The story so far: On a tour of New Zealand’s South Island, my friend Daniel and I climbed to a viewpoint 1,700 feet above the valley of the Waimakariri River in the Southern Alps. I was fascinated by the contrast between the steep mountain slopes on either side of the 2-km-wide valley and its flat floor containing a broad braided riverbed. Coming in from our right, the the canyon of the Hawdon River was floored with a similar deposit.
As we gazed at the scene in front of us, Daniel explained how this developed: During the ice ages of the past several hundred thousand years, glaciers have spread down the valleys. Fed by moisture-laden storm fronts moving in from the Tasman Sea and aided by lower temperatures, they grew and deeply filled the valleys. In many places their sculpting power is still evident. As they melted back, they left behind deep deposits of rock they had carried with them. Between glacial episodes more rocks and gravel were washed into the valley by rapid erosion.
More sediment has been dropped in the valley than could be flushed out the lower end. It has built up to an unknown depth.
Flowing on this blanket of sediment, the river spreads out into many channels which continually shift over the course of a season. In only a few places, where the riverbed is less wide and more stable, are there good places to ford or to build a bridge. In the early days of travel over Arthur’s Pass to the north, entrepreneurs built an inn at such a place up the valley. Travelers stayed there overnight before crossing the river in the morning when the water was relatively low.
Looking almost straight down on the Hawdon River coming in from the side, we could see the gray of its shifting gravelly bed along with much older channels now overgrown with varying shades of greenery.
Since coming home, I’ve studied the literature as well as my pictures to fill in the picture with more detail.
In February 1958 Maxwell Gage published the results of extensive field study in the Waimakariri drainage. In the same volume of the New Zealand Journal of Geology and Geophysics, R. P. Suggate and D. D. Wilson devoted a page to glacial evidence in the Black Range to the south. http://books.google.com/books?id=pAc4AAAAIAAJ&pg=PA1&dq=new+zealand+journal+of+geology+and+geophysics+february+1958&hl=en&sa=X&ei=ZTxZT5zqIbGNigLNycmZCw&ved=0CEsQuwUwBA#v=onepage&q&f=false
Both authors show that glacial episodes have filled this section of the valley deeply enough to overflow to the south over two passes: Lagoon Saddle at 1,200 m. and Cass Saddle at over 1,300 m.–over 700 m. or 2,300 feet above the present valley floor. How much higher the ice rose there is unknown, but moraines beyond Cass Saddle indicate that a considerable flow of ice crossed that pass. Suggate and Wilson (Pg. 44-45) note the clearly ice-smoothed surface around Lagoon Saddle as seen in aerrial photos.
Mount Horrible, the 1,234-m outlier at the corner of the Black Range, shows the outlines of a classic roche moutonnee–a resistant piece of massive bedrock over-ridden by glaciers, with a smoothly sloping side that the glacier rode up over and shaped, and a precipitous down-valley side where the glacier pulled rock away as it moved down the valley. Near its top, far from the destructive effects of streams, are several baby roche moutonnees–smoothed outcrops of resistant rock, echoes of their parent.
The surface of the ice, then, was far above the point where Daniel and I stood that day.
Farther down the valley, where it turns southward into a wide area between major ranges, broken by low divides and isolated small mountain masses, Gage (Pg. 123-155 ) has found fragments of moraines and ancient outwash plains documenting multiple glaciations. At their most extensive, the snout of the glacier extended to the southeastern edge of the mountains.
During the retreat of the last major glacial stage, a lake formed behind the end moraine. Gage estimates that it was at least 10 miles long, extending upstream past the mouth of the Hawdon River, where beaches disappear underneath its fan and the accumulating sediments of the Waimakariri.
As I try to visualize all this, I realize that what has been going on here to shape the landscape is really too big for me to fully comprehend. These mountains, the whole world, and their processes are so much larger than I–yet I’ve been made part of it all. What a privilege and what a blessing.