218 THE GEOLOGY OF MINNESOTA.. 



[Trenton limestone. 



are filled more or less with the loess-loam the topography still is so marked, 

 pertaining to and even caused by the rocky outlines, that the limits of each 

 formation are very evident to the observer. There is more or less doubt 

 about the position of the boundary between the St. Peter sandstone and the 

 Shakopee limestone. The incoherency of the St. Peter causes it to crumble 

 easily, and to leave no evidence of its final dissolution where the exact 

 contact between the formations cannot be examined; and the loam gen- 

 erally securely hides this horizon. 



The Trenton limestone. This formation, as known in Houston county, 

 consists of limestone layers that amount to a thickness of not more than 

 fifteen feet. These layers are overlain by beds of shale and fossiliferous 

 shaly limestone which reach an unascertained thickness, but probably not 

 exceeding twenty-five feet. These shaly beds have been denominated 

 " Green shales", in the reports of progress of the survey, but they seem to 

 belong to the Hudson River age, of New York. They are overlain in Fill- 

 more county, and in northeastern Iowa, by firm calcareous strata which 

 attain a thickness of fifty or sixty feet, which seem to fade into the Galena 

 formation of Iowa, as may be seen by consulting the chapters relating to 

 the geology of Fillmore and Goodhue counties. 



This formation is found in Spring Grove and Wilmington townships. 

 It runs also in a narrow, but interrupted, belt nearly to Caledonia, where it 

 may be seen distinctly in its peculiar features, and its flat-topped mounds, 

 or tables, a mile west of that village. There is reason to suppose that it 

 formerly extended much further east than it does now, covering the most 

 if not the whole of the county, and being continuous with the horizon of 

 the same formation east of the Mississippi river in Wisconsin. 



The usual characters of the Trenton, both lithological and palaeonto- 

 logical, were the only ones noticed in Houston county. It has been opened 

 for quarries only in the vicinity of Spring Grove. It generally presents a 

 stained and long-weathered aspect, as if split and dissolved by the action 

 of water. The layers are at first about an inch in thickness, but become 

 thicker, by union with each other, on being wrought to some depth, and 

 possess a blue color. 



The St. Peter sandstone. This lies next below the Trenton. Its area 

 embraces not only the slope from the high table-land of the Trenton area, 



