Nichols: The vegetation of Connecticut 237 



however, is greater than lateral, so that the stream may gradually 

 intrench itself in a narrow ravine or canyon. At this stage of 

 stream development vegetation is largely confined to the flanks 

 of the ravine. During the course of time, as the channel is cut 

 deeper and deeper, downward erosion proceeds more and more 

 slowly, ultimately ceasing altogether, while lateral erosion be- 

 comes correspondingly more pronounced; so that, as the stream 

 approaches maturity, it comes to occupy a broad valley which it 

 has carved out for itself. Vegetation covers both the bordering 

 bluffs and the floor of the valley. As the level of the stream 

 approaches the level of the water at its mouth, the current becomes 

 sluggish and aggradation largely succeeds degradation. An old 

 age river meanders about in broad, sweeping curves on the flood 

 plain which it has built up out of sediment derived from younger 

 parts of its system. A very distinctive type of vegetation is 

 developed on flood plains. 



Of course, the rate at which the topographic changes outlined 

 in the preceding paragraph are effected varies widely according 

 to the nature of the rock formations concerned. The scouring out 

 of a ravine in clay or gravel, for example, may proceed with com- 

 parative rapidity, but the erosion of a ravine in rock is extremely 

 slow. Ravines in glacial till and outwash have been developed since 

 the retreat of the glaciers, and may even now be observed in the 

 process of active formation. In most rock ravines, on the other 

 hand, the topographic changes since the ice age have been prac- 

 tically negligible. The bearing of these facts on the nature of the 

 vegetation will be referred to in later paragraphs (pp. 245, 249). 



Rock Ravines 

 Occurrence and General Features. — Throughout the region under 

 discussion .the rock ravine is the common type. It abounds in the 

 highland districts, and in the Central Lowland is frequent along 

 the trap ranges and elsewhere. Owing mainly to the nature of 

 the rocks concerned, canyons with overhanging or perpendicular 

 walls are absent. The predominant type of rock ravine (Fig. 2) 

 is somewhat V-shaped in cross section, often narrow and precipi- 

 tous below but broadening out above. Toward the bottom the 

 vegetation may be restricted to such forms as can cling to the 



