108 PUBLIC PARKS OF IOWA 



Between the steel bridge and Mill Spring ravine, nearly a mile up- 

 stream, is a noble bluff, steep everywhere and in places precipitious or 

 vertical. Trees cover it wherever they can obtain foothold and here and 

 there beetling rock precipices stand out among the foliage. The spot of 

 greatest interest in this locality is the Ice cave, which is situated per- 

 haps midway along the bluff as well as about the center of its 250 feet 

 of elevation. The local Commercial Club has built a road along the 

 face of this bluff and from here a fine view may be obtained across the 

 valley to the opposite wall, as well as up and down the river. Mill 

 Spring, at the upper end of this drive, is an unusually large one, even 

 for this land of abundant springs. It comes boiling and bubbling out of the 

 rock at the base of a great cliff which springs nearly a 100 feet straight 

 upward. Thence the stream, four or five feet wide and several inches 

 deep, dashes down over the loo'se rocks, along the narrow ravine and 

 into the river. The spring is at the mouth of a great cavern which re- 

 sembles the Ice cave, but is blocked by fallen rock masses. It undoubtedly 

 was once similar to the Ice cave in both nature and mode of origin. At 

 the mouth of Mill Spring ravine there is a deposit of tufa which once 

 evidently filled the mouth but has been partly cleared away. What was 

 the origin of this peculiar deposit? It is composed of lime which was 

 doubtless dissolved from the limestone rocks of the region and was re- 

 deposited here, perhaps by the water of the spring at some time when 

 it did not have free flow to the river. The lower part of the deposit 

 shows what seem to be impressions of stems and twigs of vegetation, in- 

 dicating that the lime was deposited around these plant fragments and 

 that they have since been removed by decay. 



The Ice cave and the Mill Spring cave seem to have been formed by 

 the slipping out on their bases of great masses of rock, which were sep- 

 arated from the main part of the cliff by joints. Thus a triangular open- 

 ing was formed between the cliff and the loosened block. Doctor Calvin 

 has explained in his discussion of the Ice cave that the country rock 

 of this region is thoroughly penetrated by cracks and fissures and hid- 

 den caverns. During the winter cold dry air is drawn into these open 

 spaces and in the spring and early summer when the sun's heat is warm- 

 ing the outer portion of the earth's crust, this air also is warmed and 

 expanded by which means some of it is driven out into the open. Where 

 this cold dry air conies into contact with the quiet warm moist air of 

 the cavern it causes precipitation of the moisture upon the inner wall, 

 and the temperature of the outcoming air is still so low as to freeze 

 this condensed moisture, forming a coating of ice several inches thick 

 on the lower part of the wall. During the latter part of the summer the 

 ice melts away and the wall remains bare until the next spring. There 

 is also near Decorah a freezing well which has the same peculiarity as 

 the Ice cave. 



Decorah is not the only locality in the Oneota valley which can 

 boast of beautiful scenery. Up the river there are especially noteworthy 

 localities at Bluffton, Plymouth Rock and Florenceville, while in the 

 lower valley, where it is deeper and the bluffs are higher, picturesque 

 spots are- plentiful. The various "Backbones" in the state which for 

 some unaccountable reason are dedicated to the evil one may be dupli- 



