202 The American Geologist. April, 189 
favoring the easy removal of material, and the calcareous 
cement in the upper portion furnishing the conditions neces- 
sary to the maintenance, for some time, of a stable roof. 
Now, if a cavern like that on the Dougherty farm is formed 
in that portion of the sandstone which is still overlain by the 
Trenton, it may for a long time remain as an unfilled space; 
but when the summit of the arched roof opens through to the 
basal part-of the Trenton, the descending waters will at once 
proceed to fill the chamber with the argillaceous and calcar- 
eous shales from numbers 1 and 8 of the Trenton section. 
The Postville well samples taken between 450 and 512 feet 
in depth are all of such material as might have come from 
the shaly members of the overlying Trenton. All apparent 
anomalies, therefore, are explained by the admission of a 
cavern at this particular point, in the Saint Peter sandstone; 
a cavern in which the arching sandstone roof was finally 
broken through up to the base of the Trenton, and into which 
easily erosible material from the overlying limestone forma- 
tion was carried by descending ground waters, until the 
entire space was tilled with the "mud rock 1 ' that was such a 
puzzle to one of the most observant and intelligent of well 
drillers. This explanation is in entire accord with known 
facts derived from observations in the field. It is inconsist- 
ent with none. 
From the very nature of the case, caverns in the Saint Peter 
sandstone beneath the Trenton must be of common occurrence, 
and the scores of deep, hopper-shaped sink holes that are so 
characteristic a feature of regions occupied by the overlying' 
limestone* show that descending waters have carried away 
enormous quantities of erosible Trenton material to be depos- 
ited somewhere along the course of the underground streams. f 
*Iowa Geol. Sur., vol. iv, p. 78, 1895. 
|Mr. John H. Dickson, of Luana, Iowa, has bored a number of wells 
throughout an extensive area in which the geological structure is essen- 
tially the same as at Postville. From what is known of the Saint Peter 
sandstone at its natural exposures it seemed that the peculiar conditions 
disclosed by boring the Postville well are sufficiently numerous to make 
it very probable that other wells might encounter the same conditions. 
Mr. Dickson, in response to inquiries, writes: "In the Tewes well, three 
miles east of Monona, we had such an opening only in a smaller degree, 
and in referring to my well -book I find, in one case in Crawford county, 
Wis., we had a similar case, only the color was buff, the same as above 
the sandstone about 30 to 40 feet. We had to go back to this well three 
limes to take up the pump, and finally had to put in 66 feet of smaller 
casing to hold the walls in proper place." 
