By about 11,000 years ago, these Clovis people had spread across the area of the United States and into Mexico. It may have been they who killed off the mammoths and then gradually assumed the characteristics of the _Folsom_ culture. The Folsom people were bison-hunters, and long were thought to have been the first population in America. It was with the use of ¹⁴C that it finally was possible to place these two cultures in proper sequence—the Clovis first—and to correlate them with major natural changes, especially the advance and retreat of glaciers across the continent. THE LONG-LIVED CLOCKS All other practical age-determination schemes are based on a few long-lived isotopes, with half-lives relatively near the age of the earth (4.5 AEONS). They are: Table III Isotope Emits Decays to Half-life Uranium-238 8 ALPHA PARTICLES[10] Lead-206 4.51 aeons Uranium-238 Spontaneous fission 2 Fragments 10 million aeons[11] Uranium-235 7 Alpha particles Lead-207 0.713 aeons Thorium-232 6 Alpha particles Lead-208 14.1 aeons Rubidium-87 Beta particle Strontium-87 4.7 aeons Potassium-40 Electron capture Argon-40 1.3 aeons ...... ...... ...... ...... Rhenium-187[12] Beta particle Osmium-187 40 aeons It is apparent that Table II on page 6, showing the long-lived radioactive nuclides, is much longer than the list of the seven shown here that are actually useful in practice. Some of the nuclides that are theoretically available are useless on a practical basis, because they are so rare in nature. Many others cannot be used for reasons that are fundamental to the whole process of nuclear age determination by “whole hourglass” (that is, parent-daughter) methods. Let’s look at these reasons. These methods are based on closed systems in which the daughter products of the radioactive decay are locked with the parent material from the beginning of the system, and nothing is added or removed thereafter. To state it in terms of our analogy, the hourglass must be in perfect working order—no leaks or cracks permitted. There is another fundamental requirement: At the beginning, the bottom part of the hourglass must be empty. If some sand were already in the bottom at the start, we would mistakenly be led to conclude that the time elapsed was longer than it actually was. That necessity places a severe limitation on the type of system we can use.