974 HISTOKICAL GEOLOGY. 



common in the Appalachians than during the First Retreat, water was less 

 abundant, and the deposits, therefore, were generally much like those of more 

 recent time. 



Deposits on the Atlantic border, partly marine and estuarine, and those 

 of similar character elsewhere, have been named the Columbian formation. 

 This formation is described by McGee (1888 and later) as occurring over the 

 coastal plain of the Atlantic slope, and ranging in height from over 100 feet 

 in the south to 400 or more in the north. It consists of a series of sub- 

 estuarine and submarine deltas and associated littoral deposits. The pre- 

 dominant and most significant phenomena are widespread stratified deposits 

 and associated terraces, newer than the Lafayette formation of the same 

 coastal region. Part of the deposits made during the earlier retreat also are 

 described as Columbian. The formation, according to McGee, is in part of 

 submarine origin. 



Another result of the melting and depositions during the long retreat was 

 the making of innumerable lakes, especially over the more level portions of 

 the glaciated region. There were probably thousands where there are now 

 scores — like the Tundra regions of similar origin, in Russia. The gradual 

 action of waters during their flood seasons have converted many lines into 

 drainage channels, while numerous others have gradually become shallowed 

 by depositions of earth and organic materials and passed to the conditions 

 of swamps ; and the swamps have ever since been drying up. 



Topographical results of abrasion by the ice and ice-made streams. — The 

 minor effects of abrasion by glaciers are scratches, groovings, polishing of 

 surfaces over the harder rocks ; making deep channels in the softer, such 

 as limestone and many sandstones ; producing forms over the surface like 

 the moldings in architecture, but yards in depth and width, where the 

 architect puts inches ; carving out roches moutonnees, an example of which, 

 from the region of the Holy Cross in Colorado, is represented on page 250 ; 

 but not boring out pot-holes, which requires a stationary tool. 



The larger effects of direct abrasion on the softer rocks are long and 

 wide trenches, one or more hundreds of feet deep ; shallow lake-basins, 

 like that of Mono Lake in Kevada, which, according to Russell, was excavated 

 out of limestone to a depth of 51 feet below the existing rim, its bottom and 

 sides being limestone ; river channels ; ridges elongated in the direction 

 of the movement of the ice ; steep fronts on the struck or stoss side of 

 hills — the side facing the ice-stream ; and long-drawn-out ridges with 

 gentle slopes on the opposite side. Another case is that of soft rocks saved 

 from removal by being under the lee of a ridge of harder rocks, the harder 

 ridge making a great cavity or notch in the ice, — as near New Haven, Conn., 

 where a ridge of weak sandstone (Sachem Ridge, on the map, page 993), 

 a mile long and 100 to 165 feet high, was left under the lee of a trap ridge 

 (Mill Rock), just as a tool with a notch in its cutting edge leaves a raised 

 line on the surface of a board. The tearing and displacing work of 

 frosts and freezing was also going on over all frosty regions, even those 



