294 P/'oceciUnr/s of the Eoijrd Plii/siral Socicfi/. 



of upheaval) ; merging down (2d) into the stage of lateral 

 terraces, as the stream begins to find lateral play (= the 

 first period of rest) ; and (3d) the stage of a new delta- 

 formation witliin the arms of the old, as the stream loses slope 

 and grows enfeebled (== later period of rest). And then 

 subsidence may recommence the cycle. The two later stages 

 are necessarily accompanied by much destruction of previous 

 terraces, and the complication must often be great beyond all 

 possibility of disentanglement, more especially as the full 

 cycle may be interrupted at any stage by fresh upheaval of 

 the coast. The relative proportions of deepening and of 

 planation that ensue must in all cases obviously depend on 

 the rapidity of upheaval, the angles of the new gradients, the 

 nature of the materials worked in, and the term of rest. 



Not the least important phase of delta-terracing through 

 repeated upheavals of the coast-line, is its tendency to extend 

 itself inland. Eiver valleys cut in soft materials may be 

 fringed for some distance from their mouth by what are really 

 delta terraces produced. " When the coast rises," says Pro- 

 fessor Hull,* " the rivers are converted into rapids and water- 

 falls, which commence to cut back their channels inland, 

 until a balance between the eroding power and the inclination 

 of the river-bed is established. Meanwhile, the channel being 

 lowered, the former alluvial flats are laid dry, and new ones at 

 a lower level are constructed." There is much reason to think, 

 however, that only in soft and somewhat uniform materials can 

 this inland extension of delta-terracing take place. A bar 

 of hard rock interposes a check, and acts as a regulator to 

 prevent spasmodic deepening in the parts of the river lying 

 above it. The facts of the case may be illustrated by a 

 circumstance related in Sir Thos. Dick Lauder's graphic 

 work on the Morayshire floods. In 1838 it was considered 

 necessary to divert the course of the Dorback — a tributary 

 of the Findhorn — across the neck of a deep loop in its course, 

 and an artificial trench was accordingly dug, saving the stream 

 a circuitous course of 730 yards. When the connection had 

 been established the water poured over a clay bank in a 

 waterfall 15 feet high. In four months thereafter the stream 



* Phys. Geol. and Geog. of Ireland, 1878, p. 114. 



