Jennings: A Botanical Survey of Presque Isle. 291 



Lakes, and from certain general principles thus ascertained the origin 

 and growth of a peninsula such as Presque Isle is easily understood. 



By far the most potent factors in determining the topography of 

 lake-shores are waves and currents. In Lake Erie the amount of in- 

 flow and outflow is hardly to be considered as a factor in the pro- 

 duction of currents, because of the relatively very broad expanse of the 

 basin of the lake and the action of other more powerful factors, but 

 investigations undertaken by the United States Weather Bureau^ have 

 shown that there is a system of currents, the general trend of which is 

 east through the North Passage and thence southeast towards the shore 

 from the vicinity of Lorain eastward. The currents must evidently 

 be due to the action of the prevailing winds, especially the more severe 

 storms. The larger axis of Lake Erie lies almost directly in the 

 normal path of cyclonic storms® and, as the bed of the lake is com- 

 paratively shallow, the wave-action thus produced is particularly 

 strong, resulting in pronounced currents. 



There is also a considerable shifting of the waters from one end of 

 the lake to the other, due to barometric pressures. The barometric 

 pressure often varies an inch within the length of Lake Erie and, when 

 this factor acts in conjunction with high winds, considerable fluctua- 

 tions may occur in the height of the water. Fluctuations of as much 

 as fifteen feet have been noted within the space of a few days at the 

 eastern end of the lake.' 



^Vhen waves from the west break upon the shelving beach of the 

 southern shore of Lake Erie, there is near the shore a movement of 

 water towards the beach, and, as the angle of incidence is oblique, 

 there is a tendency for the wave to retreat as an undertow towards the 

 northeast. The usual result is that there is formed close to the shore 

 a current running northeastward, parallel to the shore. To such lit- 

 toral currents we may in a general way attribute the formation of 

 Presque Isle. 



It has been found that in deep water the lateral movement due to 

 a wave is very slight, but when approaching shore the friction on the 

 bottom increases, so that the upper part of the wave finally topples 

 over, forming a breaker. Much of the force of the wave is here ex- 



SRussell, I. C. "Lakes of North America," pp. 32-33, 1895. 

 ^United States Weather Bureau. "Climatic Charts of the United States. Chart 

 No. 4, 1904." 



■ Russell, I. C. /. <r., p. 34. 



