QUATERNAEY DEPOSITS OF BRITISH ISLES BROOKS. 279 



oldest bowlder clay of Europe. On limestone slabs the fossils fre- 

 quently stand up in marked relief owing to the same cause. 



The colors of fluviatile and littoral sands and loams which are 

 frequentl}' exposed to the air during their formation, show a similar 

 variation from red in the tropical to yellow or brown in temperate 

 regions, which is attributed by Barrell (5) to the degree of oxidation 

 undergone. This variation of color extends into the polar regions, 

 where the prevailing tint is gray, owing to the scarcity of organic 

 iron coloring matter as well as the slight degree of oxidation. The 

 detrital deposits such as "head" and the granitic loam of Ballybe- 

 tagh in Ireland show also a prevailing gray tint. Accordingly, a 

 gi^ay deposit without fossils is of itself strong evidence for a severe 

 climate during its formation; when fossils do occur in such a gray 

 deposit, as in the gray sands of Ulster and the gray silts of the Isle 

 of Man, they are generally of arctic types. 



The relations of the coarseness of fluviatile deposits and the grade 

 of rivers have also been worked out by J. Barrell (5). An arid 

 climate tends to increase the ratio of coarse to fine material and the 

 freshness of the fine even where the land relief leads to vigorous 

 erosion. A decrease in the temperature acts very strongly in the 

 same direction as aridity, by weakening the power of vegetation to 

 produce decay but prevent erosion, and by increasing the amount 

 of frost action. Increased snowfall, however, works in the opposite 

 direction by protecting the surface from denudation and producing 

 transportation. For these reasons waxing glacial conditions are 

 normally associated with terrace formation in the neighboring river 

 valleys; waning glacial conditions with erosion. 



On the seaward ends of great rivers the question is further com- 

 plicated by oscillations of the relative level of land and sea. The 

 terraces here are sometimes attributed to one cause, sometimes to 

 the other ; probably in most cases both acted together, for the terrace- 

 building effect of waxing glaciation seems usually to have been pro- 

 longed into the waning period of glaciation by an isostatic depres- 

 sion due to the weight of the ice, as in the Cyprhia clays and Holder- 

 ness marine beds terminating the first glacial period, the 100-foot 

 beach terminating the second, and the Yoldia clays terminating the 

 third. The possibility of terrace-forming in a warm climate, how- 

 ever, makes further criteria necessary, such as large blocks, northern 

 organisms, or passage into a moraine. 



Marine terraces on rocky coasts seem only to be formed when the 

 relative level of the land and sea remains unchanged for a consider- 

 able time. When the levels are varying rapidly, noticeable raised 

 beaches are not necessarily developed. This suggests a reason for a 

 rather puzzling fact which will be noticed in the descriptions of 



