DAVIS : RIVER TERRACES IX NEW ENGLAND. 341 



distance back in a low-level re-entrant enclosed by a scarp concave 

 southward or down-valley. A little farther on, an isolated hill, M, trav- 

 ersed by the main valley road and crowned by a mansion, is sepa- 

 rated from the main slopes of the eastern side of the valley by a 

 deep -trench, N, of large sweeping curvature to the northeast and for 

 the most part apparently cut in till. The trench has a rather strong 

 under-cut slope on the outer side of its curved course, and a gently 

 terraced slope on its inner side. There can be no doubt that it 

 marks a former path of the river around a lobate spur, and that the 

 river was diverted from the trench at a comparatively modern, 

 though unrecorded, date by wearing through the narrow neck of the 

 spur at P, a little up-stream from Walpole bridge. The second one, 

 E, of the two free cusps above mentioned on the west of the river 

 is the unconsumed remnant of the neck of this spur, west of . the 

 cut-off; the isolated hill, M, is the terminal part of the spur north- 

 east of the cut-off. The following explanation of the relation between 

 the two free cusps, C and E, may be suggested. 



At an early stage of the time during which the river was making 

 its great northeastward detour around the spur, EM, its course mav 

 be represented by the curve, a, a, a, a. The normal order of change 

 in these curves would develop a later course, b, b, b, b ; thus open- 

 ing out the two large westward re-entrants, B and D, and leaving the 

 free cusp, C, as yet unconsumed between them. Had this process been 

 normally continued, the free cusp, C, would have been in time worn 

 away by the down-valley sweeping of the first b meander ; but before 

 this was accomplished, the cut-off occurred at P. Now it may be shown 

 by a study of the detailed maps of the Mississippi River Commission that 

 when a cut-off occurs, a systematic series of changes is initiated, and that 

 these changes are extended up-stream as well as down-stream from the 

 cut-off. The essence of the changes is such that the straightening of the 

 course at the cut-off tends to straighten it elsewhere also, and that this 

 tendency is most active near the cut-off, and weakens with distance from it. 

 Following this principle, the course, b, b, b, b, will be changed toe, c, c, c, 

 and to d, d, d (the present channel). The river will thereby be with- 

 drawn from both of the westward re-entrants, around whose up-valley 

 curves it was probably flowing when the cut-off took plare. Thus the 

 free cusp, C, will be left unconsumed for a time at least. It may per- 

 haps be possible, by accumulating other examples of changes similar to 

 this one, to give some degree of verity to the rather hazardous explana- 

 tion here offered. 



