84 Davis — Terraces of the Westfielil Biver, Mass, 



tude, successive terraces will remain, even without defense by 

 ledges. There can be no question that some of our terraces are 

 of this accidental kind. But the undefended lower terraces will 

 be undercut and destroyed if the river swings more strongly 

 north again. If the river again swings north until it strikes a 

 ledge, the uppermost terrace may alone be preserved, and that 

 only back of the defending ledge. It thus becomes evident 

 that in order to discover the number of times that a river has 

 swung across its valley, making laterally sloping flood plains at 

 lower and lower altitudes at every swing, we must not trust to 

 the chance preservation of flood plain remnants in terraces here 

 and there, but must seek a flight of terraces, systematically 

 grouped on a long sloping ledge, which may preserve a lateral 

 remnant of every flood plain that has been formed, as in figure 

 5. It is certainly a striking fact that the number of steps in a 

 flight of valley terraces always reaches a maximum in just such 

 situations. The preserxation of numerous terraces of moderate 

 height on long sloping ledges — however few such ledges there 

 may be in the valley and however few terraces occur elsewhere 

 on the valley sides — goes far towards excluding the theory of 

 successive uplifts and pauses as a cause of terracing. It goes 

 far also towards supporting the theory that the wandering river 

 has been swinging from side to side across its valley, always 

 degrading its channel but always acting as a graded river, 

 during the whole period of terracing, whatever may have been 

 the cause that determined the excavation of the valley drift. 

 If uplift were the cause, the uplift must have been slow and 

 relatively uniform. 



In illustration of this conclusion, we may return for a moment 

 to the broad basin east of Westfleld. No terraces at intermedi- 

 ate levels are found here to prove that the river did repeatedly 

 swing laterally while degrading its valley floor in this part of its 

 course ; yet there can be no reasonable doubt that the river really 

 did swing back and forth here, for the remnants of four flood 

 plains at intermediate altitudes are found in the terraces on the 

 west side of Prospect hill, only half a mile away. Farther up 

 the valley one may find a flight of nine defended terraces, 

 described below, whose subequal heights range from ten to 

 fifteen feet ; thus proving that even the four terraces on the 

 Westfield spur preserve a very incomplete record of the river's 

 activity. Evidently the maximum number of steps observed 

 in any terrace flight gives only the minimum number of swings 

 that the river may have made during the whole period of 

 degradation. 



8. Ledges outcrop on the icp-valley side of terrace spurs. — 

 In the terrace spur here called Prospect hill, as in many others, 

 it is noteworthy that the up-valley side of the defending ledges 



