14 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



Glacial deposits thus are characterized by a heterogeneous mixture 

 of impervious and poorly sorted deposits interbedded with well-sorted, 

 highly porous, and permeable sands and gravels. The geologist is of 

 material service to the engineer in locating deposits of sand and gravel, 

 close to their place of use, thus saving transportation costs. Moreover, 

 the buried lenses of sand and gravel are good sources of underground 

 water. (See Chapter 6.) 



Lakes are common in glacial country. In the spring and summer, 

 melting water transports a large amount of debris to the lakes. The 

 coarse particles soon settle out and form sand or coarse silt deposits. 

 In the winter, the lakes freeze so that no sediment can enter. Deposi- 

 tion is therefore confined to material largely of clay size which has 

 stayed in suspension in the water during the preceding months. Thus 

 a series of alternating coarse and fine deposits are formed (Antevs, 

 1922). These annual pairs of layers are called varves. The sand lay- 

 ers, being permeable, facilitate consolidation under load. 



Gravity 



Gravity is also an agent of deposition. In cold climates or in arid 

 regions where differences in temperature between night and day may 

 be extreme, fragments of rock are dislodged owing to differential 

 thermal expansion and contraction of the rock constituents and of the 

 water in cracks and pore spaces. If slopes are sufficiently steep or 

 smooth, dislodged fragments roll down and come to rest at some lower 

 altitude, forming piles of debris which generally have a slope of 30° 

 'to 35°. Deposits of this type are called talus and may attain con- 

 siderable thickness if the blocks are not removed by other processes 

 of erosion. Talus deposits oddly are rare in many arid regions, perhaps 

 because the rocks disintegrate and occasional large floods wash them 

 away. 



In humid and temperate climates, alternate freezing and thawing 

 of water in the pore spaces of soil or alternating periods of wetting 

 and drying of the soil result in a slow creep of soil particles down 

 slopes (Capps, 1941). In this process of creep, the constituents of the 

 soil slowly change their position with respect to other constituents, 

 thus changing the strength and stability of the soil. 



Landslides are a manifestation of the effect of gravity. For de- 

 tails see Sharpe (1938) and Chapter 13 in this symposium. Soil and 

 rock resting upon a slope start to slide when the weight of the over- 

 burden becomes greater than the strength or cohesion of the earth 

 materials. Landslide deposits commonly are heterogeneous in nature 

 because of the jostling caused as the material slides. Thus the sedi- 



