lo SCIENTIFIC SURVEY OF YORK AND DISTRICT 



If our visitor now asks why the hills rising above the Triassic plain 

 should be of Jurassic rocks to the north and of Cretaceous rocks to the 

 east, he has hit upon one of the most interesting phenomena in Yorkshire 

 geology. Near the town of Market Weigh ton the Upper Cretaceous 

 strata rest on Lower Lias. As this neighbourhood is approached from 

 north or south the Jurassic beds of North Yorkshire and of Lincolnshire 

 successively thin out and disappear. Moreover, several of them, which 

 show evidence of having been deposited, when well developed, in com- 

 paratively deep water, change their character ; instead of deep sea mollusca 

 they contain beds of oysters, clearly indicating the proximity of a shore-line 

 or, at least, of shallow water. The known area of non-deposition centres 

 round Market Weighton, and the disturbance is usually spoken of as 

 that of the Market Weighton Axis, but the direction taken by the axis is 

 a matter for speculation. Opportunity to study the variations of several 

 members of the Jurassic sequence and their successive overlaps will be 

 afforded on the excursion to Wharram, etc. 



Such, in brief outline, is the ' solid ' geology of the York neighbourhood. 

 In glacial times the Vale of York was invaded by a glacier which came over 

 Stainmore and descended Teesdale. The north-west corner of the 

 Cleveland Hills divided this stream into two branches ; one descended the 

 Vales of Mowbray and York, the other continued towards the sea and then 

 turned south along the coast, being driven inland by ice from Scandinavia, 

 which filled the North Sea. The Vale of York glacier brought with it 

 boulders of such rocks as Shap Granite, Brockram from the Vale of Eden, 

 and carboniferous rocks from the Pennines ; a fine example of the first- 

 named may be seen at the southern end of the up platform of the excursion 

 station. It was joined in its southward course by tributary glaciers from 

 the dales of West Yorkshire, Swaledale, Wensleydale, Nidderdale, Wharf e- 

 dale and Airedale, but no glaciers originated in the hills to the east. There 

 a number of glacial lakes was formed, as the natural drainage was 

 obstructed by the ice in the Vale of York and along the coast. These 

 lakes and the many channels by which they discharged have been described 

 by Prof. Kendall in a classic paper. The largest lake occupied the 

 Vale of Pickering and overflowed along what is now the Derwent Gorge, 

 from Malton to Kirkham Abbey. These will be seen on the excursions, 

 as will a fine series of overflow channels near Kilburn. On the long 

 excursion to Robin Hood's Bay those members who prefer exploring the 

 moors to studying Lias zones and boulder clay cliffs on the coast will 

 find many glacial features to interest them. 



Detailed interpretation of the glacial phenomena around York will be 

 a subject of discussion during the meeting and many of them will be seen 

 on the excursions. We do not wish to anticipate what will be said then, 

 but mention must be made here of the two great crescentic ridges, composed 

 in part of gravel but more largely of boulder clay, which cross the Vale, as 

 indicated in the previous chapter. The outer, or more southerly, of these 

 merges to the west in the marginal deposits. The inner ridge, known as the 

 ' York Moraine,' after curving round through York, passes westward into 

 a complex of ridges and mounds. From York narrow ridges of gravel also 

 extend southwards and south-westwards towards the ' Escrick Moraine.' 



