22 A NEW FLORA OF 



sequence of these superficial deposits is not easily determinable, 

 but we may recognize two groups of difi'erent 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 ti'avelled 

 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 di'essed surfaces have been observed 

 on the limestone at Middleton, near Belford, on the basalt of the 

 Fame 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 de- 

 posit. Near to Dui'ham, 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 Islcmdica, 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 Eng- 

 land 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 



