332 bulletin: museum of comparative zoology. 



northern side there are four groups of defended terrace cusps, forming 

 what may be culled the Pochassic (just to the left of Figure 39), Perry's, 

 K, Brown's, F, and Prospect spurs, A, while curved re-entrants have been 

 excavated between the spurs where ledges are rare or wanting. The 

 re-entrants show that the river has everywhere attempted to widen its 

 valley, while the terraces on the defended spurs show that the widening 

 has been locally prevented by the outcropping ledges. Wherever free 

 cusps occur they exhibit the patterns deduced as of common occurrence 

 on page 308. Xone of the combinations there deduced as rare are 

 found. The cusps are usually more closely trimmed on the up-valley 

 than on the down-valley side. It would be difficult to imagine a more 

 complete confirmation of Miller's theory than is here presented. 



Special mention may be made of a few features. Just east of Po- 

 chassic Street a series of at least nine terraces, H to M, may be counted. 

 They range in height from eight to fifteen feet, and thus suggest a 

 rough measure for the amount of valley deepening during a swing of the 

 river southward across the valley and back again. This maximum num- 

 ber is evidently dependent on the numerous ledges here discovered at all 

 levels from highest to lowest. Although no other part of the valley 

 shows so many terraces, it must be concluded that flood plains, con- 

 tinuous with the remnants here preserved, were made far up and down 

 the valley ; and hence that the river was essentially at grade during the 

 whole process of valley degradation. Two terraces at the top of this 

 flight, in the re-entrant east of Pochassic Street, exhibit minor re-en- 

 trants of small radius and large arc near H and H', comparable to the 

 curves of the present river, thus indicating that no significant change of 

 volume has occurred since the work of terracing began. A broad 

 terrace plain stretches back of Perry's spur, K, and four low terraces 

 rise above it to higher levels, showing that four broad northward swings 

 were here executed. The fifth terrace (counting from the top of the 

 series) runs forward to Perry's spur, because the highest ledge of that 

 spur was discovered when the river was making its fifth northward 

 swing. It is worth noting that several defending ledges in this spur 

 would be unseen but for road and railroad cuts. The fourth terrace 

 swings forward in a long sweeping curve to the apex of Brown's spur, 

 because the summit ledge was there found by the fourth northward 

 swing. Only two distinct terraces occur on the high plain back of 

 Prospect spur, because the ledges in that spur rise still higher than in 

 Brown's spur. In a word the river has always shown a capacity for 

 broad swinging until it became hampered in its movements by coming 



