CHAP. xii. GEOLOGICAL PRINCIPLES. 123 



strata thrown up at a high angle, and which naturally 

 weathers into a jagged or ridged surface. 



(3) The smoothed rocks are often found to be covered 

 with numerous striae, deep grooves, or huge flutings, and 

 these are almost always in one direction, which is that of 

 the course of the glacier. They may often be traced in 

 the same direction for miles, and do not change in har- 

 mony with the lesser inequalities of the valley, as they 

 would certainly do had they been formed by Avater 

 action. These striae and smoothed rocks are often found 

 hundreds or even thousands of feet above the floor of the 

 valley, and in many cases a definite line can be traced, 

 above which the rocks are rugged and jagged, while be- 

 low it they are more or less rounded, smooth, or polished. 



(4) Erratic blocks are among the most widespread and 

 remarkable indications of glacial action, and they were 

 the first that attracted the attention of men of science. 

 The great plains of Denmark, Prussia, North Germany, 

 and Russia are strewn with large masses of granite and 

 hard metamorphic rocks, and these rest either on glacial 

 drift or on quite different rocks of Secondary or Ter- 

 tiary age. In parts of North Germany they are so abun- 

 dant as to hide the natural surface, and they are often 

 piled up in irregular heaps forming hills of granite 

 boulders covered with forests of pine, birch, and juniper. 

 Many of these blocks are more than a thousand tons' 

 weight, and almost all of them can be traced to the 

 mountains of Scandinavia as their source. Many of the 

 largest blocks have been carried furthest from the parent 

 rock a fact which is conclusive against their having 

 been brought to their present position by the action of 

 floods. 



