Monday, December 17, 2007

Mount Talbert and Landslides

Built by volcanic outpourings, carved by ice age floods, Mount Talbert has some very steep slopes. Naturally enough some landslide potential exists. This mountain seems to consist mostly of grey andesitic flows which are quite resistant to erosion. The rock bones of the mountain are mostly near the surface. Only where deep soils have accumulated is land slippage likely.

The mechanics of most landslides are fairly simple. The section of ground destined to slide cracks loose from the remaining slope along a nearly vertical but curved surface and gravity tugs until the slide block rotates downward. The falling mass moves most quickly along the curved surface next to the remaining slope, so the top angles back towards the slip surface. The rotating block begins to break up from the bottom. This process often continues until the block disintegrates, but an equilibrium can be reached, stopping the process - part of the block stops in place, freeze-framing the process at any point. I recently located such a slide on the west side, above some apartments.

Just below the Loop Trail is the sudden drop characteristic of a slip surface. The drop is some thirty feet. A game trail tracked by deer passes over a seepage area onto the hummocky top of the frozen-in-place slide. Many hummocks are made as the slide block breaks up. Some firs rode the slide and survived, slanting strongly towards the mountain. Adjacent trees on different hummocks slant at different angles while those on a single block angle the same. The trees in the decades since this land movement grew upright, bending the trunk.

Park Improvements in West Linn

West Linn Wilderness Park has just been linked to the adjacent Camassia Natural Area with a new trail. The city park and what is The Nature Conservancy's first purchase in Oregon have always been contiguous, but old tracks connecting the two have slowly closed in with poison oak and other shrubbery. The new route starts at the low point of the trail in the section of city park west of Clark Street. Slanting down a steep slope it passes a view of a boggy pond before joining the main trail system on the southeast corner of the scabland portion of Camassia. TNC has provided signage.

Joining these two makes a more useable open space for those wishing a bit of exercise. Both have their unique botanic qualities which complement one another. This rapidly developing area is blessed by these prime examples of nature in the city.

The Wilderness Park features a fir forest with much larger trees than usual. This area is just uphill from the first sawmill west of the Mississippi, and the woodland has been growing back from very early logging of probably 160 years ago. The biggest Douglas firs take three people to link arms around - nearly 5 feet wide and 15 feet around. The largest grand fir are nearly as big.

A hearty attempt to eradicate English Ivy began last summer but less than half the area is done. There are still remnant populations of many native woodland plants so hopefully they can spread once the ivy is gone.

Camassia Natural Area is positioned lower on the slope and has large areas with only a little soil over basaltic bedrock. Massive displays of deep blue camas peak in April, and other flowers, some very rare, add to the spring show. Dwarfed oaks are scattered about, but much of the thin soil area is treeless. Wildflowers in Western Oregon are usually limited by the dense dark forests, but here they grow lustily and provide a solid display in the sunshine.

Licorice fern, Oregon grape and mid-sized oaks at Camassia Natural Area

Google Map of the two natural areas