Three Successive Records Of Glaciation
By Ranger-Naturalist E. L. Clark
Along the inner rim slope midway between Sinnott Memorial and Discovery Point occurs a record of three successive glaciations. There in the great outdoor laboratory of vulcanism and glaciation are found three separate layers of glacial drift. The accompanying diagram illustrates the arrangement of the several glacier evidences. A section by W. R. Atwood has been included as a means of comparing the evidence of glaciation found at different localities within the rim of the crater.
It will be readily seen that the record of glaciation found at "Glacier Point" is of much significance inasmuch as it represents two distinct outpourings of lava after the appearance of the first glacier. The question may be raised; Are the glacial tracks really the marks made by moving masses of ice or are they pseudo-glacial markings formed by the solidification of hot, seething masses of lava pouring out of a volcanic crater and down the mountain side?
Glacial striae are superficial scratches or grooves made on a rock surface by the tools of a glacier as it slides over the bedrock. These markings are usually arranged in parallel groups. The direction in which the striae are running is indicative of the general trend of movement of the glacier at that particular place. It happens that the rock surfaces will be somewhat polished and at least smoothed by the abrasive action of the ice and detritional rock fragments.
Banding due to flowage of the lava is the internal arrangement of particles within the lava flow. These particles may occur singly or in combination.
They may be crystals that had started to form before the ejection or outpouring of the lava from its conduit. They may be included material that were picked up by the lava as it moved over loose fragments of other rocks; or they may happen to be certain portions of the lava flow rich in volatile constituents which on release may leave cavities in the solidified lava and produce the frequent occurrence of lines or layers of bubble marks in a lava. One portion of this structure may vary in its relation to its remainder. Furthermore, one set of flow banding need not agree in attitude with the other sets of flow banding in a single lava bed. More often it happens that one flow structure can be traced to a place in the lava flow where it intersects a second flow banding. Fracturing of the lava during its cooling may produce smooth curved surfaces, but these will never be straight and polished as produced by glacial scour.
One who walks along the path of Nature to Discovery Point shall have a wonderful opportunity to read the story recorded in the rocks and distinguish between these confusing structures. Along this trail one first meets with glacial striae on the upper surfaces of many lava flows. Next a pseudo-striation produced by both flow banding and fracturing of the hot molten mass will be noted. And further along the trail Mother Nature has placed these two confusing structures together. The glacial striae are seen to be at right angles to the flow banding of the lava, and again, within the same lava flow, the striae are seen parallel to the banding. Truly, it is here where Mother Nature has solved the problem and completed the lesson for the day.
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