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explanation may be found in tlie action of sea waves, either 
with or without the assistance of coast ice. (Plate TV.) 
The above-named blocks, as regards their dispersion, can- 
not be separated from the boulders which occur lower down 
in a deposit of clay. Beginning with the river Wharfe, we 
find that it has excavated its channel in boulder clay, washed 
away the smaller stones and left the larger. Here and there 
it has deposited shingle, mainly derived from the stones of 
the boulder clay. I say mainly, because in some parts of its 
channel, there are many limestone pebbles, which look as if 
they had been washed down by the river from the limestone 
rock in the upper parts of the vaUey. On the side of the 
Alexandra Road cutting, leading out of Ilkley towards the 
bridge, there is the appearance of clearly- washed river shingle 
abutting against a deposit of boulder clay. But the rounded 
stones, so far from being confined to the river shingle, are 
often quite as characteristic of the boulder day in the lower 
part of the valley. They are nearly all millstone grit ; lime- 
stone, though not absent, being very rare. They gradually 
become less round and more angular as we ascend the slope 
towards the base of the cliffs of Rombold's Moor, where 
some of the stones still retain their roundness though most of 
them are angular. The clay about Ilkley and towards the 
north-west forms a flat plateau slightly sloping towards the 
river. About a mile to the west of the town it comes up to 
the base of a fine cliff line with enormous fallen blocks, which 
runs along the base of the terraced escarpment of Eombold's 
Moor ; some of these blocks weigh not less than 100 tons, 
hundreds of acres being covered with them embedded in 
diluvial detritus, from 15 to 20 feet deep, containing great 
quantities of rounded pebbles, showing unmistakably that a 
tremendous quantity of dehrk must have been swept down 
the valley during the ancient sea-period, and singular enough 
the configuration of this portion of the valley along with the 
