242 C. P. BERKEY PALEOGEOGRAPHY OP SAINT PETER TIME 



level, with retreat of the sea, an erosion interval, a readvance of the sea, 

 each with its characteristic deposits. Just as in displacements by folding 

 or faulting, such disturbances are dissipated by ready adjustments among 

 the easily shifted grains, so that occasionally all traces of the movements 

 are wholly obliterated, so in like manner may an erosion interval be swal- 

 lowed up and almost wholly lost in the shifting and worked over sands 

 that finally constitute the formation. It may easily happen that sands 

 spread out by a retreating sea should be wholly worked over by the read- 

 vance, and yet, on a very uniform surface, leave no trace except a rather 

 abrupt transition. In such a case the underlying bed would carry most 

 of the marks preserved at all and might exhibit prominent erosional un- 

 conformity in limited areas. The overlying bed, in this instance the 

 Stones Kiyer limestone, should be perfectly conformable, as descriptions 

 show it to be (see figure -I). 



Breccias are developed to a limited extent at some of the Saint Peter- 

 Shakopee contacts, according to the records of both the Wisconsin and 

 the Michigan reports. The Magnesian is represented as wedging out in 

 an enveloping upper and lower sandstone wherever an original margin 

 is preserved. 



In Michigan this edge is the remnant of a bed partly destroyed by the 

 encroaching Saint Peter sea, as at one time interpreted by Eominger, 

 and beyond its present margin the Saint Peter should be expected to 

 merge into the Great Basal sandstone ("Eastern sandstone") of the 

 region. This interpretation, and indeed the presence of Saint Peter in 

 Michigan at all, seems to have been regarded with doubt by many writers ; 

 yet there is no inconsistency about the occurrence in any respect, and 

 the later state reports* under Doctor Lane add to the Saint Peter data. 

 It is probable that the same type of edge was developed along the whole 

 length of the ancient shoreline. In the extensive denudation of the re- 

 gion, however, this, together with all later deposits, has been destroyed 

 except in the most protected localities. 



. In short, it is held that in the case of the Saint Peter evidence gathered 

 from a study of its extent as well as its contacts and transitions is in sup- 

 port of a considerable erosion interval. 



Source of Supply of Material 



The Saint Peter sands are widely and rather evenly spread. Consid- 

 ering the possible sources of supply, they are carried to surprising dis- 

 tances. There is no doubt at all of the sedimentary character of the 



* Geological Survey of Michigan, Annual Reports for 1901 and 1903. 



