ANNUAL OSCILLATIONS. 277 



or elsewhere in the neighborhood of the old shore-hnes will pi-obably cease 

 to exist within a century, or in some cases within a score of years. On the 

 other hand, many artificial groves sun-onnding farmhouses, and lines of trees 

 cultivated on the divisions of property or of adjacent fields, will probably 

 more than replace such loss, making the coimtry more beautiful and less 

 liable to be swept heavily by winds. But the extensive views enjoyed 

 by the writer and his assistant rodman as they advanced along the course 

 of the beaches, mapping them and determining their elevation, will be 

 then hindered by the cultivated groves, tree rows, and hedges. Only upon 

 a prairie country, such as this was when its shore-lines were first traced, 

 can the grandeur of the proofs of existence of glacial lakes, held by the 

 obstruction of the departing ice, be taken in by an imimpeded vision of 

 the smooth lake bottom on one side stretching out to a distance of 10 or 20 

 miles within sight, of the bordering beach, running as one unbroken ridge 

 of gravel and sand in a nearly du-ect com-se discernible for several miles, 

 and of the broad, slightly higher expanse of more undulating and knolly 

 glacial drift outside the lake area. 



From these descriptions of the beach ridges and eroded shores of the 

 old lake, its levels at the time of formation of these shore-lines are deduci- 

 ble approximately. The elevations of the crests of the beach ridges, as 

 recorded in these notes, are commonly 5 to 10 feet, or rarely 15 feet or 

 more, above the level held by the lake when the beaches were heaped up 

 by the waves, chiefly during storms. Where the descents of the slopes of 

 these gravel and sand ridges are noted, the lake level was nearly always 

 below the depression which borders the landward side of the beach and 

 was near the foot of the lakeward slope. Cliffs eroded by the lake waves 

 give more definitely the plane of the water surface which cut into the base 

 of the eroded escarpment, usually consisting of till, undermining it and car- 

 rying away its material to form a very gently descending slope, which was 

 covered by the margin of the lake. 



Fluctuations of the lake level, which doubtless rose in summer a few 

 feet higher than in winter, because of the variations in the volume of water 

 supplied from the melting ice-sheet, have given a variability within limits 

 generally 5 feet and perhaps sometimes 8 or 10 feet apart to the heights of 



