FLUCTUATIONS OF CRILLON GLACIER SYSTEM,SOUTHEAST ALASKA

Abstract

Crillon Glacier system (Lat. 58° 37′ N, Long. 137° 23′w) consisting of two long feeding arms and a common T-shaped ending along the Fairweather Fault, with two cliffed termini, has been advancing. La Perouse's sketch map in 1786 suggests that North and South Crillon Glaciers were back far enough to have separate termini in Lituya Bay, so they must have advanced at about 30 m/yr for 150 years. More detailed maps and photos show that North Crillon terminus did advance 28 m/yr into open water positions from 1894 to 1933. Some of this 4.5 km advance may well be just the result of consolidation of two long calving ice cliffs into one deep water front.

South Crillon front in the other arm of the “T”, where no junction of fronts is involved, also advanced into Crillon Lake 10 m/yr between 1929 and 1961. This implies that the basic cause was some increased snowfall on the high mountains. At the same time glaciers from lower peaks just to the east of Glacier Bay lost nearly all accumulation by a rising snowline and receded 5 to 11 km.

Since 1926 the Lituya Bay terminus has been more or less stable where it lies in water; however, the land portion has advanced 25 m/yr since 1948. Between about 1920 and 1961 a large gravel delta appeared and grew to 145 × 10⁶ m³, so this slow ice advance on one side of the front may be due to ice shearing over the new land of the fan where ablation is slower than watercliff calving.

Prehistoric advances of Crillon Glacier system are recorded in stratigraphic deposits dated by C-14. Trees buried in the prominent extensive outer moraine system around Crillon Lake and Lituya Bay were annihilated in the maximal push of the Little Ice Age 400 to 1000 years ago. The first advance came 1500 to 1800 years ago when logs became buried in glacial outwash, now under Little Ice Age till. Because it is regional, this was a climatic event involving a lowering snowline due to cooler summers.

Still further back, in hypsithermal time, these Crillon Glacier termini were probably as retracted as today, because many outcrops of buried forest 3000 to 9000 years old occur in alluvial deposits in Lituya Bay. Underneath all these is outwash gravel and till recording the last push of Wisconsin ice, ending at least 9000 years ago. Still earlier Wisconsin advances of North Crillon Glacier carried the Lituya Bay ice tongue a little further to sea, yet all through the pleistocene, unglaciated refugia endured between the ice streams.