Apparent Anomalies of the Postville Well. — Calvin. 201 
time caverns of large dimensions may be formed in the sand- 
stone by means purely mechanical. That caverns are so 
made is clearly shown at a number of exposures. About one 
mile south of Werhan's mill, and about seven miles north of east 
from Postville, there is a large natural cavern in the Saint Peter, 
opening by a relatively small entrance into the side of the 
bluff from the level of the roadway. The chamber is at least 
sixty by forty by twenty feet in dimensions. A little sand 
may have been hauled away from here for building purposes, 
and some small amount of artificial excavation may have 
been made in adapting the cavern to its present use as a great 
natural cow-shed, but the chamber, in its main features, illus- 
trates the possibilities of cave formation, by mechanical 
erosion, in this particular formation. The arching roof of 
the cave is of sandstone, in which the grains are cemented 
together by calcareous material carried down by infiltrating 
waters during the process of solution and removal of the 
overlying Trenton limestone. The bond is just sufficient to 
enable the arch to stand in opposition to gravity. The roof 
of the cavern is intact. The only opening is from the side. 
The erosion of the valley, which is followed by the roadway, 
barely cut into the edge of the natural excavation. 
On the farm of Mr. James Dougherty in French Creek 
township there is another cavern in the Saint Peter sandstone 
even more suggestive than that just described. In this case 
the opening, a few yards in diameter, is from above. The 
cave is entered by means of a ladder. The chamber is some 
forty to sixty feet in diameter. It is over-arched by a dome 
like roof of sandstone, the roof being complete except for the 
small opening through which entrance is effected. -This cave 
has probably been excavated since the Trenton, that originally 
overspread the region, was removed. At all events, the open- 
ing in the roof is comparatively recent, and has been formed 
since the disappearance of the limestone. 
Many other caves in the sandstone bear testimony to the 
Fact already stated that the Saint Peter is especially liable to 
become cavernous, hut those already noted will serve for pur- 
poses of illustration. Erosion by underground streams may 
begin anywhere below the upper surface of the formation and 
may result in producing a dome-shaped cave; the friability 
of the deposit in its lower portions arFording the conditions 
