38 UNIVERSITY OF COLORADO STUDIES 



stationary for a few years a high ridge is formed. Such ridges abound 

 in the glaciated valleys of the Rocky Mountains and Sierras, and in 

 many places where the fluctuations of the ice-tongue have been numer- 

 ous the ancient moraines are compHcated, intersecting each other at 

 various angles. An excellent and easily accessible example of this may 

 be seen on North Boulder Creek, Boulder County, Colorado, below 

 Silver Lake, and others are equally good. The morainal material 

 varies much in fineness, boulders weighing tons being included in the 

 finest of glacial mud, without assortment. Rock ridges resembling 

 terminal moraines in form, but of different origin, will be discussed 

 further on. 



" Rock-fiour." — The moving ice, dragging boulders frozen in the 

 bottom and sides, scours off the rocks upon which it rests, polishing 

 and striating them and producing a fine sediment which has been 

 called "rock-flour." This sediment gives to the water which pours 

 from beneath the glacier a greenish-white or milky appearance, also 

 furnishing the fine mud which is found mingled in large proportion 

 with the rocks of the moraine, often giving it a fresh appearance as 

 if it had just been dumped out in a wet condition by a gigantic steam 

 shovel. This is one of the best indications of present activity of a 

 glacier, as the mud would be washed off by a very few seasons' storms, 

 leaving only the larger rock fragments exposed. Such sediments 

 may not always be produced continually even by true glaciers, for 

 they do not always move with the same velocity and may indulge 

 sometimes in periods of comparative rest. Similar mud may be 

 produced on a much smaller scale by the neve, and hence would not 

 of itself be determinative. 



A very different kind of mud, composed of wind-blown dust, frag- 

 ments of vegetation, insects, etc., and possibly somewhat coarser 

 rock fragments washed on to the ice from surrounding cliffs, must 

 be distinguished from glacial mud. It mingles with the winter's 

 snows and is left as a residue when the snow melts. Consequently it 

 is found in many situations where even perpetual snow does not exist. 



Size and Depth. — Manifestly the difficulty of determining just 

 when a glacier ceases to exist, leaving only its neve as a reminder. 



