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4. Final Denudation of the Drift- Deposits of the West 
Riding. — I have hinted that the shape of the surface of drift 
deposits may have been partly left by deposition ; but the 
frequent cropping out of the laminae on the side of a sand- 
hill, as if they had all been cut off, the capping of sand hills 
with boulder-clay, the existence of two or more drift-forma- 
tions in one hill, irrespectively of the contour or shape of the 
hill, and other facts, would seem to show that the drifts 
collectively must have been subjected to a sweeping denuda- 
tion, which, according to the form of the valleys, and the 
direction of the currents, must have left flat terraces or 
expanses of drift in certain positions, and gently undulating 
knolls or abrupt eminences in others. The only cause with 
the mode of operation of which we are acquainted which 
could have left the numerous undulations in the drift-deposits 
of the West Eiding is the deflected and eddying action of 
oceanic currents. The boulder-clay and boulder- gravel knolls 
of the plain of Craven display an almost perfectly curvilinear 
outline, which here and there graduates into hollows with 
no outlet for the surface-drainage. The knolls diversify a 
plain which could never have been subjected to the action of 
a valley- glacier, supposing a land glacial period to have 
succeeded to the deposition of the latest drifts of the West 
Riding. Atmospheric action could never possibly have given 
rise to the geometrical exactness of the outlines presented by 
these drift-hills. Atmospheric action is now making un- 
sightly ruts down their smooth and continuous slopes. But 
it is difficult to resist the belief that the current-action which 
accomplished the last great denudation of the drift- deposits 
was much more powerful and sweeping than the ordinary 
action of tidal currents in existing estuaries and bays. 
Without, however, asserting that manifestations of marine 
agency capable of accounting for the final great denuda- 
tion of drift- deposits may not yet be discovered in certain 
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