28 trask. PRINCIPLES OF SEDIMENTATION [Ch. 1 



material accumulate. In the wooded areas the material consists prin- 

 cipally of the partially decomposed fragments of wood embedded in a 

 matrix of more or less completely decomposed woody material. This 

 matrix has some of the characteristics of a colloidal gel of organic 

 material. With time, such deposits give rise to ordinary coal. In the 

 open areas pollen and spore material may collect. Such deposits ulti- 

 mately form cannel coal. In places, particularly in open water, algae 

 may develop luxuriantly and give rise to beds of boghead coal or oil 

 shale. The essential feature in the formation of any particular type 

 of coaly material is the relative abundance of woody, spore, and algal 

 material preserved in the sediments. Thus distribution of wooded 

 and open areas, types of plants present, length of time stagnant con- 

 ditions prevail, rate of influx of terrigenous material, and subsequent 

 geologic history are essential factors in the formation of coal. 



Lakes 



Lake sediments are influenced materially by the nature of the 

 water in the lake. Geologists interested in lacustrine deposits should 

 be familiar with the fundamental principles of limnology, as there 

 are many different types of lakes (Welch, 1935; Bradley, 1948). Criti- 

 cal factors are average temperature of water, extremes of tempera- 

 ture, quantity of rainfall, seasonal and cyclic distribution of rainfall, 

 activity of microorganisms, concentration of dissolved materials, size 

 and shape of lake, depth of water, and supply of detritus. The sea- 

 sonal variations in temperature of the water profoundly influence the 

 type of deposits. Fresh water attains its maximum density at 39° F. 

 As the surface water approaches 39° F. it becomes heavy and sinks 

 to the bottom of the lake, forcing the bottom water upward. In the 

 more northern latitudes this phenomenon happens twice a year; once 

 in the spring as the surface water warms up to 39° F., and again in 

 the fall when it falls to 39° F. As the deeper layers of water com- 

 monly are relatively rich in organic matter and mineral nutrients, 

 plant life tends to increase when this deep water rises to the surface 

 in the spring. However, if during the summer months the activity of 

 microorganisms has consumed all the dissolved oxygen in the lower 

 water, hydrogen sulphide is formed, which, when it rises to the surface 

 during the spring and fall turnovers, may kill a considerable part of 

 the life in the lake. 



In more tropic areas, temperature of the surface water never falls 

 below 39°, with the result that the deep water does not rise to the sur- 

 face. Thus, a permanent state of stagnation may be produced in the 

 deep water, in which hydrogen sulphide can be continually present and 



