[Illustration: Figure 55. _East face of Signal Mountain showing Bivouac Formation (upper Pliocene or Pleistocene). Tilted ledge is rhyolitic welded tuff 2.5 million years old, and slopes above and below it are conglomerate. National Park Service photo by W. E. Dilley._] After Teewinot Lake was filled with sediment, the floor of Jackson Hole became a flat boulder-covered surface. Nearby vents erupted heavy fiery clouds of gaseous molten rock that rolled across this plain and then congealed into hard layers with the general appearance of lava flows. Under a microscope, however, the rock is seen to be made up of compressed fragments of glass that matted down and solidified when the clouds stopped moving. This kind of rock is called a _welded tuff_. One of these forms the conspicuous ledge in the Bivouac Formation on the north and east sides of Signal Mountain (fig. 55), and is especially important because it has a potassium-argon date of 2.5 million years. More of this _welded tuff_ flowed southward from Yellowstone National Park, engulfed the north end of the Teton Range (fig. 53), and continued southward along the west side of the mountains for 35 miles and along the east side for 25 miles. [Illustration: Figure 56. _The final 3 million years on our yardstick of time, enlarged to show approximate dates of major events._] THE LAST HUNDREDTHS ABSOLUTE TIME IMPORTANT EVENTS OF AN INCH OF THE (Years ago) YARDSTICK 0 0 Last glaciation followed by faulting ¹/₁₀₀₀ 50000 Second glaciation ²/₁₀₀₀ 100000 ?—First glaciation ⁶/₁₀₀₀ 700000 ?—Second Quaternary lake ⁸/₁₀₀₀ 1 million ?—Tilting and faulting of southern part of Jackson Hole ¹¹/₁₀₀₀ 1.3 million ?—First Quaternary lake ¹²/₁₀₀₀ 1.5 million } Complex series of volcanic eruptions in southern Jackson Hole ¹⁵/₁₀₀₀ 1.9 million } ¹⁶/₁₀₀₀ 2 million ?—Development of Hoback normal fault ²/₁₀₀ 2.5 million Eruption of welded tuff in Bivouac Formation ²⁴/₁₀₀₀ 3 million QUATERNARY—TIME OF ICE, MORE LAKES, AND CONTINUED CRUSTAL DISTURBANCE