Geology field trip to metamorphosed marine lava and sediment at Burlington Hill, Skagit Valley, Washington

Burlington Hill looking south from Cook Road. Click to enlarge any image.

Burlington Hill is a small but conspicuous isolated prominence rising  above the flat Skagit River valley immediately north of Burlington, Washington. Interstate 5 traffic at the Chuckanut Drive exit buzzes by the hill’s western skirt. Sterl;ing Hill is a similar, slightly smallers 2.5 km to the northeast, plainly visible to I-5 travellers near the Cook Road exit. The two forested hills contrast strongly with the flat Skagit and Samish flood plains. The rocks are Mesozoic  metamorphic greenschist and phyllite of the Haystack-Helena Melange. These rocks appear indistinguishable from the better known and older Darrington phyllite and Shuksan greenschist exposed on Blanchard Mountain west of Alger. A short loop drive can be made over Burlington Hill’s 137 m (450′) summit, where you’ll find fine views over the valley. Along the way, you can examine excellent outcrops of these marine rocks. The paved road winds through a residential neighborhood, but there are plenty of street-side outcrops. We’ll examine these rocks and also discuss their anomalous location. This article is based on a visit by Dave Tucker and Doug McKeever on a sunny day in early January, 2014.

Google Earth image looking southeast over the Cook Road I-5 exit.

Looking west from near the switchback on Hillcrest Drive.

Getting there: You can traverse Burlington Hill either from the north or the south. The route is simpler to describe if we begin on the hill’s north side. From I-5 exit 231 (Burlington-Chuckanut Drive-WA 11) head east to a roundabout. Spin around the traffic circle and go north alongside I-5 on Old 99. After a half mile (just shy of a km) turn east on North Hill Blvd through a light industrial center. This street curves south.  At Hillcrest Drive turn left and head up the hill. The rest of the traverse is described below. There are two possible loops. The short loop is 4.5 miles, and returns you to the roundabout by way of downtown Burlington. The longer loop is 8 miles, and returns you to I-5 at the Cook Road exit.

Geologic overview

HHM rocks are purple on this figure from Tabor and others (2000) Snoqualmie 30×60 map. Note the curving trend of the Devils Mountain fault zone.

The Helena-Haystack Melange (HHM) consists of scattered outcrops strewn along the north and east margins of the Devils Mountain fault zone. Similar rocks are found on the opposite side of the fault about 35 miles NE of Mount Rainier (see figure at left). The term ‘melange’ refers to the mix of lithologies in the unit, not necessarily related in age or even provenance (original source), thrown into proximity by intricate, subduction-related faulting.

Zircon mineral grains in some HHM greenstone units yielded U-Pb  ages of 160 to 170 Ma (Jurassic), and radiolarians from chert associated with HHM greenschist on Butler Hill are also Jurassic. Metamorphism is therefore younger than that. Elsewhere, the HHM rocks contain tectonic blocks of early Cretaceous Shuksan Greenschist, so the melange must have been assembled between early Cretaceous and late middle Eocene time (the age of overlying Barlow Pass volcanic rocks- we can examine them on a later field trip).For more on the Helena-Haystack, see Tabor and others (2000) and Tabor (1994). Other examples of the HHM are described in the Bald Mountain/Big Rock field trip east of Mount Vernon.

The field guide

Smooth joints cross cut the foliation planes in Helena-Haystack Melange greenschist at the second switchback on Hillcrest Drive. Hey Doug, don’t spill your coffee! This view looks roughly east. Click to enlarge.

As soon as you begin climbing the hill you’ll come to greenschist outcrops on the left. But, continue up the road a bit further- there are larger and better exposures around the corner at the second switchback. Let’s be clear: these are not beautiful rocks, but they are still interesting. The parent rock was submarine basalt erupted at a spreading center or at a seamount volcano. The seafloor rocks were eventually carried into a subduction zone, compressed and reheated. Olivine and pyroxene crystals in the lava were altered by the low temperatures but high pressure of subduction to green chlorite and epidote, flattened, and realigned during the compression. The result is a foliated, green metamorphic rock called greenschist. (Learn more about foliation elsewhere on this website.) You can see the foliation very well at the second switchback. The foliation is nearly vertical and runs north-south. Following metamorphism, the rocks were cut by parallel joints that run east-west and dip steeply toward the south. Joints often form as rocks expand from decompression during uplift to the surface. If you walk up to the top of this outcrop, you’ll see the rock has been planed off by glaciation to form a wide ledge. See if you can find some glacial striae. Continue up the road. Take the first right and then the second right  to the upper cul de sac- you are pretty much at the summit of the hill. Walk a couple hundred feet past the blue water tower and find yourself beneath the big cross and a big vista to the south, east and west. On a good day you can see far to the south-west across the Skagit flats. You may spot a few small forested islands twelve miles (19 km) away just on the edge of the delta near La Connor. The largest two are Ika (on the east) and Goat. They are slowly becoming surrounded by alluvium as the Skagit delta migrates outward into Skagit Bay. Goat is HHM greenschist, though Ika is both greenschist and some Eocene sedimentary rocks called the Bulson Creek Formation. We’ll visit the Bulson Creek sedimentary rocks another time.

Ika Island south of La Connor is being surrounded by the Skagit delta.

Burlington Hill is similarly surrounded by alluvium. Most of the sediment is outwash deposited by streams running out of the receding great Vashon glacier at the end of the Pleistocene. At that time, the mass of the glaciation depressed the crust into the mantle, and much of what is now the Skagit flats was below sea level. When the glacier melted and unplugged the Straits of Juan de Fuca, the sea flowed back into the lowlands and would have surrounded the isolated Skagit hills. Post-glacial rebound and sedimentation from the Skagit River just south of the hill have now made both hills part of the mainland, and the delta has built far to the southwest. Samish Island is another example of one of these isolated hills, barely attached to the rest of the mainland.

The phyllite quarry on Grandview Court.

When you are ready, drive back down the hill to Hillcrest and turn right. Go right at Grandview Ct. and gape at the big quarry walls. You have crossed out of the greenschist and into a phyllite unit within the HHM. The quarry is fenced, but when I visited with Doug some of the chain link was down and we just waltzed in. The rocks look really different. They are silvery black and shiny due to alignment of microscopic mica flakes. The parent rock for phyllite is fine-grained marine mud. Perhaps the sediment was deposited on top of the basalt lava that became the greenschist, or perhaps the lava didn’t cover this part of the seafloor. There is no exposed contact between the phyllite and greenschist. Dethier and Bowles (1981) mapped a fault with unknown offset separating the two units, which persists on less detailed maps, but it is not exposed and a fault is not necessary to account for the change in lithology. Foliation direction on either side of the contact between the two units is essentially the same. If you like phyllite, check out this field trip to Darrington phyllite.

Foliation is evident in this phyllite.

(Some readers sent comments with  more info on this big quarry. See below, and thanks Readers!)

That about does it for Burlington Hill.  To complete  a loop, return to Hillcrest, turn right and head downhill to N. Skagit St. Turn right to reach Highway 20. Had enough? To complete the short loop option and  return to I-5, turn right on 20 into Burlington and follow signs to I-5. To make a longer loop and get a view of Sterling Hill, turn left on Highway 20. In 1.4 mile (2.3 km) turn left on District Line Road. You’ll drive over Gage’s Slough, an old meander channel of the Skagit River occupied by an oxbow lake (the river is less than 0.5 mile, 0.8 km, to the south) . The forested  hill on the west is entirely private. There is an active rock quarry on its southern tip that mines greenschist for crushed rock and stone walls. Continue north across the flats 1.3 miles (2 km) to Cook Road. Turn left here and head west 2 miles (3.3 km) to return to I-5. Along the way, you can really appreciate the isolation of both hills, surrounded by the alluvial plain of the Skagit River.

Silvery phyllite and greenschist.

References:

Dethier, D.P., and Whetten, J.T., 1981, Preliminary geologic map of the Mount Vernon 7.5 minute quadrangle, Skagit County, Washington. USGS Open-File Report OF-81-105. Scale 1:24,000.

Tabor, R. W., 1994, Late Mesozoic and possible early Tertiary accretion in western Washington State: the Helena-Haystack melange and the Darrington- Devils Mountain fault zone: Geological Society of America Bulletin v. 106, p. 217-232.

Tabor, R. W., Frizzell, V. A., Jr., Booth, D. B.,Waitt, R. B., 2000, Geologic map of the Snoqualmie Pass 30 x 60 minute quadrangle,Washington: U.S. Geological Survey Geologic Investigations Series Map I-2538, 1 sheet, scale 1:100,000, with 57 p. text. [http://pubs.usgs.gov/imap/i2538/]