Page:Transactions of the Natural History Society of Northumberland, Durham, and Newcastle-upon-Tyne (1867).djvu/38

22 sequence of these superficial deposits is not easily determinable, but we may recognize two groups of different ages.

1. The Plistocene, the oldest of these deposits, includes the boulder clay, which is usually tough, red or blue, and sometimes above 80 feet in thickness, but either unstratified, or with an irregular and imperfect kind of stratification. Through this are irregularly distributed fragments of rock, both small and large, many of them weighing several tons; and some, which are angular or but slightly rounded, are polished and striated. Generally, these blocks have been derived from rocks in situ in or near to our district; but a few, such as granite, have travelled a considerable distance. The surface of the rocks on which this clay rests has also been found polished like marble, striated, and grooved, the striae and grooves having the same general direction, and evidencing the movement of a powerful abrading agent in a southerly direction. These dressed surfaces have been observed on the limestone at Middleton, near Belford, on the basalt of the Farne Islands, on limestones at Swinhoe, Dunstanburgh, Little Mill, Hawkhill, Belsay, and on sandstones at Berling Car, south of Alnmouth, and on the banks of the Irthing. Mr. Howse has also noticed such dressings on the Magnesian Limestone near South Shields. Along with this clay, and apparently belonging to the same era, are gravel and sand beds, and fine laminated clays, which are sometimes interstratified with the greater clay deposit. Near to Durham, boulder clay, containing large polished and striated blocks, overlies stratified sands, which are about 30 feet in thickness. Excepting at Tynemouth, where Mr. Howse found small pieces of Cyprina Islandica, a marine shell still living on our coast, I know of no fossils which have been discovered within our district belonging to the boulder clay era.

So far as is at present known, moving ice is the only natural agent which produces such dressed surfaces; and therefore it may be inferred, that during the boulder clay era, the North of England had an arctic climate. Not only do the glaciers on the Alps, in Iceland, and other mountain regions, polish and striate surfaces over which they move, but even the ice-covering of an