The Potomac, in Virginia and Maryland, completely cuts through the Blue Ridge Mountains on the way to the Atlantic, forming an impressive gorge at Harpers Ferry. But the Blue Ridge, as large as the range is, is only one range of the Appalachians and the Potomac does not cut completely though the Appalachians.
In California, the San Joaquin River, the state’s second longest, cuts completely though the immense Sierra Nevada Range, the major mountain range of that state with many peaks over 14,000 feet elevation. The San Joaquin is the major river of California Southern Central Valley, and it eventually flows into the Sacramento River before emptying into San Francisco Bay.
The San Joaquin knifes dramatically, from east to west, through the Sierra Nevada between Yosemite and Kings Canyon. However, immediately east of the San Joaquin headwaters, there is an immense, 11,000 foot volcano unrelated to the Sierra Nevada, called Mammoth Mountain. Mammoth Mountain prevents the San Joaquin headwaters from eroding further into the great basin and perhaps capturing the headwaters of the Colorado.
It is the gap created by the San Joaquin that causes the abundance of snow of Mammoth Mountain, in an area that normally would be desert, making it one of the world’s great ski areas.
There was also considerable discussion of mountain building processes. The three major kinds of mountain building, according to Art, are (1) volcanic, (2) compression (where the land is squeezed together) and (3) tension (where the land is pulled apart). All three are represented in Southern Oregon with the Cascades, the Siskiyous and the interior basin ranges such as the Warmer Mountains.