244 



HARD WICKERS SCIENCE-GOSSIP. 



with bluish-grey, not arranged in regular layers, but 

 forming a massive deposit full of chalk debris, together 

 with some larger stones and boulders. 



This kind of material is known to geologists by 

 the name of "boulder clay," and this particular 

 deposit is known as the " Hessle boulder clay," from 

 the name of the place where it was first studied and 

 described. Fifty years ago all such deposits exhibit- 

 ing signs of tumultuous arrangement used to be 

 attributed to the agency of Noah's flood, but 

 subsequent inquiries showed that this was a mistake, 

 and that movi7ig ice was the real agent concerned in 



along over the rocky bottom with resistless power ; 

 they have seen also that the rocks over which the ice 

 passes, are likewise deeply grooved and scratched by 

 the same means. When therefore we find a deposit 

 full of such ice-scratched stones, we know that ice 

 must once have moved over the ground which it 

 occupies, and it is now an ascertained fact in geology 

 that there was a time when the British Isles were 

 surrounded and smothered with ice ; this time has 

 received the appropriate name of the Glacial Period, 

 and it is to the later part of this period that the Hessle 

 boulder clay belongs. 



Fig. 179. — Map of the Lincolnshire Marsh Land. Scale about five miles to one inch. 1. Marsh and Fen ; 

 2. Hessle clay ; 3. Sandbanks ; 4. Gravel and sand ; 5. Chalky clay ; 6. Cretaceous rocks. 



the formation of such clays. But how, it may 

 naturally be asked, can geologists be sure of this? 

 The answer is simple, because the stones and boulders 

 in the clay exhibit marks which can only be made by 

 the action of moving ice. These stones are to the 

 geologist what coins are to the antiquary — they bear 

 inscriptions which are records of the times to which 

 they belong. Men who have seen ice at work on 

 Arctic shores, or who have been into ice-caves under 

 the glaciers in Switzerland, have seen how such marks 

 are made, — how the blocks anil stones, frozen into 

 the ice, are scratched and ground as they are forced 



A mere inspection of the clay itself, however, will 

 not tell us all that we want to know. There are 

 several different forms of moving ice on the surface 

 of the globe. The glacier which sweeps down a 

 mountain valley ; the iceberg, or ice-floe, which 

 grounds in shallow wMter ; the ice-foot, or coast-ice, 

 which surrounds all shores in the northern regions ; 

 all these forms of ice are supposed to be capable of 

 producing boulder clay, and further inquiry is 

 necessary before we can discover which has been the 

 agent in the formation of this Hessle clay. 



Now there are several remarkable points connected 



