392 THE ICE AGE IN NORTH AMERICA. 



abandoned water weirs at an elevation of approximately 1,100 

 feet above tide. These are just such channels or "cols" as 

 would be cut by the escaping water impounded by an ice-dam 

 which obstructed the northerly drainage of the Monongahela. 

 The terrace deposits of clay, quicksand, sand and gravel, bor- 

 dering the river valley for more than 100 miles below Weston 

 in West Virginia, range from 1,020 to 1,068 feet above tide, 

 while that at Pittsburg is 990 feet above tide. The present 

 river falls 290 feet between Weston and Pittsburg, while 

 the terrace falls only forty feet in 200 miles at Pittsburg, and 

 eight feet at Geneva, 117 miles below Weston. In tabular 

 form the facts are as follows. 



MILES 





PRESENT RIVER 



A.T. 



TOP OF DEPOSITS 

 A.T. 







Weston 



990 



1,030 



40 



Clarksburg 



916 



1,020 



75 



Fairmont 



851 



1,067 



101 



Morgantown 



787 



1,038 



117 



Geneva 



772 



1,022 



206 



Pittsburg 



700 



990 



These terraces are specially prominent wherever a tributary 

 comes into the main valley to form a delta at the level of 

 the impounded water. Frequently the clay topping of the 

 terrace is more than sixty feet thick, furnishing material for 

 numerous pottery establishments. 



The following plants were identified by Dr. F. H. Knowl- 

 ton, as occurring in these clay deposits. 1 . Equisetum arvense 

 L.; 2. Cyperus sp.; 3. Potamogeton robbinsii Oakes; 4. Liqui- 

 dambar styracifolia L; 5. Platanus occidentalis L (sycamore); 

 6. Ulmus racemosa Thomas (the white elm); 7. Quercus 

 falcata Michigan; 8. Betula nigra L. (black birch); 9. Fagus 

 ferruginea Ait (beech) ; 10. Castanea pumila Mill (chinquapin). 

 All of these are living species, and most of them are still 

 found in the vicinity. But special interest attaches to Pota- 

 mogeton robbinsii, of which a great number of fragments of 



