Correspondence — Colonel George Greenwood. 431 



■waterfall. But I believe that it was called Hjaa Foss. I could not 

 hear of Mr. Hall's Mork Foss, so I presume that I have been to the 

 wrong Aardal. Balf the bridge below Hjaa Foss had been swept 

 away, which brought me to a full stop. So as I sat and gazed on 

 the fall, I transferred my deck-load of luncheon to the hold, and 

 then retraced a walk of loveliness, such as of itself alone would have 

 repaid me for my journey from here by Hull to Stavanger. 



In my letter (Geol. Mag., April) I have agreed with all Professor 

 Kjerulf's facts, and with all his theories, except that I think that the 

 level of the terraces depended, not on the level of permanent " water- 

 surfaces," but on the level at which the river which carried the 

 materials overflowed on to the land. I do not, however, think with 

 the Professor, that we need suppose the rising of the land which 

 placed the marine terraces above the overflow of the river to have 

 been sudden and " with several shocks." As the land rose gradually, 

 the river gi-adually deepened its channel, and not only ceased to 

 overflow and deposit on its Delta, but in floods cut away the banks 

 which it had formerly built, which it drove to the hill-side in the 

 form of two parallel terraces. The existing slopes of the sides and 

 ends of these terraces, on which the Professor founds his theory of 

 " sudden shocks," are not the slopes at which they rose from the sea 

 thousands or millions of years ago. These slopes have been receding 

 from atmospheric erosion during all this period, and they are receding 

 now. But they retain universally the angle at which their incoherent 

 materials wOl rest — an angle not very unlike that taken by the 

 sands of an hour-glass. For these materials, like those of the Scot- 

 tish Karnes, the Irish Eskers, and (query) of every terrace on earth, 

 are simply sand and pebbles ; and where they are accidentally bared 

 of herbage, you sink ankle-deep in going up or down the slopes. 



The Delta which the river is forming now at Aardal is not at the 

 level of the sea, or even of the unflooded river, but, like every 

 other Delta in the world, above both. The great universal mistake 

 is to suppose that Deltas formed by rivers in the sea, or in lakes, 

 cease to rise when they reach the water-level. Can any one point 

 to a Delta at the level of the sea or of a lake ? Deltas universally 

 rise by overflow of the rivers above the water-level, and they con- 

 tinue to rise as long as the land and water maintain the same relative 

 level ; though all Deltas slope downward to the end at the water's 

 edge. All alluvial fiats formed on land by the annual overflow of 

 rivers are just as level as the Deltas which are formed above the 

 sea-level or lake-level. 



On the new Delta at Aardal there are a number of terraces at 

 different levels, and the boundary of any one which is not over- 

 flowed by any particular flood is swejDt by the escaping flood-water. 

 Six terraces may be counted on the left bank of the river at the 

 bridges. These may probably, by unusually high floods, be cut back 

 against the hill-side in the form of one high terrace, and then sup- 

 posed to have been thrust up at one " shock." The correspondence 

 of terraces in number, on opposite sides of the river, will often vary 

 from this destruction of terraces by unusual floods. 



