CHANGES OF LEVEL OF NORTH AMERICAN COASTS. 153 



degradation, and where the rivers in ])lace of havinsj; normal deltas enter 

 the sea by estuaries, the shapes of wliieh ean only be explained by the 

 supposition that the margins of the marine reentrant are tlie steeps of 

 the old river valley, we are justitied in assuming the subsidence of the 

 land. 



It is true that there is some difhculty encountered when we come to 

 apply tliis topograi)hic index to ghiciated districts. Thus in the fiord 

 zone, where the plane of the sea cuts the glacially worn, hard rocks, we 

 may always assume that the ice-streams could have excavated the chan- 

 nels for some distance outward beyond the point where the sea, if freed 

 from the ice, would have come in contact with the land. It is very likely, 

 indeed, that much of the fiord topography was due to ice erosion, accom- 

 plished below the ocean level. Where, however, the valleys are broad, 

 in the manner of that of the Saint Lawrence below Montreal and in 

 other similar instances, the most reasonable supposition generall}^ is that 

 the indentation is due to the flooding of a river trough. This supposi- 

 tion can often be verified by the fact that rivers occupying preglacial 

 channels converge in a normal digitating manner toward the axis of the 

 valle}'. This evidence is beautifully shown in the case of such flooded 

 valleys as that of the Chesapeake. 



Changes in Altitudes of the Shores. 

 north american coasts. 



General Statement concerning Them. — On the Ijasis laid down in the fore- 

 going statements I shall now proceed to note some steps in the process 

 which it seems to me we are prepared to take in the general interpreta- 

 tion of the changes of elevation which have occurred in the recent geo- 

 logic ages along the coast of North America, and incidentally on the shores 

 of tlie other continents. In tliis task I shall not undertake to present 

 in any detail the results of certain studies which I have of late years 

 made wliile in charge of the Atlantic Coast Division of the United States 

 Geological Survey, and tliis for the reason that to set forth my observa- 

 tions, tliough they are as yet incomiilete, would recjuire excessive; sjjace. 

 In tliis writing my aim is merely to state certain general conclusions, 

 which ai)pear to me to be well supi)orted by the topography of the shore- 

 line, along with a few notes drawn from observations on the land area. 



Isthinii.-i (if Darlcn to Florida. — Beginning the consideration of this con- 

 tinent with the isthmus of Darien and proceeding northward we observe 

 that the coastline shows little evidence which can be inter})reted as 

 indicating flooded valleys, or, in other words, a recent de})ression of the 

 shore, until we reach the northern border of Mexico. Thence along the 



