The deciduous-leaved trees are almost a negligible element in the present park flora, being confined to an occasional cottonwood (Populus angustifolia) at the lower elevations, along the Yellowstone River, and small groves of the quaking aspen (Populus tremuloides). Along the streams and in wet places there are many species of willow (Salix) and several alders (Alnus), and in mountain bogs and valleys there is a small birch (Betula glandulosa). There are, of course, many small shrubs, such as gooseberries, currants, and roses.
AGE OF THE FOSSIL FORESTS.
The question is often asked, How old are the fossil forests? It is, of course, impossible to fix their age exactly in years, though it is easy enough to place them in the geologic time scale. The stratified rocks that make up the crust of the earth, from the oldest we know to the most recent, have been divided by geologists into a number of major divisions or systems, each—except perhaps the oldest—containing the remains of certain kinds of plants and animals. The accompanying diagram ([fig. 16]), shows these major time divisions, arranged in their proper sequence from the lowest to the highest. The star (*) in this geologic time scale indicates the age of the rocks in which the fossil forests were entombed. It shows that they were buried during the Tertiary period. This period is divided into four epochs, the oldest called Eocene, having been succeeded in turn by the Oligocene, the Miocene, and the Pliocene, which just precedes the Pleistocene or glacial epoch. The forests of the Yellowstone National Park are found in the Miocene series of the Tertiary. As compared with the eons of geologic time that preceded it the Miocene is relatively very recent, though, if the various estimates of the age of the earth that have been made by geologists are anywhere near correct it may well have been a million years ago. It must be remembered, however, that this estimate involves more or less speculation based on a number of factors which may or may not have been correctly interpreted.
A study of the fossil trees themselves gives at least a rough approximation as to the length of time it may have taken to accumulate the beds in which they are now buried. As already mentioned, there is a succession of forests, one above another, through a thickness of 2,000 feet of strata. The unit of the measure of the time is the time taken by each forest to grow. Pine trees of the types represented in the fossil trunks require 200 or 300 years to reach maturity, and redwoods may require from 500 to 1,000 years. Twelve or more of these forest levels have been found. By multiplying this number by the minimum age of the trees (200 years) we shall have 2,400 years, and by multiplying it by the maximum age of the redwood (1,000 years) we shall have 12,000 years as the possible time during which these forests flourished. It is possible that the truth lies somewhere between these extremes.
| Fig. 16.—Geologic divisions. | ||
|---|---|---|
| Era. | Period. | Epoch. |
| Cenozoic. | Quaternary. | Recent. |
| Pleistocene (glacial). | ||
| *Tertiary. | Pliocene. | |
| *Miocene. | ||
| Oligocene. | ||
| Eocene. | ||
| Mesozoic. | Cretaceous. | |
| Jurassic. | ||
| Triassic. | ||
| Paleozoic. | Carboniferous. | |
| Devonian. | ||
| Silurian. | ||
| Ordovician. | ||
| Proterozoic. | Cambrian. | |
| Algonkian. | ||
| Archean. | ||
CLIMATE DURING THE LIFE OF THE FOSSIL TREES.
A final word may be added regarding the probable climate of the region during the lifetime of these fossil forests. It is obvious that the present flora of the Yellowstone National Park has comparatively little relation to the Tertiary flora and can not be considered the descendant of it. It is also clear that the climatic conditions must have greatly changed since Tertiary time. The Tertiary flora appears to have come from the south, but the present flora is evidently of more northern origin. The climate during Tertiary time, as indicated by the vegetation, was temperate or warm-temperate, not unlike that of Virginia or the Carolinas at the present time, and the presence of numerous species of figs, a supposed bread-fruit tree, cinnamons, bays, and other southern plants indicates that it may have been almost subtropical. However, the conditions that were favorable to this seemingly subtropical growth may have been different from the conditions now necessary for the growth of similar vegetation. It may be that these supposed subtropical plants were at that time so constituted as to grow in a temperate land, and that they may have become tropical in recent times. Following this general line of thought it may be said that although the Tertiary vegetation of the Yellowstone National Park would now be regarded as indicating a temperate or even warmer climate, the actual climate may not have been subtropical. It is certain, however, that the conditions were very different from those now prevailing in the park.
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