292 
YORKSHIRE DRIFTS. 
dwelling. There are accumulations of this drift in valleys, proving 
the latter were excavated previously to its deposition. The scratches 
on the included boulders are ascribed to the action of icebergs or of 
glaciers ; whilst the absence of these scratches in the higher beds 
indicates the deposit was accumulated under violent aqueous action 
in shallow water. He agrees with Gumming, Trimmer, and others 
that at the commencement of the period of the boulder clay forma- 
tion, the sea and land had the same relative level as at present ; that 
the climate was of an Arctic character, due to a great current 
originating probably in the North Polar area, together with a modifi- 
cation of the present Gulf Stream ; that this was followed by a 
gTadual submergence of the area of the British Isles to an extent in 
some places of 1,600 feet ; that subsequently a gradual emergence of 
the land took place to the same extent, of which he considers suf- 
ficient evidence is afforded by the elevated terraces or extended plat- 
form of rolled boulders and gravel, sometimes consisting of re-distri- 
buted stones from the boulder clay, in others, of adjacent rocks 
regularly stratified. During the uprising of the land the severe cold 
was modified ; erratics from distant localities were dropped b)^ the 
melting of icebergs, while the scratching and grooving action of 
glaciers in a great measure ceased, and a period ensued which marked 
the close of the glacial epoch in this country. The gravels extending 
along the sides of the rivers Aire and Galder were derived from sand- 
stones of the coal district localities in which they occur, intermixed 
with occasional fragments from the mountain limestone of Graven. 
This gravel does not extend much north or south of the valleys of 
the Aire and Galder. Leeds stands upon it, and it occurs on the 
higher grounds near Rothwell Haigh, at Oulton, and Ferrybridge. He 
considered that these masses of material were derived from the strata 
of the district by the waters which were driven eastward during the 
time of emergence, and down the lines of valleys which then formed 
the easiest retreat for the retiring waters. The gravels noticed by 
Mr. Thorp are at a much higher level, and must not be confounded 
with those previously described by Mr. Glay. which occupy only the 
bottom of the valley, and contain many boulders which have not been 
derived from the adjacent strata. 
