Layers in the Chuckanut- a walk to visit conglomerate, coarse- and fine-grained sandstone.

With special acknowledgement to my geologist buddy, Scott Linneman (WWU), my friend Grace who asked questions and waited patiently for us to come up with a good story, and Scott’s two black dogs, Aggie and Archie- they are entertaining and also good for scale in pictures!

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Change in sedimentary facies occurs as conglomerate rich in 3-inch cobbles rapidly transitions to much finer-grained, slightly younger sandstone. This indicates a decrease in energy in the stream that deposited the sediment. A boisterous river rolled the cobbles along its bed while carrying off sand. Later, the energy rapidly decreased and sand was deposited, burying the cobbles. The stray cobble near upper right may have rolled off a bank into the sandy river bed.

This is a 3 mile round trip hike on a gated logging road, open to walkers. The road is lined with outcrops of Chuckanut Formation rocks. It offers a great opportunity to see a variety of sedimentary facies within the Chuckanut, from remarkable boulder conglomerate to fine-grained and thinly layered sandstone. All the rocks dip northward, mostly, meaning the layers are tilted. Some are nearly vertical. This is the best place I know to see multiple sedimentary facies changes within the ca. 50 million-year-old Chuckanut Formation. Long ago I wrote about tilted Chuckanut rocks, and how they are described. Questions? Please leave a comment.

Note- biology happens. Trees and brush along the road may obscure outcrops, then be removed by road crews or an enterprising geologist. Ahem. Geology happens, too. Raodside exposures change appearance as slabs of rock fall into the roadside ditch. So if you don’t see what I describe, don’t think you are crazy. I hike this road regularly for fun and exercise so will try to keep this guide up-to-date.

GETTING THERE: This road hike begins near the I-5 North Lake Samish Exit 246. If you drive:  from the north (Bellingham) exit and turn right. Go past the convenience store and take a left on Old Samish Road, just before the freeway overpass. From the south (Burlington), exit and turn right on Samish Way. Cross over the freeway and turn right on Old Samish. Everyone– once you are on Old Samish, go 0.25 mile to a gravel road with a yellow gate on the left. Park here; there is room for three cars max. There is no place to park on Old Samish itself. Please do not block the gate.

Chuck Rd map

Red line is the road you will walk. The yellow scale line is 1/4 mile long.

On your bike from Bellingham, ride out Old Samish 4.25 miles from the intersection with Chuckanut Drive.

THE HIKE: This road climbs steadily but not too steeply for the first 1/2 mile, reaching a saddle above Lake Samish. It then bends around the nose of an east-west ridge of resistant Chuckanut Formation rocks and traverses the south side of it. It runs through young clear cuts virtually the entire way, and is mostly south facing. On a sunny day it can be pretty warm so be prepared. Rock outcrops are pretty continuous beyond the 3/4 mile mark. The last short stretch is downhill to a quarry. Return the way you came. The adventurous, seeking a longer hike, can continue beyond the quarry on a road that circles around the west side of the ridge in forest, up to the microwave towers at the ridge top. You can return to the road via a mountain bike trail. All of this shows, including the bike trail, quite clearly on Google Earth. The summit towers are at 48.681305° and -122.413139°   .

Outcrop Map

Outcrop map of Chuckanut Formation (CK), courtesy of George Mustoe, WWU. The large unit in the lower right is the correlative Swauk Formation (SK).

A bit about the geology: Quite a number of trips on this website visit the Chuckanut Formation; it is the local bedrock around my home in Bellingham. For a bit of background, the Eocene Chuckanut Formation consists of a phenomenally thick sequence (9000 m!) of stream-deposited sandstone, conglomerate and mudstone, originally deposited in flood plains in subsiding basins near the coast of Washington- or at least, where the coast was around 50,000,000 years ago. Sediment sources were the highlands of the Rocky Mountains and the southern interior of  British Columbia.  As millions of years passed, rivers  meandered across the landscape, and the sediment they deposited fluctuated back and forth through a range of clast, or particle sizes, from fine sand and silt when rivers were relatively sluggish to boulders when rivers were faster- and back again. The transition between layers with different clast sizes in the Chuckanut is the facies transition. This transition represents differences in the depositional environment and sediment sources. Depositional environments ranged from fast streams to placid, organic rich swamps.  This variation in clast sizes and the transition between sedimentary facies is a main focus of this hike.


Geologists like to categorize things. How big is a boulder? 256 mm is 10 inches.

The climate in the Eocene was warm and moist- this area was covered with subtropical forest. Leaf fossils are common in the Chuckanut, and swampy areas eventually became coal deposits. Deposition was prior to rise of the Cascade volcanic arc or subduction-accretion of the Crescent Terrane (broadly, the Olympic Peninsula rocks). Uplift of the Cascade Range effectively shut off the sediment supply. Docking of the Crescent Terrane deformed the Chuckanut sedimentary rocks. Most people know that Chuckanut rocks are found in west-central Whatcom County. However, scattered units of these rocks also extend along the Darrington-Devil’s Mountain fault zone through Skagit and Snohomish Counties. A succinct introduction to the Chuckanut is “Geology and paleontology of the early Tertiary Chuckanut Formation” by G. Mustoe, R. Dilhoff, and T. Dillhoff, 2007.

Chuckanut folds LiDAR with road and labels

Here is a LIDAR image showing the great curving fold of the Chuckanut Anticline. The heavy black line is the axis of the fold. The originally flat-lying layers of the Chuckanut Formation have been folded and arched upward. The whole fold has itself been tipped downward to the northwest (upper left) so that a given hypothetical layer can be represented by the yellow line. The field trip is along the road shown in red. Interstate 5 shows clearlyacross the top of the image. Bellingham is just off the  the upper left corner. Scale- about 4 miles from east to west. To see more LIDAR imagery go here

The Field Trip: Walk up the road for a quarter mile. Go around a bend and see the first outcrop- a dark north-facing wall that will not impress you at first.

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Stop 1.

This is massive sandstone, with no evident layers or features other than fractures. But if you look carefully at the extreme right edge of the outcrop, you should see some steeply tilted thin layers. So this whole mass of originally flat lying rock is actually tipped to the north, toward you. The fractures are ‘joints’, developed in the rock as the whole mass of the Chuckanut sediment was folded. These rocks weren’t buried very deep in the crust then, so they deformed in a brittle fashion. Fractures formed in order for the brittle rock to accommodate the compression and deformation. Also look for 2″ diameter holes

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Scott points to a drilled shot hole. Note the radiating fractures.

drilled into the rock. I spotted 4 or 5. These remain from blasting to remove rock and allow the road to sneak across the hillside here. One of these holes has large fractures radiating outward, so you can see how the rock was broken up during blasting. One reason to stop here is to compare the rock in the outcrop with the rock used to make the road (also known as ‘road metal’). They are very different.

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Road metal at stop 1. Not Chuckanut. I don’t know for certain where this rock was quarried. The Siper Quarry north of Nugents Corner produces rock like this- metasedimentary rocks that are 150,000,000 years old or so. They were once on a seafloor, subducted beneath North America and accreted to the margin of the continent. Note the thin quartz veins. None of that in the Chuckanut.

Onward. A couple hundred yard along note a smooth brown slab on the left beside the road. It is covered in shallow, subtle scratches parallel to each orther, running north-south. I believe these must be glaical striations, left behind when the last great lobe of ice flowed south; it buried the Chuckanut hills entirely in ice. There are other, deeper gouges but these are probaby left by road machinery. Soon you will pass a junction with a secondary road off to the left- continue straight up the hill.. At the next sharp bend to the right, note the angular Chuckanut blocks to the right Some of these are nearly car size. Where did they come from? I puzzled over this with my geologist friend Scott as we worked on this field trip. There is no nearby cliff to be blasted from. Look at this place on Google Earth. 48.683774°, -122.401961° You will see they are only found here. There is no evident glacial moraine here. We could only conclude that they were dumped here during road construction.  At the top of the next leftward curve, a bike track enters on the right. This goes up to the top of the ridge crest and would get you to the fine views on the upper road. If you did the loop from the quarry at the last stop on this field trip, you would come out here. Cauthion- this is a mountain bike trail. It is steep. Watch for zoomers! Not to be done in wet weather- very slick.

Just beyond is the start of nearly continuous outcrop along the road for most of the remainder of the hike. This is Stop 2. We are walking around the east end of this ridge. When you face the roadside outcrop here you are looking westward, so now the northward dip of the layers becomes apparent. As energy in the stream that carried and deposited the sediment fluctuated (floods, changes in rainfall, changes in the material

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At Stop 2, looking west. My yellow notebook rests on a large cobble surrounded by sandstone. You can see crude layering of the conglomerate facies. The beds here are pretty obviously dipping downward to the north- into the slope. (‘Dip’ direction is the direction layers are tilted toward.)

eroded by the stream) the size of sediment carried also varied. So, the sediment preserved in the Chuckanut varies as this energy waxed and waned. This is the variation in sedimentary facies we are here to see, in the road cuts for the next half mile or so.

Mosey along. The road soon makes a bend. Fifty yards beyond is a distinctive high wall at Stop 3, see the photo below.

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My friend Grace at Stop 3, in front of conglomerate interlayered with sandstone. Another fine example of changing facies within the Chuckanut Formation. There is a rapid transition above the shadowed overhang, where sandstone predominates higher on this wall.

We have walked around the end of the ridge and are now heading westward, looking into the dip of the layers. They have been tilted during folding and now angle downward away from you into the hillside, at about the same angle we saw at Stop 1, where they angled downward toward you. The dip of the beds here is accentuated by the changing facies. In many other places the Chuckanut Formation is massive sandstone, many tens of feet thick in outcrops, and the tilted layers are not so evident. The high wall is pale sandstone, with a few intervening layers of conglomerate that reflect short-term facies changes- a quick flood perhaps. The lower part of the exposure here is almost entirely cobbly conglomerate and with thin interbeds of sandstone.

The road is more or less straight for a bit. Don’t forget to admire the view of Lake Samish to the south, in case you actually have a view, as we did:P1040814 rsz markNote the sandy soil on the slopes- this is what happens to sandstone when the cement holding grains together is weakened during weathering. After a few hundred yards the road makes another swooping curve south, a mile from the yellow gate. Look for some boulders in the roadside bushes on the right at this bend- the brush has grown up a lot since I orginally wrote this and the boulders are not as obvious. This is a cluster of glacial erratics; none of them are Chuckanut Formation. You may not spot the one in the picture below. It has a pale weathered surface, which makes it difficult to figure out what it really is. P1040813 rsz mark

It is very hard- I had to really whack it with my rock hammer to get inside the weathering and break off a chip to see what it really looked like under the weathering. Here it is:

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The weathering rind on the erratic is about 5 mm thick.  Freshly broken rock inside is very different from the outer surface- a very good illustration of why a fresh rock surface is needed to truly describe a rock.

The fresh rock is dark greenish gray inside. There are a few dark mineral grains,  1 mm or so across. This is a slightly metamorphosed basalt or perhaps andesite. When hot water penetrates lava flows on the seafloor as they cool from eruption temperatures, the original minerals content is subtly altered to predominantly green minerals like chlorite. There are fine examples of these greenstones up the Nooksack just beyond Glacier. See the Wells Creek Volcanics field trip I wrote up a decade ago. However, given what we know about flow directions of Pleistocene glaciers, this boulder came from somewhere out of British Columbia rather than up the Nooksack.

Continue through the next couple of bends. Stop at the outcrops below where the uncut trees along the edge of the clear cut reach the top of the bank along the road. I noted two interesting features here at Stops 4 and 4a on the map above. At Stop 4, there is a very sharp contact between sandstone and a very sudden facies change to conglomerate. If you see  a few 3″ tree stumps next to the road you are at the right spot. They mark the precise spot you should be looking at.

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Scott points to the contact at Stop 4. The rocks here dip 50 degrees to the north.

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This is a knife-sharp change from one form of river deposition to another, and the contact is straight. There is no transition from deposition of sand on the river bottom to cobbles, as I would expect. We haven’t seen this sort of sudden transition elsewhere on our geo-hike. I think this sharp facies change could be due to one of two things. This could be a minor erosional unconformity, formed when river currents stripped away the gradual facies transition to produce a locally planar surface, and then a load of gravel came down the river. Or it could actually be due to a minor fault, which slide these two subunits of the Chuckanut into contact from a few feet or yards apart, destroying the transition. The fact that there is a visible and planar ‘parting’ between the layers lends a bit of credence to the faulting hypothesis. But it is a very small exposure and I couldn’t see any shearing features- which we will see at the quarry at the end of the field trip- 0.3 mile from here.

The dip of the rocks along our walk today is pretty consistent- down toward the north. The angle of the dip varies, but this is because deformation of the Chuckanut produced folds within folds. Also the folding was not in the form of smoothly curved folds as you would get bending in soft clay. The rocks were shallow in the crust and brittle. The folding was actually accommodated by a myriad tiny fractures and very local shearing along faults, as you might get if you let that clay dry out for a while before you bent it. As a consequence, angles of dip are not consistent.

Stop 4a is just 50 feet further. We are in sandstone now, with very little conglomerate. This stop has been radically changed. The block Scott is pointing at collapsed in May 2023 and moved across the road by a bulldozer. It is no longer useful for measuring dip. But the method described below may interest you. I’ll find a nother place along the road where we can try the technique. Patience please.

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Scott points out a very steeply dipping, smooth sandstone surface we will use for a measurement of the angle of dip. Gone now.

This is a good place to explain how geologists measure the dip of layers in the rock, the better to understand the amount of deformation that has occurred and to assist mapping out the extent of geologic rock units, or formations. It is also useful in the mining industry to predict where and how deep a coal seam might be intersected underground by a mine shaft or tunnel. The angle and direction of dip is measured with an inclinometer combined with a compass- geologists generally use a ‘Brunton Compass’ to do this.

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A Brunton. The upper circle shows the inclinometer level bubble. The lower bubble shows the vernier angle indicator.

The flat edge is placed against the surface whose vertical angle (this is the ‘dip’) you want to measure. First off, you need to have as smooth a surface as you can find.

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You may need to average out bumps using your field book, and place your Brunton on that. Then you rotate a lever on the back of the Brunton until the level bubble (circled) is…well.. level. These rocks are dipping very steeply…the steepest we have seen along this road.

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Read off the vernier scale at the bottom. Lousy photo, sorry. But my field notes say the dip is 88°. Once you get to dips this steep you have to wonder if these are rocks actually overturned beyond vertical by extreme folding. That takes a bit more sleuthing and we won’t go into that here. My gut tells me that these rocks aren’t overturned.

The dip angle is recorded in field notes and on the map. Also the ‘strike’ of the rock layers, but I won’t get into that here. Again you can visit the ‘sedimentary rock structure‘ page for that.

Just past Stop 4a the clearcut edge crosses the road so that there are larger trees along both side. Which provide some welcome shade on a hot day. Stop 5 is 300 yards further, and about 100 feet downhill. Go past a dead end spur to the right, and stop at the rocks at the junction with another spur branching very sharply back to the west. The Chuckanut Formation here is thinly bedded sandstone.

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Stop 5. Note the overhang on the right. This marks the base of a bedding plane. The rock beneath has collapsed. This rock dips much more shallowly then at Stop 4a..

The sandstone on the surface here has been weathered to the usual tan color. But freshly exposed, unweathered sandstone in the Chuckanut is gray. In this particular place we also see yet another facies change, this

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Thinly bedded sandstone at Stop 5.

time within the sandstone. The Chuckanut here consists of thinly-bedded layers, representing successive layers of sand on those 50-million-year old stream bottoms. It takes pretty quiet water for this, without erosion of earlier beds by swirling water currents. Unfortunately, these rocks are just a bit too coarse grained to preserve the leaf fossils that are found in finer-grained Chuckanut shales, deposited in the calm water of ponds. I didn’t see any shale along this road.

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Weathering change at Stop 5.

The other attraction at Stop 5 is the extreme difference in weathering at this outcrop. Water (rain, snowmelt) percolating into these rocks dissolves cement, leaches some minerals right out of the rock, and chemically alters other minerals, such as feldspars, to clay minerals. Leaching changed the color, and the other weathering effects greatly weaken the rock. Note that this entire outcrop is tan except for the small bit revealed at the little collapse. The depth of weathering is also very well defined. Either water can enter the rock, or it can’t. The very same layers that are gray and intact are, only inches away, weakened. The layers are broken up into chips, some of which you can crumble in your hands, and the chemical effect on darker minerals is striking- there aren’t any left.

Stop 6, our last, is another 300 yards, at a quarry. BEWARE!! This is a periodically active quarry. Watch out for (usually yellow) detonating cord. Small fragments may be found lying around. It is a thin, flexible

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Detonating cord fragment. This is left over after blasting. It isn’t connected to a blasting cap or a source for an explosion. Generally just leave it alone.

plastic tube usually filled with pentaerythritol tetranitrate (PETN, pentrite). It is detonated by electric shock. Just don’t touch it, OK? On one visit, we saw a long strand of it attached to a blasting cap hanging down the back face of the quarry.

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Detonating cord hangs down a wall in the quarry. Note the boulders- these are over 1 foot across.

But we came here to see the fantastic conglomerate. Here is a remarkable facies change. I think this is the coarsest and thickest conglomerate I have seen anywhere in the Chuckanut Formation. Some of the clasts here are boulders, not pebbles and cobbles we saw further east. Why was this material deposited here? Where did such big stuff come from? What kind of river could transport these big things? For one thing these are all pretty well rounded, meaning they have been carried quite a long way from wherever the rock was originally exposed to erosion. And they are pretty hard- boulders of Chuckanut sandstone, for instance, wouldn’t survive much river transport, bouncing along a river bottom and banging against other rocks.

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Twelve inch boulder eroded out of the conglomerate.

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Its not all boulders here. My GPSr is 160 mm (6 inches). The clasts in this conglomerate are cobbles, as we have seen all along the road. Note weathering rind at upper left.

Two other things you can see in this quarry. Look around for masses of black organic

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Coal in the conglomerate at Stop 6.

material, usually part of a block of conglomerate. This is coal, from chunks of wood buried along with the river sediment. If you are really lucky you might find a large piece of very dense coal, like I did. So here we have yet another sedimentary facies, though

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This chunk of coal has weird anastomozing light gray features. I think these were layers of very find silt, or maybe clay, buried in the organic mat that became the coal, and easily squished and deformed under the weight of later sediment.

represented by only isolated blocks rather than layers in the rock. The Chuckanut Formation is known for its coal deposit, actively mined into the middle of the 20th Century. This rock represents swamps or wetlands where thick accumulations of rotting leaves could build up. These organic deposits became coal under pressure. And that’s a whole other Chuckanut story, best told by my friend George Mustoe. Read it here.

One last thing here. The Chuckanut conglomerate shows evidence of faulting as the formation was folded and deformed. Remember I said that the rocks were shallow and brittle as that occurred. Well, some of the rock slipped past adjacent rock to accommodate the folding, leaving slippery, smeared out slickensides from friction as the rock on either side of the fault plane sheared. You can find plenty of this on the sides of conglomerate blocks here. P1040829 rsz markP1040833 rsz mark

From here, you can walk back the way you came, 1.5 miles. Or make a loop in the forest on a road for 0.8 miles to the ridge crest. Turn uphill (right) at a junction a couple hundred yards beyond the quarry. Reach a second junction just as you enter the clear cut again near the ridgecrest. Turn right to go to the microwave towers (no views) or left 150 yards to the top of the mountain bike trail descending to the road near Stop 2. Elevation gain from the quarry to the ridge crest is 400′. Oh, and enjoy the 360 degree view!

On your way back, ponder this. What once lay above the Chuckanut? I mean, these rocks are 50 million years old. Something happened after that. The Cascades rose up, streams flowed west toward the sea, so presumably younger rocks covered the Chuckanut Formation. What? No clues, really. All stripped away by even more recent streams, not to mention the profound landscape-changing effect of the six continental glaciations that have rasped away the surface over the past million years.

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