the cooling process.
More recently, ice ages have eroded the volcanoes of the Cascade Mountains. Glaciers remain on many volcanoes, particularly Mount Rainier and Glacier Peak. You can gain an idea of the comparative age of volcanoes from their appearance. Younger ones, including many of the small volcanoes that you see, tend to have the classical conic shape, whereas older volcanoes have been eroded during the ice ages, leaving only the towering crags of their more resistant cores.
There are three main types of glacial erosion.
Plucking: glacial melt water freezes around lumps of cracked and broken rock. When the ice moves downhill, the rock is plucked from the back wall.
Abrasion: rock frozen to the base and back of the glacier scrapes the bedrock.
Freeze-thaw: melt water or rain penetrates cracks in the bedrock. At night it freezes, expands and enlarges the crack, eventually breaking the rock away.
Volcanic rocks are easily eroded but granite is very resistant. Only where it is highly fractured or has been subjected to deep weathering is it easily eroded. Such weakened rock is easily excavated by glaciers, which leave basins of resistant granite that fill with water as the glaciers retreat. These are the corrie lakes that make granite mountain landscapes so attractive. Volcanic mountains lack the resistant rocks that allow lakes to form but in places glaciers have peeled them right down to the granite beneath, allowing the formation of lakes.
There are very few sedimentary rocks in the mountains through which the PCT passes.
Weather patterns
The mountains of Southern California, the Sierra Nevada and the Cascades form a highly effective rain shadow. Very little rain falls to the east of the mountains and desert conditions predominate as a result.
The PCT generally keeps to the crests or western slopes of the mountains to avoid the desert areas. This is epitomised by the long detour west of Mount Shasta, in Northern California, to avoid the dry, waterless hills east of the massive volcano. The main exception is the crossing of the western corner of the Mojave Desert, which has very little rainfall.
Rain and snow come either from frontal storms (often tropical storms developing well to the south-west) or from thunderstorms. Fortunately for PCT hikers, the frontal storms, which dump large quantities of snow in the mountains, occur mainly in winter. Storm frequency and the duration of the storm season increase as you head north. In Southern California most of the storms occur from January to March; by Northern California they have spread from late-September to May; and in Washington you can expect frontal storms at any time of year.
Debris from the storm of 2003 (Section 96)
Thunderstorms develop mainly as a result of convection currents caused by the power of the sun and are concentrated in the summer months, with July being the peak. The frequency of thunderstorms diminishes as you head north. Storm clouds usually start to build in the early afternoon, with the storm arriving in the late afternoon or evening.
Yucca at Vasquez Rocks (Section 18)
A thru’-hiker can expect hot, dry weather in Southern California and the High Sierra. Northern California is hiked at the peak of the thunderstorm season but there are fewer here than further south. Oregon in August is generally warm and dry but an occasional frontal storm or thunderstorm can be expected. In Washington in September you can expect periods of frontal rain but you can also get long sunny periods.
The weather’s boring!
Liz Willis, a British thru’-hiker in 2002, after weeks of continuous sunshine in California.
The weather does not always follow these patterns. You must be prepared for rain or snow at any time in the mountains.
Plant and wildlife
Vegetation
The mountain vegetation of California and Oregon has to deal with a difficult climate. Most of the precipitation at higher altitudes comes in winter, in the form of snow, and there is very little rain in summer. It has to survive high summer and low winter temperatures, as well as the poor quality soils in areas where granite is the predominant rock type.
The first thing Northern Europeans will notice on the PCT is the absence of grass and the poor quality of the meadows. Grass requires a lot of water and it isn’t until you reach Washington that it seems to thrive.
The second feature Europeans will notice in Southern California is the absence of trees at lower altitudes, except along creeks. Only at higher altitudes are temperatures cool enough and water plentiful enough for trees to survive. Below tree-line chaparral predominates. Chaparral is composed of broad-leaved shrubs, bushes and small trees, usually below 8ft high, which have evolved to cope with hot dry conditions. At the lowest altitudes, in the drier areas, only true desert vegetation survives and you will see various species of cactus and yucca.
Just some of the flowers and cacti you may see along the Trail
There are many different species of tree on the PCT and each seems to have evolved to fill a particular niche in the ecosystem. Deciduous trees predominate at lower altitudes where there is sufficient water but in the mountains you see a wide variety of pine.
There is also an upper limit to tree-line, above which conditions are too harsh for any tree to survive. In Southern California the hardiest species eek out an existence at 9000ft on exposed ridges but in Washington you will find that trees struggle to survive on ridges at 6000ft.
You will see many dead or dying trees. In many areas they are suffering from beetle infestations, as well as problems caused by pollution and forest fires.
In Washington you will spend much of your time in what is described as temperate rainforest, with the profusion of vegetation you would expect when there is plenty of rain throughout the year.
For much of the PCT you will hike through areas of forest that are only very lightly managed. Often this consists of little more than keeping trails clear for hikers. Fallen trees are allowed to rot, providing habitat for insects and returning nutrition into the soil. Only in Northern California will you hike through areas of commercial forestry, and even here authorities try to ensure a ribbon of natural woodland remains along the route of the PCT.
The variety and abundance of flowers increases as you head north. In Southern California the desert flora waits for rain before producing flowers and if you arrive in a wet spell you can expect to see the desert bloom. In a dry spell, however, there will be few signs of flowers. As you head north you will see plenty of flowers in the damper meadows.
Animals and birds
Some of the animals you will see are mentioned in the Wilderness Hiking section of the introduction; others will be featured in the map sections.
Birds have generally evolved separately in the Americas, and Europeans will find few species with which they are familiar. Migration patterns are mainly north-to-south rather than east-to-west and the greatest species-overlap is among waterfowl and waders (shore birds). Notes on the birds you are most likely to see appear in the map sections.
What
