26 THE GLACIAL LAKE AGASSIZ. 



SHORE-LINES. 



Considered in relation to the general topography, the shore-lines of 

 Lake Agassiz are inconspicuous, though they are very distinctly traceable. 

 They are usually marked by a beach deposit of gravel and sand, forming 

 a continuous, smoothly rounded ridge, such as is found along the shores of 

 the ocean or of our great Laurentian lakes wherever the land sinks in a 

 gently descending slope beneath the water level. The beach ridges of 

 Lake Agassiz (fig. 6) commonly rise 3 to 10 feet above the adjoining land 



Till __.— g r . ' „-^^:-.^..^-^ -^^^^^^^^^ rut, suCHTLrr/fooeo. 



— r^,„-i— ^ r-,^,-,-^^,^,^-c=T, iCi'SLOriAAC ASASSI2. 



i"iG. 6.— Typical section across a beach ridge of Lake Agassiz. Scale, 100 feet to an inch. 



on the side that was away from the lake, and 10 to 20 feet above the 

 adjoining land on the side where the lake lay. In breadth these ridges 

 vary from 10 to 25 or 30 rods. In some places they have been cut through 

 and carried away by streams, and occasionally they are interrupted for 

 a quarter or a half mile, or even 2 or 3 miles, where the outline of the 

 lake shore and the direction of the shore cun-euts prevented such accumu- 

 lation. Throughout almost the whole extent of Lake Agassiz examined 

 witliin the United States the regular outlines of the surface, its gentle slope 

 toward this lake, and its material, which nearly everj^vhere is till, were very 

 favorable for the formation of beach deposits. Many beach ridges, record- 



FiG. 7. — Eroded terrace marking the shore of Lake Agassiz. Scale, 100 feet to an inch. 



ing the successive reductions in the elevation and area of this lake, have 

 been traced in continuous, approximately parallel courses along each side 

 of the Red River Valley. PI. YI shows an excejitionally massive beach 

 ridge marking the highest shore-line of Lake Agassiz close noilhwest of 

 Maple Lake, about 20 miles east-southeast from Crookston, Minn. 



Another type of shore-lines is presented where the lake has formed a 

 terrace in the till (fig. 7), with no definite beach deposit, the work of the 

 waves having been to erode and carrv awav rather than to accumulate. The 



