PLANTS OF YELLOWSTONE NATIONAL PARK 
3 
DEVELOPMENT OF THE PARK VEGETATION 
The area now occupied by Yellowstone National Park has not 
always been as we see it today. Geologists tell us that millions of 
years ago, before there were any Rocky Mountains or any birds or 
mammals, this entire region was an inland sea, an arm of the ocean. 
That sea existed here for a very long time, but finally there came a 
period of great earthquake activity during which the whole area 
was gradually pushed upward, far above sea level, and the Rocky 
Mountain system was formed. But throughout the entire history 
of the world, whenever there has been a period of mountain building, 
such a time has been followed by a period of volcanic activity, and 
the Yellowstone region was no exception. Great quantities of 
molten lava were poured from volcanoes or from fissures in the 
earth’s surface and spread out over the area burying the former 
bottom of the sea to a depth of hundreds of feet. Following this 
activity there must have been a time when the whole region had a 
surface of dry, bare rock with absolutely no soil and no plants of any 
kind. There would, of course, be depressions filled with water but 
otherwise nothing but hard, lava rock. 
How would nature go about it to develop the beautiful vegetation 
that we have in the park today on such a rock surface as we have 
assumed? The only plants that can grow on dry, bare rock, with¬ 
out any soil at all, are certain lichens. A lichen is not a single plant, 
but is a combination of algae and fungi growing together, and it is 
an interesting fact that, while practically all common algae grow 
in water and most of the common fungi grow in places that are at 
least moist, when the two are together in the sort of structure that 
is called a lichen they can grow in drier places than any other plants. 
They are always the pioneer plants on dry, bare rock and many 
of the brilliant colors that we see on rocks and boulders throughout 
the country are due to the lichens that are growing on them, for 
lichens occur in a variety of colors such as red, yellow, orange, green, 
black, brown, or gray. 
There are three general types of lichens. The first type that ap¬ 
pears on the naked rock is very thin and crustlike, appearing much 
like a smear of paint, and for this reason it is called a crustose lichen. 
The crustose lichen grows very slowly, but it gradually brings about 
a slight disintegration of the rock surface, thus initiating the proc¬ 
ess of soil formation. As soon as a thin layer of dustlike soil has 
been formed the second type of lichen may appear. This second type 
is more or less leaflike, like foliage, and so is called a foliose form. 
It also grows very slowly, but it continues the process of rock disin¬ 
tegration, and it has body enough to catch some wind-blown dust. 
Thus after a long period of time soil enough will have accumulated 
