292 PRINCIPLES OF STRATIGRAPHY 



in many of the geologic periods of the earth's history. Autorudytes 

 of glacial origin, when unaffected by other agents, are characterized 

 by the polished and striated surfaces of the boulders, larger pebbles 

 and cobbles. Flat blocks generally have only two sides striated, 

 while the margins may remain angular. The characteristic striation 

 is soon lost through subsequent wear by streams from the ice, the 

 material thus becoming hydroclastic (potamoclastic). The charac- 

 teristics of glacial sands, whether derived from the crushing of 

 igneous or of clastic rocks, lie chiefly in their angularity and fresh- 

 ness of grain, unless subsequent weathering has attacked these. If 

 the material is derived through crushing of igneous or other crystal- 

 line rocks, it will show a variety of mineral grains. The quartz 

 grains will be sharp-edged and pointed, with strongly vitreous and 

 conchoidal surfaces, while the cleavable minerals will show fresh 

 cleavage faces as well as sharp outlines. ( Sherzer-26:di'5.) 

 When pure clastic rocks such as sandstones are crushed by ice the 

 resulting material will be pure, with sharply angular grains, which 

 may or may not be derived from originally rounded grains. 



Glacial boulders and sands are frequently reworked by the 

 glacial streams and so become hydroautoclastic or aqueo-glacial 

 (Sherzer). The finer rock flour, etc., may be reworked by wind 

 and so become anemo-autoclastic or aolo-glacial (ex. loess). See 

 further. Chapters XII and XIII. 



3. The Atmoclastics. These comprise rocks broken in situ, 

 either by chemical or mechanical means, and recemented 

 without further rearrangement by wind or water. Most of 

 the rocks of this type are of complex composition, and there 

 is a characteristic angularity in the coarser material which 

 shows the absence of water. Stratification also is coarse or 

 absent altogether. Characteristic examples are found in talus 

 breccias, which when consolidated form typical atmorudytes ; 

 in the extensive subaerial accumulations of waste along 

 slopes of most mountains, and in many of the Tertiary and 

 earlier subaerial deposits, which were neither windlaid nor 

 deposited in water bodies. Remains of land plants and 

 animals are often characteristic of these rocks. The kaolinite 

 and laterite, i. c, decomposition products which mantle the 

 rock in unglaciated regions, when consolidated, also form 

 typical examples of atmoclastic rocks. 



The composition of atmoclastic rocks varies, of course, greatly, 

 according to the nature of the rock from which they are derived, 

 and the complication of the atmospheric processes involved. Under 



