

 exposed in a similar manner and to a greater extent. The cut through the 

 Red Beds is nowhere more than 200 feet deep, and the width of their out- 

 crop is not greater than two miles. In some of the side branches and dry 

 canons, however, the Red Bed exposure penetrates the bluffs for a distance 

 of several miles. 



The following cross-section will give a fair idea of the structure of the 

 Belle Fourche Valley : 



Valley of the 

 Belle Fourche 



J 



Fig. 24. — Section across the valley of the Belle Fourche near Bear Lodge. 

 1. Red Beds. 2. 2. Jurassic strata. 3. Dakota sandstone (Cretaceous). 



The Red Beds are here so sandy as to form a tolerably coherent shaly 

 sandstone, but in places they resume their usual clayey character. The 

 following section, observed at the spot represented by the illustration, shows 

 the strata from the summit of the lower plateau to the water of the river. 



