MATURITY IN ALPINE GLACIAL EROSION 571 



lengthened in down-canyon order. In that order, also the phe- 

 nomena of the faintly reversed grade and of the rock-basin lakes 

 rapidly failed. Apparently, at the canyon head, the last touch of 

 vanishing glaciation had been so recent that filling had not been 

 initiated, while down-stream, incision of the step cliffs and aggrada- 

 tion of the fiats had made at least a beginning in the immense task 

 of grade adjustment; the tread of the step was graded forward, but 

 so insensibly, as a rule, that its draining stream lingered in meanders 

 on a strip of meadow, as though approaching base-level. These 

 deep-sunk ribbon meadows, still thousands of feet above the sea and 

 miles in length, reflecting in placid waters their bordering walls or 

 abnormally steep slopes, presented an anomaly of the longitudinal 

 profile in erosion no less impressive than that of the upright canyon 

 heads. 



In ground plan, the canyon heads crowded upon the summit 

 upland, frequently intersecting. They scalloped its borders, pro- 

 ducing remnantal-table effects. In plan as in profile,- the inset arcs 

 of the amphitheaters were vigorously suggestive of basal sapping 

 and recession. The summit upland — the preglacial upland beyond a 

 doubt — was recognizable only in patches, long and narrow and irregu- 

 lar in plan, detached and variously disposed as to orientation, but 

 always in sharp tabular relief and always scalloped. I likened it 

 then, and by way of illustration I can best do so now, to the irregular 

 remnants of a sheet of dough, on the biscuit board, after the biscuit 

 tin has done its work. 



In large part, apparently, a preglacial summit topography had 

 been channeled away. By sapping at low levels, by retrogressive 

 undercutting on the part of individual ice-streams at their amphi- 

 theater heads in opposing disorderly ranks, the old surface had been 

 consumed, leaving sinking ridges, meandering dulled divides, low 

 cols or passes, and passageways of transection pointing to piracy and 

 to wide shiftings of the glacial drainage. There was not wanting a 

 scattering of the more evanescent sharp forms of transition which 

 the hypothesis would require, as thin aretes, small isolated table caps, 

 needle-pointed Matterhorn pyramids with incurving slopes, and 

 subdued spires (in the massive granite tracts) with radiating spurs 

 inclosing basin lakes. The broader areas of this deep erosion, where 



