362 



EARTH FEATURES AND THEIR MEANING 



When the elevation of the land in the vicinity of this shore had 

 caused a recession of the waters, there was formed a coastal plain 

 on the borders of the oldland like that which is now found upon 

 our Atlantic border between the Appalachians and the sea (Fig. 

 272, p. 246). The rivers from the oldland cut their way in narrow 

 trenches across the newland, and because of the harder limestone 

 formations, their tributaries gradually became diverted from their 

 earlier courses until they entered the trunk stream nearly at right 

 angles and produced the type of drainage 

 network which is called " trellis drainage." 

 It is characteristic of this drainage that 

 few tributaries of the second order will 

 flow up the natural slope of the beds, but 

 on the contrary these natural slopes are 

 followed in the softer rock nearly at right 

 angles again to the tributaries of the first 

 order of magnitude (Fig. 387). Thus are 

 produced a series of more or less parallel 

 escarpments formed in the harder rock and 

 having at their base a lowland which rises 

 gradually in the direction of the oldland 

 until a new escarpment is reached in the 

 formations. Such 

 with inter- 



next lower of the hard 



FIG. 388. Bird's-eye view . 



of the cuestas south of flat-topped uplands in series 



Lakes Ontario and Erie mediate lowlands and separated by sharp 

 (after Gilbert). escarpments are known as cuestas (see p. 



246), and the Lewiston Escarpment limits that formed in Niagara 

 limestone (Figs. 387 and 388). 



Episodes of Niagara's history and their correlation with those 

 of the Glacial Lakes. Of the early episodes of Niagara's history, 

 our knowledge is not as perfect as we could desire, but the later 

 events are fully and trustworthily recorded. The birth of the 

 Falls is to be dated at the time when the ice front had here first 

 retired into what is now Canadian territory, thus for the first time 

 allowing the waters from the Erie basin to discharge over the Lewis- 

 ton Escarpment into the basin of the newly formed Lake Iroquois 

 (Fig. 364, p. 334). Since the level of Lake Iroquois was far above 

 that of the present Lake Ontario, the new-born cataract was not 

 the equivalent in height of the escarpment to-day. The Iroquois 



