By Dave Tucker, with information from Jean Maier

May 6, 2011

Reader Jean Maier of Seattle reports on a trail-side granodiorite erratic in Seattle’s Ravenna Park. Granodiorite stones are very common in northwest Washington’s glacial deposits. They most likely all came to the Salish Lowland via the Cordilleran ice sheets that descended the Fraser River out of the interior of BC, along the eastern margin of the BC Coast Mountains and west edge of the Cascades. Probably all of these erratics are from the final advance, the Vashon, which sat over the Seattle area for only a couple thousand years, between 18,000 to 16,000 years ago. The maximum ice thickness was around 3000 feet at Seattle. Here is an animation by Ralph Haugerud of the USGS of advance and retreat of the Vashon ice.

Granodiorite is very common in the BC Coast Mountains. However, ice that came down the west side of that range into the Straits of Georgia appears to have mostly turned west and gone out the Straits of Juan de Fuca, rather than south into the Puget basin. There is plenty of g’dio in the northern Cascades. However, for any of that rock to get to Seattle, or anywhere in the lowlands, a glacier would have had to descend one of the major rivers, such as the Nooksack, Skagit, or Skykomish, joining up with the main glacier lobe in the lowlands. The conventional wisdom based on distribution of erratic pebbles in till is that the Vashon glacier’s Salish lowland lobe sent arms UP these valleys, rather than glaciers flowing down them. The northern part of the range was eventually overtopped by thickening ice flowing directly south over the border. Consequently, Cascade rocks are not usually documented in lowland tills. For more photos of the erratic, including closeups, visit Jean and Doug’s photo page.

Getting There:

I-5 Northbound:

Exit at NE 65th St.

Turn right on to Ravenna Blvd to 20th Avenue NE.

Turn left on 20th Avenue NE.

The parking lot is on the right hand side of the street on the corner of NE 58th St and 20th Avenue NE.

I-5 Southbound:

Take the N 85th St/NE 80th St exit.

Follow the sign to NE 80th St.

Go east on NE 80th St to 15th Avenue NE.

Turn right on to 15th Avenue NE to Ravenna Blvd.

Turn left on to Ravenna Blvd.

Take Ravenna Blvd to 20th Avenue NE.

Turn left on 20th Avenue NE.

The parking lot is on the right hand side of the street on the corner of NE 58th St and 20th Avenue NE.

Jean says: we usually park at the hidden lot at the end of 20th NE to the south of the (pedestrian / bike) bridge. Walk across the grassy play field, and take the left hand trail and plunge down into the ravine. Go left along the upper ravine trail (toward the NW), pass under the bridge, and then to the right are steps heading down to the boulder.

For a longer walk through the ravine, find your way to Cowan Park on Brooklyn Avenue- the main access to Cowan-Ravenna Park that winds through the park begins here, north of the intersection with NE Ravenna. Follow the main path which goes from Brooklyn Ave NE all the way to NE 55th through the bottom of the ravine. The boulder is just to the west of the 20th Ave NE

Bridge.

There are two overhead bridges as you go along the trail. If you follow the path from the Cowan Park / NW side you go under the 15th NE Bridge, and then there will be the boulder off to your right, just before the 20th NE Bridge.

If you come from the SE side (NE 55th), the first bridge is the 20th NE, then just to your left will be the boulder.