Yol. 52.] MAMMALIAN REMAINS IN DERWENT RIVER-GRAVELS. 505 



action by the rivers. The highest are, consequently the oldest, 

 and the lowest the most recent. 



The Allenton gravel forms one of the lowest of the upper series 

 of terraces, and, since it was spread out by the river, the Derwent 

 has deepened its valley from 15 to 20 feet, and by excavating 

 horizontally as well as vertically, and leaving behind it a bed of 

 gravel and brick-earth, has formed a broad low-level alluvial plain. 



The question of the relationship of the river-gravels to the 

 surrounding deposits merits more detailed notice, for there are 

 -considerable masses of silty boulder-clay in their immediate 

 vicinity. 



The elevated land to the south and south-west of the Allenton 

 iterrace is capped by two varieties of boulder-clay and associated 

 gravel and sand. The oldest deposit, the Pennine Boulder Clay, is 

 a stiff, blue, silty ' clay containing numerous well-striated and 

 polished rock-fragments from the Pennine Chain. These boulders 

 may have, in part at least, come down the Derwent and Wye 

 valleys, and in part by way of Staffordshire, Cheshire, and Lan- 

 cashire from the Yorkshire Fells. Flints are absent except in the 

 4 trail.' A mass of this boulder-clay spreads as an irregular 

 patch over the low ridge separating the Allenton terrace from the 

 depression of Sinfin Moor, as shown in the map (fig. 2). As 

 already pointed out, it rests upon Keuper Marl. The same boulder- 

 clay is exposed in an open section at Sheldon Wharf, a little farther 

 south. For a description of this section, see Quart. Journ. Geol. Soc. 

 vol. xlii. (1886) p. 449. It here passes beneath another and newer 

 >boulder-clay, namely, the Great Chalky Boulder Clay. 



At Spondon, on the opposite side of the valley, the hill is covered 

 by another mass of Pennine Boulder Clay, which reaches a thick- 

 ness of more than 60 feet. 



The Great Chalky Boulder Clay covers the greater part of Chel- 

 laston Hill to the south of Allenton, where it attains a great thick- 

 ness. Unlike the Pennine Boulder Clay, the Chalky Boulder Clay 

 contains an abundance of rock-debris only to be found in situ to the 

 east of the Pennine Chain. The boulder-clay is capped by a thick 

 bed of clean, stratified, current-bedded sand at a height 256 feet 

 above Ordnance-datum, or 130 feet above the alluvial plr.in of the 

 Derwent. The sand is about 17 feet thick, and does not resemble 

 river-gravel. 



Boulder-clays of both kinds have been found rapping the high 

 grounds and river-escarpments of the Trent and its tributaries. At 

 some points they come down to levels within a few feet of the flood- 

 level of the Trent. They were deposited in the pre-Glacial river- 

 valleys which they filled to a depth of at least 100 feet ; and when 

 the rivers again commenced to flow, on the disappearance of the 

 ice, their first task was to clear away the Glacial deposits. The 

 high-level terraces were formed while this was taking place. 

 Indeed the pebbles and boulders which they contain are, in many 

 instances, only such as could have been derived from the boulder- 



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