166 REV. THOMAS BROWN ON THE OLD RIVER TERRACES 



Here, again, the facts point to these river currents as the agent which built 

 up the terraces. In estimating the facts, however, care must be taken to 

 include some considerable length of the river course. A short reach, where 

 the water runs level and slow in the midst of rapids, may be the very place 

 where the deposits most accumulate. But take any considerable distance, and 

 it will be found that in proportion as the gradient is steep'"" these old terraces 

 rise in height, and as the gradient gets low the terraces diminish. 



If we could only suppose a time when the river had the power of rising in 

 flood to the requisite height, the whole phenomena would admit of the most 

 easy and natural explanation. The simple key to the whole would be the 

 principle, that just as the river deals now with its present channel and present 

 banks, so it dealt in the old time ivith those high lying ter races. \ 



Putting together then the whole of these facts, we can now see what this 

 threefold system of terraces means. It is simply a record of the different levels 

 at which the most powerful river floods stood at different periods of the past. 

 The highest of these lines of deposit is evidently the oldest, a gravelly and 

 sandy terrace which runs along our valleys at a height of from 35 to 60 feet. 

 Then we find a descending scale as the floods grew less and less powerful, 

 subsiding towards the present state of things. Bather more than half way 

 down the scale there is an intermediate terrace about 16 to 24 feet above the 

 river, and forming an outstanding feature of these deposits. It seems to 

 indicate that when the descending waters reached that stage a pause of con- 

 siderable duration took place, during which the action of the highest floods 

 went on at that level. Subsequently there was a descent from the middle 

 terrace to the present banks. Between each of the stages indeed, there are 

 intermediate lines of deposit occurring here and there in different localities, 

 and putting them all together it would be possible to construct a whole series 

 of graduations by which the highest terrace would be found to descend to the 

 second, and the second to the lowest. Still there can be no doubt that the 

 three terraces form the prominent feature of these deposits. 



The general result thus seems to be, that along the sides of these river 

 valleys, we read the history of various ages during which the floods gradually 

 ran at a lower and lower level, and in that record there are three great lines 

 which stand out from the rest as indicating each some considerable period 

 during which the waters remained stationary, till at last the intervals were all 

 passed over and the present state of things was reached. 



* This does not apply when one follows the stream up among the mountains, where for the most 

 part the terraces are absent. Is it that denudation has swept them wholly away? or is it that during 

 the epoch of these old floods there were glaciers still lingering in the upper portions of the river- valleys ? 



t More than twenty years ago, Mr Milne-Home described the terraces above Perth as haughs 

 or river flats, but he seems to connect their formation with the bursting of lakes. See " Trans. Boy 

 Soc. Ed.," vol. xvi. p. 418. I do not refer to other districts. 



