The timeline seen in Figure 8.1 shows the part of the geologic time scale covering the period when the Appalachian Mountains started forming, 500 to 350 million years ago. The mountain range originated during three pulses of mountain building—called orogenies; the black bars in this figure show the two earliest orogenies. Orogenies do not start and stop abruptly, but the Taconic Orogeny occurred around 480 to 445 million years ago, and the Acadian Orogeny occurred about 390 to 360 million years ago. The third major Appalachian orogeny, the Alleghanian Orogeny (not shown in Fig. 8.1), occurred around 325 to 260 million years ago. Each of the three orogenies created a tall mountain range near where the present-day Appalachians are found. But, between orogenies, and since the last one, a great deal of erosion occurred. Thus, most of the mountains that were once there are now gone.