This is one of three caches which are 'virtual docent hikes' through Edgewood Natural Preserve, presenting the natural history of the park to geocachers. Their information is based on the guided tours given by the Friends of Edgewood during the spring and summer wildflower season. This cache features the geology of Edgewood.
The cache creators were members of the Edgewood docent team, and invite you to join one of the 'real' docent hikes in additional to seeking this cache. The other caches in the series are Perennials and .PurplePeople's Posey Perambulation. With some careful planning you can do this cache and 'Perennials' on one hike, with one or two backtracks depending on your starting point.
Access for this cache is from ample parking at the listed coordinates, via the Edgewood Trail. The stages can be done in any order, and can also be reached via the Clarkia Trail, starting from more limited parking near GCRZZT, or the Sunset entrance, near GCNWBA. The best time of year to enjoy Edgewood's wildflower displays is mid-March through May, but the rocks are there year-round. NOTE: Everything on the Edgewood preserve is protected, including the rocks. Stay on trails, and take nothing but pictures and notes.
Waypoints:
1. N 37 27.857 W 122 17.833 The geology of the Bay Area in general, and Edgewood Park in particular, is dominated by faults that edge the Pacific Ocean. Standing here, you are on the North American continental plate. The ridge to the West is on the edge of the Pacific Plate, which is being shoved along and under the North American plate by continental drift. This subduction takes place along the San Andreas fault, which lies between you and the ridge, in a valley now occupied by San Francisco's water supply. One sign here will introduce you to Edgewood Park, and contains a map. How many tenths of a mile is it from this location to Ridgeview Trail? . Write it down as 'F'.
At Post 14 go left a bit to:
2. N 37 27.979 W 122 17.391 Faults often bring subsurface water to the surface. The water-loving riparian vegetation here - willows, sedges and rushes - is made possible by seasonal springs along the base of the Central Ridge ahead of you. Contrast the dry land vegetation you will see on the remainder of the walk. Here you will also find a 'paddle' type sign at the edge of the trail. It will have one letter and a two digits. Add the 2 digits to the letter's position in the alphabet,and write down the result as 'A'.
Return to Post 14 and continue on the Sunset Trail.
3. N 37 27.852 W 122 17.189 Look near the top of the Central Ridge to the East, between here and waypoint 4, and you will see patchs of exposed reddish soil. This tells you that the underlying rock is greenstone. Huh? Greenstone contains iron, and the red results when it weathers and oxidizes. The Central Ridge is a plug of greenstone that was forced up through the surrounding rocks by the fault. The soil here was eroded from this Central Ridge, and supports growths of tall (non-native) grass and shrubs such as the silver-leafed bush lupine. Look around for another of those trailside paddles. Add 5 to the number you find and write it down as 'B'.
4. N 37 27.792 W 122 17.034 The small watercourse just to the West follows a fault trace, an offshoot of the main San Andreas fault line. The boulders exposed in the ditch behind you and in the field to the East are serpentine, a mineral found only near faults. It is formed deep in subduction zones, and contains metals such as magnesium and chromium. Serpentine is malleable – for a rock – and is squeezed to the surface along the faults. The resulting sinuous folded layers of stone give serpentine its name. When it weathers off, the resulting metal-rich soils are inhospitable to non-native plants that did not evolve to cope with them. Compared to the coverage at station #3, the imported grasses growing on serpentine soils between this station and #5 are stunted, and offer less competition to the native wildflowers that bloom profusely in the spring, giving us a glimpse of the California flora as it was before settlement. Once again, find a trail-side paddle. Subtract two from the number, and write it down as 'D'.
5. N 37 27.730 W 122 16.699 Look for chips of bluish rock at the edges of the road nearby. This is glaucophane schist, a metamorphic rock formed from shale when it is compressed and heated in the depths of a subduction zone. If you have a hand lens, you may be able to see small crystals of garnet embedded in the schist. Count the number of individual hand out bins attached to the kiosk here. Write that number down as 'E'.
Backtrack to Post 20 and take the Serpentine Trail.
6. N 37 27.827 W 122 16.720 From here you have a great view to San Francisco Bay and on a clear day to the East Bay hills. The Bay is a valley that was drowned when sea level rose after the Ice Ages. The distant hills were produced by the Hayward fault, which runs at their base. You may be able to see the heights of Mount Diablo in the distance. Like the Central Ridge, Mount Diablo is a (much larger) plug of rock forced up through the surrounding strata by fault pressure. Here you will also find a newly dedicated bench. Count the number of letters in the 1st name of the dedicatee on the sign, and write that down as 'G'.
At Post 19 go left onto the Live Oak Trail, and at Post 17 head up the hill to the summit.
7. N 37 27.835 W 122 16.831 You are now standing on 'Inspiration Heights', the southern end of the Central Ridge. Nearby, you may see gopher castings whose color again shows the red of oxidized iron from greenstone. However, the soils below to the east, west, and south are all serpentine. At the right time in spring, you may see them carpeted with wildflowers. Here's a convenient bench where you can sit and figure out the final location. Before you do, count the letters in the first three words of the quotation. That's 'C'.
Return to Post 17 and take the Ridgeview Trail to the Sunset Trail.
6. Final: The cache is a cameoed nut can, located at N 37 2A.BCD W 122 17.EFG. Remember that you gathered the letters in a different order! The final is within a few steps of an access route. If you find yourself having to climb, or enter serpentine grassland, you have the wrong access or the wrong coordinates. There is poison oak near the final, but it can be avoided.
You can validate your puzzle solution with certitude.