32 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
at the two extremities of the 225-foot plain, where the rock outcrops 
produce a series of stepping terraces. No such definite arrangement 
of cusps, one back of another, could exist by accident. A better 
example is found (plate 1) west of the meadow, in the vicinity of the 
farm buildings, where the curving terraces terminate and are replaced 
by the rock ledges which—as will be later explained—form the con¬ 
trol for the systematic arrangement. 
Terraces of such complications frequently show a perfectly developed 
system of terrace drainage. This system exists upon the Retreat plain - 
and the meadow plain. A line of drainage follows the back of the 
Retreat plain to its lower end. There the brook has eroded for itself 
a valley to the meadow plain and deposited on the floor below a fan¬ 
like delta. The Retreat plain slopes to this line of drainage, indicating 
that the plain was built while the river occupied the position of the 
present brook and that the river was finally withdrawn by short-cut or 
partition. At the foot of the Retreat scarp on the meadow is another 
brook, following what may have been the old course of the West River 
and now emptying into the present river. 
Such complexity of terrace pattern as exists within the West River 
valley is typical of the middle stage, because rock ledges were discovered 
to defend and preserve positions of the many terrace plains. More¬ 
over, the remnants of the higher terrace plains are few and small, 
since they have been repeatedly undercut and destroyed; and the 
lower plains are larger, more numerous, and better preserved because 
time enough has not elapsed for them to be frequently attacked by 
the river. 
Terrace 'pattern, late stage .— When the power of a river to degrade 
weakens, it devotes its energy to swinging repeatedly to and fro across 
its valley. In the absence of rock ledges, even the basal terraces dis¬ 
appear, and the intermediate plains may be completely swept away. 
The river thus swings until it reaches the limit of its wandering belt; 
and the whole descent from the high-level terrace to the existing flood 
plain is made in one deep escarpment. 
Three conditions may bring about this weakened ability to degrade, 
as Davis ( 02) has shown: first, the attainment of nearly fixed values 
of volume and load, such as might be reached when a glacial climate 
had given way to a milder climate and the latter had become well 
established; second, the cessation of any slow uplift by which degrada¬ 
tion had been initiated or aided; third, superposition of the stream 
