CHAPTER XXV. 



DEFOEMATION OF SHOKE LINES. 

 By Prank B. Taylor. 



POSSIBLE CAUSES. 



Old shore lines may have been deformed in several ways. 



1. The water surface may not have been perfectly horizontal while the beaches were 

 being made, as where the water of a lake rested against the front of the ice sheet and was 

 drawn up toward it by the gravitative attraction of the ice. This effect was slight in the Great 

 Lakes region but was always present. 



2. Shore lines may be deformed after they are made by elevation or depression of the 

 land arising from one or more causes. 



According to the theory of isostasy the larger inequalities of the earth's surface express 

 differences of density of segmental parts of the earth, the less dense high or continental parts 

 being balanced against the more dense low or oceanic parts. As a sequence of this theory, 

 erosion from the land surface removes weight from that part and deposition of sediment in 

 the sea adds weight there, and on account of this shifting of weight the equilibrium of the 

 parts affected is progressively disturbed. The deposition of sediments causes the ocean floor 

 to sink and erosion of the land causes the continental areas to rise. De Geer has ascribed the 

 rising of the land and consequent deformation of shore lines of Sweden in part to the erosion 

 of the old (pre-Cambrian) land surface, and he extends the same idea to America. 



A popular view, in some sense a corollary of the theory of isostasy, is that the earth move- 

 ments which raised and deformed the old shore lines were due to resilience of the land following 

 depression by the weight of the ice sheet. There are many reasons for regarding this idea 

 with favor, and it has found more adherents than any other among students of glacial geology. 



All forms of earth movement, without regard to cause, may deform old shore lines. This 

 includes the movements involved in continental and mountain growth as well as the more 

 local warpings, sinkings, and upheavals. 



3. Eustatic movements, like those described by Suess, tend to raise or lower the whole 

 ocean with reference to the land. Such movements would cause the shore lines to be sub- 

 merged as though the land were depressed, or abandoned as though the land were elevated, 

 according as the movement of the sea was positive or negative. 



Shore lines may also be in effect raised or depressed (according to Suess) by oscillations 

 of the ocean which may at one time cause an actual excess of water at the poles and an actual 

 deficiency at the equator, and at other times may reverse these conditions, producing the effect 

 of raised and submerged shore lines in both high and low latitudes. 



AH these possible causes should claim attention in any exhaustive discussion of the subject. 

 In this monograph, however, it will suffice to consider a few of them in a general way. 



HINGE LINES AND AREAS OF UPLIFT. 



The map (fig. 14) is diagrammatic and is intended to show only the principa. facts relating 

 to the ice sheet and the uplifting movements. The boundary of the last or Wisconsin ice sheet 

 at its greatest extent is marked by the heavy line extending entirely across the map, and 

 defines by its sinuous course the outlines of the several strongly marked lobes and reentrant 

 angles of the ice front. The Dakota and the Minnesota-Iowa or Iowa lobes are sharply 

 developed in the west. Between the northeast angle of the Driftless Area in Wisconsin and 

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