672 
PROFESSOR PRESTWICH ON THE ORIGIN 
larger delta,” and is “ out of all proportion to the size of the stream.” He describes 
it as “some furlongs wide,” with “a front rising at its centre 90 feet above the bottom 
of the valley—a striking proof how long the lake must have existed. The fan-shaped 
margin shows that the water it dropped into had no strong set or current.”* 
This mound is certainly a remarkable object, and has attracted the attention of all 
observers since Macculloch’s first description of what he considered to be alluvial 
lake-deltas terraced by later river action. Its surface, however, is not an inclined 
plane, nor is there any sorting of its materials from its head at the Turret Pass to its 
extremity on the Poy. On the contrary, the mass of debris is less at the foot of 
the Turret Pass, where there is the following section (fig. 1) on the river bank, than 
lower down, and its component parts are no finer at the one place than at the other. 
Fig. 1.+ 
a. Gravel horizontally stratified. 10 feet? 
m. Light grey loam with angular debris and blocks in inclined rough bedding. 20 feet F 
From this point the surface of the mound at first falls slightly, and then gradually 
rises until it attains, at the further end near the Poy, at the distance of a mile, a 
height of 80 to 90 feet above the river-—the very reverse of the form that a delta should 
exhibit. Its mass, too, is out of all proportion to the size of the Turret, for it covers 
an area fully equal to a mile square, and averages probably a thickness of 50 feet. 
It consists, also, in larger proportion of an unstratified detritus, with a small capping, 
of very variable thickness, of stratified water-worn debris. 
Fig. 2 shows the upper water-worn portion of the mound at its extreme end. 
Fig. 2. 
T 
Section of top of mound facing the Roy, half a mile north of its junction with the Turret. 
1. Surface soil.6 to 8 inches. 
2. Greyish clayey sand with worn fragments and a few large angular pieces 
of the local rocks.6 feet. 
3. Fine angular and subangular debris of the same.1J „ 
4. Very finely laminated yellow clay.3 „ 
5. Very finely laminated grey clay.3 „ + 
t. Talus from the beds above covering the unstratified detritus m. 
The thickness of No. 5 shown here is uncei’tain: it appears not to exceed 4 or 5 feet. 
Quart. Journ. G-eol. Soc., vol. x., p. 242. 
f The sections from nature throughout the paper are on the same vertical scale of 40 feet to 1 inch 
unless mentioned to the contrary. 
