86 ALASKAN MINERAL RESOURCES IN 1906. 
PHYSIOGRAPHY. 
The front range is a maturely dissected ridge, modified and sharp- 
ened in its details by recent glaciation. The valleys on the coastal 
side present distinct evidence of two cycles of valley development. 
The older cycle is indicated by shoulders on the spurs at an elevation 
of 1,200 to 1,500 feet; these are interpreted as representing the bot- 
toms of broad valleys. Above the shoulders the average slope is not 
steep and the ample tributary gulches have occupied the entire field. 
Beneath the shoulders the more recent canyons belonging to the second 
cycle are intrenched. Along the front of the range a series of rocky 
terraces corresponds to and merges into the high shoulders just men- 
tioned. Another and less continuous line of terraces and flat-topped 
hills of rock stands at an elevation of about 100 feet along the border 
of the mountain front. Both sets of benches are attributed to erosion 
by the waves of the Pacific when it stood farther inland than now. 
The following summary of the physiographic history of the region 
conveys the writer's interpretation of the observed facts. It is pre- 
sented as a suggestion for more detailed and critical work by future 
students of the region : 
1. Early erosion cycle. — Mai ure dissection of a broad west-northwest uplift." Formal 
tion of open main valleys with divides 1,000 to 2,000 feet high, and the production 
of broad marine shelves by waves cutting on the seaward front of the mountains 
2. Canyon erosion cycle.— Rejuvenation of drainage by an uplift amounting to about 
1,200 feet. As a result the development of V-shaped canyons within the older 
valleys. In most places the rejuvenation has not, yet reached the heads of the 
older gulches. Formation of high sea cliffs and low cut terraces on the outer 
spurs of the mountains. 
3. (Uncial erosion cycle. — Maximum extension of glaciers; excavation of cirques in the 
heads of the gulches. Ice mounted 1,200 to 1,500 feet higher on the slopes of the 
valleys than now, and was proportionately more extensive. Most of the large 
glaciers discharged into the ocean, which skirted the mountain front. 
4. Glacial retreat. — Uplift of about 100 feet, resulting in the partial uncovering of ihe 
coastal plain. Yakutat Bay and Russell Fiord glaciers formed moraines upon the 
flat before retreating. Other glaciers probably receded before the ocean was 
excluded from their valleys, and consequently formed no moraines. Plain 
gradually extended by the deposits of shifting streams. The glaciers decreased 
tn nearly their present size and many of them entirely disappeared. 
PROSPECTING. 
The district described has been explored to some extent by pros- 
pectors since the early nineties, but as yet no deposits of proved value 
have been discovered. The beach placers about Yakutat Bay have 
been described by previous observers. 6 Deposits of black sand on 
Black Sand Island contain small amounts of gold and have been 
a From results of studies by Russell and by Tarr and Martin it seems probable that this is a rising 
fault block. 
*> Tarr, R. S., Bull. U. S. Geol. Survey No. 284, 1906, p. 64. 
