256 ROLLIN T. CHAMBERLIN 



Falls. Other terraces occur at 880 feet in Taylor's Falls, and 875 

 feet at its highest point in St. Croix Falls. 



The significant feature of these terraces is that none of them is 

 continued below the Dalles. The highest, and that at 880 feet, end 

 against the Keweenawan barrier near the Taylor's Falls schoolhouse; 

 the 750-foot flat, upon which most of the town is built, terminates 

 with the trap-rock in the interstate park; the 875 and 810-foot flats 

 disappear less abruptly on the wooded river banks not far from the 

 diabase ledges; the lowest bench is stopped at the rocks close to the 

 toll bridge. The hard trap-rock at the Dalles acted as a barrier, 

 while the stream above formed a flood-plain. As soon as the falls 

 which developed from the more rapid cutting below the barrier, cut 

 a stope through the barrier and reached the drift above, a trench was 

 quickly cut in the alluvial plain, producing a terrace. At a later 

 time, when the stream-level to the south had lowered sufficiently, 

 falls again developed and produced another terrace ; and so on. The 

 pronounced 780-foot terrace, instead of extending upstream from the 

 Dalles, starts with the narrows at the present falls, nearly a mile 

 farther north, where another, but much lower, barrier checked the 

 stream's progress. Falls were therefore here at the time of the forma- 

 tion of that terrace, and with the same reasoning it is safe to predict 

 that the next terrace to appear will be just above the present falls. 



The highest of the terraces, at 920 feet, represents approximately 

 the original level of the drift at this point, showing that soon after 

 drainage adopted this course, the igneous rock was encountered, and 

 the stoping process begun ; the younger benches, lower upon the slopes. 

 of the trench, mark the stages or repetitions of this method of rock 

 erosion wich alternate filling and cutting upstream. These post- 

 glacial terraces, intimately associated with, and dependent upon,, 

 the receding falls, indicate that the cutting of the deep, but short,, 

 gorge in the Keweenawan has all been accomplished by the St. Croix 

 since the retreat of the last ice-sheet. 



