Nature's Notes 



105 



grazing. Bridger crept closer and fired again. Nothing happened. 

 Three, four, five times he aimed very carefully at the elk and shot, each 

 time creeping closer. He then suddenly found himself face to face with 

 Obsidian Cliff. He had been shooting at the elk, he said, right through 

 the mountain of glass and of course his bullets glanced off the moun 

 tain and missed the elk !" 



Another curious formation that amazes many people is that of the 

 pentagonal columns which hold up the cliffs on both sides of the Yellow 

 stone River near Tower Falls. These columns are lava that cooled and 

 cracked into remarkably symmetrical geometric figures. There are miles 

 of them and were it not for the many other wonders of the park they 

 would be an outstanding attraction in themselves. 



"Ranger, what is the cause of the heat under the park?" 



"That is quite a question. The story the geologists tell us is that 

 Yellowstone was once a part of a great shallow sea. There are evidences 

 of fossil shells and sea animals found in parts of the park. The theory 

 is that ages ago there occurred a mighty upheaval of mountain masses, 

 forming the Rockies. This was followed by a long period of volcanic 

 activity. The Yellowstone region included several volcanoes, which, 

 with their lava outpourings, formed the great plateau of the park. 

 Blankets of lava spread out over the area, cooled, were covered with 

 forests. Later, becoming active again, the volcanoes again belched forth, 

 repeating the building-up process. 



"The great volcanoes spent their energy, finally, and became quiet, 

 but in many places their furnaces have not yet cooled. They are dying 

 out slowly, but it may take thousands of years for them to cool entirely. 



"Why do geysers erupt ? Well, you have watched a coffee percolator 

 in action. The geyser works much the same way. The water of geysers 

 and hot springs comes originally from rain and melting snow. Flowing 

 down and coming in contact with lava flows which have but partially 

 cooled, it is heated. The geyser's tube is so long and so narrow that as 

 the water is heated at the bottom it cannot rise to the sur 

 face in natural hot springs. Hence it collects at the bottom, 

 the very hot water held down by the pressure of other water 

 on top. Steam is formed below while there is still some 

 water at the upper end of the tube. The 

 steam finally forces its way upward through 

 the water in the tube, emptying the tube 

 through a volcanic eruption. After the steam 

 and hot water are out, the geyser is quiet until 

 enough water and steam gather to repeat the 

 process. In the museum a miniature geyser, 

 has been built to illustrate this action." 



