DISTINCT GLACIAL PERIODS 



39 



the curious thing is that three of these have all occurred in 

 the quite late geological period which we call "Pleistocene," 

 since (as an instance which will have some familiarity for 

 English readers) the accumulation of the shelly marine 

 deposits in the southern area of the North Sea which we 

 call the Red Crag and Coralline Crag of Suffolk, and the 

 Yellow and Black Crag of Antwerp. One glacial period 

 immediately preceded the Red Crag. These deposits 

 (called " Pliocene ") are not very old or deep in a diagram 

 representing the thickness of the various strata of the 

 earth's crust. They have only some 200 ft. above them, 

 whilst below them there are (before we come to the chalk, 

 a great landmark in the geology of this part of the world) 

 2400 ft. of the sands and clays which are called Miocene 

 and Eocene ! Passing on downwards from the uppermost 

 "chalk" to the Silurian strata (without going any further 

 down), there are 29,000 ft. of deposit ! So we see that 

 the three or more glacial periods and interludes of the 

 Pleistocene period corresponds to a very brief chapter of 

 geological history, and that the latest. In fact, we know 

 that man chipped his flint implements and dwelt in caves 

 in Europe before the greatest of these glacial periods. 

 When we examine the deposits of the periods preceding 

 the latest Pliocene, and the fossilised remains of plants and 

 animals contained in them, it is the fact that we get no 

 indication of other and earlier " glacial periods " in this 

 part of the world, until we get into a very remote period 

 before that of the chalk and the oolites. The " breccias," 

 or deposits, of large angular rock fragments of the new red 

 sandstone or " Triassic " period bear indications, in the 

 form of scratches and polishing of the stones, of the action 

 of glacier ice. But the negative evidence is not conclu- 

 sive, and it may well be that glacial periods earlier than 

 those of the latest epoch (the Pleistocene) have come and 

 gone, but left no evidence of their occurrence in the much 



