288 Mr Maclaren's Remarks on Natural Terraces 



hurgh New Philosophical Journal, No. 54, for Oct. 1839) led 

 him to study the formation of sea-beaches in valleys with much 

 care. He has favoured the author with some remarks in ex- 

 planation of the difficulty arising from the inclined position of 

 the terraces in the valley of the Tay." 



*' I can positively assert," he says, " from barometrical mea- 

 surements, and from finding sea-shells on the surface, and em- 

 bedded, that the sea in retiring from a rising valley does leave 

 sloping terraces on the sides, like those described by you along 

 the Tay. Such sloping terraces occur in Glen Spean, beneath 

 the true parallel roads, and are laid down in Sir T. Lauder 

 Dick's excellent paper. Nor do I think their explanation diffi- 

 cult, on the principles adopted by you, although, at first, I re- 

 member thinking it not obvious. The case, I believe, is this, 

 — ^that in places where detritus is freely brought down, as 

 where a river or torrent enters the head of a creek of the sea, 

 the effect of an equable rise of the land, owing to the check 

 given to the transport of the detritus at an infinite number of 

 successive levels, will be to produce a smooth slope, the incli- 

 nation of which will be regulated by the amount of transported 

 matter, and the form of the underlying rock. If a central slip, 

 and here and there the sides, were removed, your terraces 

 would be left. The ordinary effect of a period of rest in the 

 elevatory forces, would simply be to render the bottom of the 

 valley flat during such period. There is, however, an early 

 check to any great extension down the valley of these flat 

 places, so as to become prominently visible ; for, as soon as 

 the stream has to flow over a small breadth, where flat, it must 

 drop its detritus, fill up its bed, change its course, and so on, 

 until the entire flat surface or terrace is converted into a talus- 

 like slope, regulated by the force of the water in transporting 

 matter from the valley above. As far as my experience goes, 

 it is only when valleys are broad, generally near their mouths, 

 that the sea eats out during periods of rest, the detritus accu- 

 mulated during a rise, and thus forms cliffs. High up valleys, I 

 believe, it is the river which has removed the central strip, and 

 has left the fringes of sloping terraces. Such fringes extend up 

 nearly every valley in the Cordillera of Chile, to the height of 



