180 
IOWA ACADEMY OF SCIENCE Vol. XXIV, 1917 
TABLE SHOWING VARYING THICKNESSES AND THE SUMS OF 
THE THICKNESSES OF THE PRAIRIE DU CHIEN 
AND ST. PETER FORMATIONS AT PIKES 
PEAK AND PICTURED ROCK. 
Trail. 
Thickness of 
Prairie du Chien 
Thickness of 
St. Peter, 
Sum of Thick- 
nesses of Prairie 
du Chien and 
St. Peter. 
Short trail up 
nose of peak 
177 ft. 
123 ft. 
300 ft. 
Long trail by way 
of falls and 
84 ft. 
223 ft. 
307 ft. 
spring 
Middle trail 
115 ft. 
178 ft. 
293 ft. 
(3) Not only are the sums of the thicknesses of the two 
formations approximately constant around 300 feet at specific 
points, but the sum of their average thicknesses is approxi- 
mately the same. The greatest known thickness of the Prairie 
du Chien formation is 268 feet and the least known thickness 
is 80 feet. Corresponding figures for the St. Peter are 238 
feet and 50 feet respectively. The average of thirteen known 
thickness of the Prairie du Chien formation is 167 feet. The 
average of an equal number of known thicknesses of the St. 
Peter is 146 feet. The sum of these two averages is 313 feet. 
(4) In several places, notably in the vicinity of Church, 
the basal portion. of the St. Peter sandstone contains fragments 
of chert which came from the Prairie du Chien dolomite. This 
shows that calcareous materials had been deposited, cementa- 
tion, dolomitization and silicification had been accomplished, 
and the dolomite had been exposed and partly disrupted before 
the deposition of the St. Peter. 
The four points discussed above seem to demonstrate that the 
St. Peter formation lies unconformably on the Prairie du Chien. 
The irregular surface of the Prairie du Chien is due to ero- 
sive agencies operating after the withdrawal of the Prairie du 
Chien sea and before the deposition of the St. Peter sandstone. 
The basal portion of the St, Peter, where the formation is 
thick, is quite different from the lowermost beds where the sand- 
stone is thin. That is, there are two phases of the St. Peter in 
Iowa; namely, a valley phase and an unland phase. The sand- 
