290 Proceedings of the Royal Physical Society. 



ill the Figure), which appears to have been substituted for 

 these two by the same process of growth exemplified in the 

 bank C, which, twenty years ago, was a sand-bank, but is 

 now covered by coarse grass and osiers. The terrace-front 

 bordering the haugh B on its landward side is equal in height 

 to the destroyed terraces combined, a vestige of one of which 

 may be that preserved on its flank at D.* 



Fan Terraces, or Lateral Delta Terraces. 



Eiver terraces in valleys are usually found to slope up 

 towards the mouths of tributary streams, and in these 

 positions, having some distinct characters of their own, they 

 have been termed by Hitchcock delta terraces. It would be 

 well, however, that the term delta terrace should be restricted 

 to terraces formed by intersected deltas at or near river 

 mouths. In this paper lateral delta terraces will be referred 

 to as fan terraces. 



All deltas are due to a building-up of alluvial debris 

 around the mouth of a valley or ravine. It needs scarcely to 

 be said that they are not necessarily or mainly submarine 

 or sub-aqueous. Shot d^hris spreads in the shedding, whether 

 under water or above it. The fan-shaped talus round the 

 mouth of a rocky notch ; the alluvial cones that are shed 

 around the mouths of tributaries on valley sides ; and great 

 river-deltas spread over miles of seaward-flattening valleys 

 like the Nile or Ganges — are all primarily due to the same 

 principle. A point has been reached at which the d6hris can 

 be carried no further ; it is accordingly laid down at some 

 point in front of the point of exit, or '' yatel' as the Americans 

 term it ; and if the " gate " open at right angles to the bank, 

 the materials extruded arrange themselves in a semicircular 

 fan or delta. The formation of these deltas has been described 



* All the evidence which the writer l^as obtained goes to show that the 

 above is the correct reading of the terraces represented in the Figure ; at the 

 same time it is used more as an illustration than taken as a fact. It will be 

 observed that the curve A bears somewhat the same relation to a larger curve 

 of the river that a shifting curve bears to a persistent one in the formation of 

 the amphitheatre terrace. This docs not seem, liovvever, to be an essential 

 feature of this case, seeing that it might i»roduce similar results without it. 



