Corbet t : Glacial Gcoloi^y of Doncastcr. 49 



There is another small patch of Boulder Clay about three- 

 quarters of a mile north of the Balby deposit. No distant 

 erratics have been found in this. 



About four miles further up the Don valley, at Conisboroug-h, 

 is a third deposit of Boulder Clay. This is of very small extent 

 and caps the Lower Mag-nesian Limestone at the Ashfield Brick- 

 works. The contained boulders are nearly all of limestone from 

 the immediate locality, but there are also a few well-striated 

 Mountain Limestones with Crinoids. 



At Cusworth boulders up to a ton in weig-ht are turned up in 

 the fields by ploug-hing-. These consist of Grits, Gannisters, 

 Mountain Limestone, Whin Sill, Quartz Porphyry, and Basic 

 rocks that weather with a deep red crust. Of these the Whin 

 Sills and Limestones are rounded as if water-worn, while the 

 others are sub-ang-ular. Along- with these are quartzite pebbles 

 from the Bunter Sandstone. 



Eastward of Balby and spreading- in a wide fan-shape between 

 the river Don on the north and the outcrop of the Permian rocks 

 on the south is an undulating^ country, all of which below 

 25 feet O.D. is warp and peat, and all above 25 feet and reach- 

 ing- to a maximum altitude of about 80 feet being'- covered with 

 g-ravels. These g:ravels may be divided into three kinds : — ■ 

 I St, Coarse g-ravels, chiefly composed of Carboniferous rocks. 

 2nd, Finer g-ravels, but with many large stones in them, com- 

 posed of a mixture of Carboniferous rocks and pebbles from 

 the Trias. 3rd, Fine gravels, composed almost entirely of 

 pebbles from the Trias. The range of these three gravels may 

 be seen by reference to the map. 



The Carboniferous gravels fairly follow the right bank of the 

 Don from Balby to Barnby Dun. Wherever they are exposed 

 in section they are found to be strongly current-bedded with 

 the dip varying from 30° downwards invariably from N.W. to 

 S.E., i.e., towards Trent. This is the case even where they are 

 on the Don side of the watershed between Don and Trent. 

 The contained stones are largely Sandstones, Clay-ironstone, 

 Grit, and Gannister, but along with these are a considerable 

 number of Cherts. Many of these are deeply weathered, this 

 being particularly the case with those composed in a great 

 measure of casts of Crinoids. The more compact ones con- 

 tain Corals, Brochiopods, etc., and some are crowded with 

 spicules of Hyalonema parallclla. A very few Lake District 

 rocks have been found also. 



1903 February i. D 



