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892 STRATIGRAPHICAL GEOLOGY.  [Boox VI. 
horizontal strata of clay and sand, other similar strata have been — 
violently crumpled, while horizontal beds lie directly upon them. 
These contortions may have been produced by the horizontal pressure 
of some heavy body moving upon the originally flat beds, such as 
ice in the form of an ice-sheet or of large stranding masses driven 
aground in the fjords or shallow waters where the clays accumulated, 
or possibly in some cases sheets of ice, laden with stones and earth, 
sank and were covered up with sand and clay, which, on the subse- 
quent melting of the ice, would subside irregularly. Another indi- 
cation of the presence of floating ice is furnished by large boulders 
scattered over the country, and lying sometimes on the stratified 
sands and gravels, though no doubt many of the so-called erratics 
belong to the time of the chief glaciation. 
The sands and gravels which overlie the boulder-clay or older 
diluvium present some curious problems. Covering the lower ground 
in a sporadic manner, often tolerably thick on the plains, they rise 
up to heights of 1000 feet or more. In some places they cannot be 
satisfactorily separated from the sands and gravels associated with 
the boulder-clay, in others they seem to merge into the sandy 
deposits of the raised beaches, while in hilly tracts it is sometimes 
hard to distinguish between them and true moraine-stuff. Their 
most remarkable mode of occurrence is when they assume the form of 
mounds and ridges which run across valleys and plains, along hill- 
sides, and even over watersheds. Frequently these ridges coalesce 
so as to enclose basin-shaped hollows, which are often occupied by 
tarns. Many of the most marked ridges are not more than 50 or 60 
feet in diameter, sloping up to the crest, which may be 20 or 30 
feet above the plain. A single ridge may occasionally be traced 
in a slightly sinuous course for several miles. These ridges, known 
in Seotland as kames, in Ireland as eskers, and in Scandinavia 
as Osar, consist sometimes of coarse gravel or earthy detritus, but 
more usually of clean, well-stratified sand and gravel, the stratifica- 
tion towards the surface corresponding with the external slopes of_ 
the ground, in such a manner as to prove that the ridges are usually 
original forms of deposit, rather than the result of the irregular erosion 
of a general bed of sand and gravel. Some writers have compared 
these features to the submarine banks formed in the pathway of tidal 
currents near the shore. Others have supposed them rather to be of 
terrestrial origin, due to the melting of the great snow-fields and 
glaciers, and the consequent discharge of large quantities of water 
over the country. But no very satisfactory explanation of them has 
yet been given. 
Second Glaciation—Re-elevation—Raised Beaches.— 
When the land re-emerged from its depression, the temperature all 
over central and northern Europe was again severe. Vast sheets of 
ice still held sway over the mountains, and continued to descend into 
the lower tracts and to go out to sea. ‘To this period are ascribed 
certain terraces or “ parallel-roads” which run along the sides of 
