YELLOWSTONE NATURE NOTES

THE MODERN PARK UNFOLDS

As the volcanic cover cooled, hydrothermal action with its consequent chemical alteration of the overlying rocks and the creation of early hot spring and geyser action took place. The stage was now set for the last decisive event, glaciation. Alpine glaciers, nourished by the snow fields of the surrounding mountains sent tongues of ice along pre-existing valleys, these lobes in places coalescing to create ice sheets that have scoured and planed off the topography. Occasionally hydrothermal action extended up through the ice to create interesting hills of tillite to be seen today in Twin Buttes and Porcupine Hills.

The final shaping of the Yellowstone landscape has been accomplished largely by water, both hot and cold. Streams fed by melting snow fields and rainfall begin their long tortuous journeys to both the Atlantic and Pacific dropping vertically over a mile and a half until sea level is reached. Streams, motivated by the force of gravity and laden with their cutting tools of sand and gravel have gashed the plateau with deep youthful V-shaped canyons.

Perhaps less impressive, but of equal importance, is the chemical work of hot waters. Ground water seeping through glacial gravels and fractured rhyolite, contacts the rising heat from the magmatic mass below. These hot waters are ever busy dissolving mineral matter at depth, transporting it up ard in solution and then, after depositing some at the surface, the balance is carried on eventually to the oceans to add to their dissolved salt content. The Mammoth Hot Spring travertine terraces and the glaring white siliceous sinter deposits of the geyser basins have all been formed by this process. Great solution channels and cavities must honeycomb the underlying rocks rendering them highly susceptible to fracturing and collapse by earthquake tremors.

Allen and Day (1935, p. 129) made chemical analyses of these waters, estimated their rate of discharge, and concluded that 390 tons of mineral matter from the geyser basins are carried away in solution daily by streams in Yellowstone Park. Project this rate of solution into the geologic past and the conclusion seems warranted that the hot waters resources have played an important part in shaping the topography of the thermal areas.

PLATE 4. Pitchstone Plateau lava flows southwestern corner Yellowstone Park. Park Aerial Survey photos 2-169 and 2-170.

A study of aerial photographs of the park permits interpretation of features that escape detection on the ground. Coniferous forests of lodgepole pine trees obscure the bedrock and mask the surficial features of the lava flows. Far in the interior of the Pitchstone Plateau there are rhyolite flows that appear as fresh as if formed yesterday. These flows seem to have moved like glaciers with great lobate swirls and flow lines accentuated by stands of timber.

Recent work done by the U. S. Geological Survey¹ has shown that a short distance west of the park, lava flows have been found on top of glacial deposits proving that volcanic action may have subsided no less than 10,000 years ago.

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¹Earthquake correspondence file, Yellowstone Park Library.

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