302 Glacial and Moditied Drift 



Hastings. Castle rock originally had a height of 70 feet above 

 the lowest ground at its base, and its upper 20 feet was a slen- 

 der rock column, which, by the effect of subaerial erosion of its 

 lower part, fell down several years ago. The Chimney rock 

 here mentioned, one of several in Dakota county bearing this 

 name, is the most picturesque and perfect example of columnar 

 rock weathering in Minnesota, or indeed, as I believe, in our 

 entire country. It is a vertical pillar, measuring 34 feet in 

 height and ai)out 6 and twelve feet in its less and greater diam- 

 eters, being no thicker near the base than in its upper part. 

 Plate VIII gives a view of this sandstone column, of which no 

 former description or illustration has been published. 



Such spires of easily crumbling sandstone could not en- 

 dure the envelopment of this area by the slowly moving ice- 

 sheet, which is known to have once existed there by the con- 

 tinuation of the very old drift many miles beyond these rock 

 pillars. During the deposition of that glacial drift, knolls or 

 small plateaus of the sandstone, capped by an exceptionally 

 hard layer or by the next higher Trenton limestone, and hav- 

 ing sufficient area to withstand the pressure of the ice current, 

 doubtless occupied the sites of the Castle and Chimney rocks ; 

 and by subsequent erosion of weathering, through the agencies 

 of rain and wind, cold and heat, the sandstone slowly crum- 

 bled away, leaving only these columnar masses. How long a 

 time would be required for this result, we can only vaguely 

 conjecture: but it seems probable that the 50,000 or 100,000 

 years which have been variously computed to have passed 

 since the culmination of North American glaciation, in the 

 Kansas stage, would suffice. It is evident that the relatively 

 short time since the Illinoian and lowan stages of glaciation 

 w^ould be inadequate. 



The Ice age thus was very long in comparison with the 

 Postglacial period. Indeed, the whole Quaternary era may 

 have measured 150,000 years, or more, in which time were 

 comprised the gradual oncoming of the ice-sheet, its repeated 

 fluctuations, and at last it? most energetic accumulation of 

 marginal morainic hills, whenever its final melting and retreat 

 w^ere temporarily interrupted. 



With the departure of the ice-sheet, while it was being 

 melted back from one marginal moraine to another, yielding- 

 its ground in general by a recession from south to north, the 



