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 (Fic. 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 
