Geogj-aphy of Ohio 197 



Whenever a tongue of ice stood for a long time at a given point 

 in a valley, its sides and front were marked by a morainic loop. 

 The streams issuing down through the valley from this position 

 were heavily laden, and in consequence aggraded the valley floor 

 for several miles beyond. The washed deposits thus accumulated 

 down stream from a morainic loop are called "valley trains." 



Outwash plains. The intervalley portions of retreatal moraines 

 are often bordered by gradually sloping bands of gravels and sand 

 called "outwash plains." The material of these plains was deposit- 

 ited by streams or sheets of water issuing from the ice-front. Local 

 irregularities of relief in this intervalley area introduced corres- 

 ponding irregularities in the shapes of these outwash plains. The 

 genesis of valley trains and outwash plains is practically the same, 

 except that the former deposits are confined laterally. 



Eskers. Sometimes in the thinner areas, near the front of the 

 ice sheet, surface water may be directed towards a crack or open- 

 ing in the ice, and then flow along beneath the glacier to its mar- 

 gin. This condition could not exist if the ice were very active ; it 

 obtains only in stagnant or semi-stagnant ice areas. Streams 

 gathering on the surface and flowing into a depression to the bot- 

 tom of the glacier carry with them much debris. The}- also con- 

 tinue to transport material as they flow beneath the glacier. 

 In reference to carrying a load, subglacial streams behave just as 

 do surface rivers ; they aggrade and erode. As they aggrade their 

 beds, the stream itself is lifted against the arch above and tends 

 to melt the ice further. This process continuing, the channel 

 becomes more aggraded, and the size of the arch gradually grows. 

 At the mouth of the channel, that is, at the glacier margin, finer 

 materials, which the stream was able to carry through, are depos- 

 ited. Sub-glacial streams, in one respect, differ from surface 

 rivers; they may flow up hill. This is possible through hydro- 

 static pressure. Water accumulating on and beneath the ice may 

 form a reservoir of sufficient volume to give pressure that will 

 lift a stream over a considerable altitude. 



The courses of these streams are indicated to-day by the long 

 sinuous ridges of washed drift. The outline of these ridges shows 

 the shape of the ice arch that once was above them. Their verti- 

 cal range defines the irregular grade of the sub-glacial stream. 

 Ridges of this origin are called ''eskers." They usually have a 

 serpentine course; and because they consist almost entirely of 



