20 SIXTEENTH REPORT. 



mentioned, is illustrated; the ponds, instead of being formed from the entire 

 lake by sedimentation, are cut off from the parent body by sand and gravel 

 barriers, built up by the action of the waves and currents and often also of 

 the ice. There are two types of these pools: those separated mainly by 

 barrier beaches, and those bounded by true bars (10), and it may not ):)e 

 out of place here to include a slight digression into the dynamics of these 

 two classes. 



/. Barrier Beach Types. Barrier beaches are embankments of sand, etc., 

 thrown up at right angles to the direction of the prevailing strongest winds 

 and usually more or less parallel to the shore. They are built up by the 

 direct action of the waves, combined with the shifting action of the inshore 

 currents; when they connect with the shore at the ends, they enclose small 

 beach pools. On account of this method of formation, they can be raised, 

 by the action of the water alone, only as high as the limit of action of the 

 largest waves, and from depths no greater than this same limit of action; 

 their height and depth are thus proportionate to the size of the waves, and 

 consequently to the size of the lake in which they are formed. Secondarily 

 wind action may raise the barrier beaches even higher, even building large 

 sand dunes in front of the pools, altho this usually only happens where the 

 surface level of the lake is falling, because only in such cases is enough dry 

 sand exposed. The ice also often helps in building these barrier beaches, 

 and the freezing of the water in the sand allows greater movement of the 

 sand by the wind. 



It has sometimes been stated that barrier beaches could only be formed 

 along retreating shore-lines, that is, in lakes, the surface level of which was 

 falling. The maps (maps 3, 4, 5 and 6) show an example of the formation 

 of a beach pool while the surface of the lake was actually rising. It was 

 very evident, to an observer, that this barrier beach was actually pounded 

 up above the surface of the water by the direct action of the waves; in fact, 

 during a heavy storm the barrier beach would actually increase in height 

 while it was being watched. There could be no doubt that the beach could 

 be easily built up while the water remained at the same level. In addition, 

 during the next summer a continued study of the same region was made, 

 and it was found that the formation of the barrier beach was effectually 

 prevented when the surface of the lake fell rapidly, because the line where 

 the waves broke, and where the action was most severe, did not remain long 

 enough in the same place to completely effect its construction. Time and time 

 again during the summer, barrier beaches were started and then abandoned 

 for a line farther out, after which the .outwash, that takes place in shallow 

 water between waves, scoured away the partially developed ridge. 



A similar abandonment might occur at the time of a rise in surface level, 

 as is shown by the series of maps. Map 4 shows the beginning of a barrier 

 beach, which was later abandoned, and finally destroyed when the rising 

 water caused the line of greatest wave action to move farther inshore. In 

 conclusion, it might be stated that the best conditions for the formation 

 of barrier beach pools appeared to be a fairly constant water level, in very 

 shallow water which deepened very gradually out from the shore, in a lake 

 where large waves were produced during storms. 



Along Saginaw Bay (5), this method, combined with wind action, separated 

 long-lived, sand dune pools, which attained a length of nine hundred meters, 

 a width of from fifty to sixty meters, and a depth of not much over a meter; 

 Long Lake on Sand Point was such an example. At Douglas Lake, however, 

 as may partially be seen from the maps, they were usually not more than 



