Whidbey Pleistocene Stratigraphy, Part 2: Blowers Bluff
By Dave Tucker
January 6, 2010
This is the second installment in the series of field trips to glacial and interglacial stratigraphy on Whidbey Island. The thick stack of geologic strata at Blowers Bluff are spectacular! Here is colorful and intricately stratified sediment from Whidbey interglacial times right above the beach, overlain sequentially by the Possession glacial till and glaciomarine drift, thin Olympia interglacial deposits, Vashon advance outwash (Esperance ‘sand’), Vashon till and, at the cliff top, the Everson glaciomarine drift. However, the sequence is in places deeply eroded, with Vashon sediments sitting directly on Whidbey, or even right at beach level. Refer back to the table listing Pleistocene glacial and interglacial units on Whidbey Island.
Unless the tide is way out, the beach is mostly cobbles and boulders, rather than the easy sand at Useless Bay, but the walk is shorter. Use caution and do this trip on a falling tide. Check the tide using the Coupeville table. I visited on a +6 foot tide and there was plenty of beach width. There is a brief description of the geology here in Don Easterbrook’s 1994 GSA guidebook (see ref below). This doesn’t discuss most of the detail seen on the following trip, however, and my interpretations may or may not agree with his. Remember that UTM grid coordinates are in NAD 27, but a GPS is certainly not necessary for this trip. I estimate that the trip to my turn-around point at a wonderful clastic dike is a shade over 1 mile each way.
Getting there: Blowers Bluff is reached from the beach access at Monroe Landing. To get there from the north, drive State Route 20 to Oak Harbor. Turn west at the light at the south edge of the Oak Harbor retail district. Take note of the fork with SW Fort Nugent Avenue, which goes off to the right, signed to “Joseph Whidbey State Park and beach” Stay to the left on 20 and continue 1.5 miles; turn left on Monroe Landing Road. Go 1.8 miles to the end of this road at Monroe Landing on Penn Cove.
From Coupeville and the south, go north on 20 and turn right on Holbrooke Road in the community of San de Fuca, 4.6 miles north of Coupeville. At the next intersection, go east (left) on Scenic Heights Road. Monroe Landing is 1.9 miles further.
Monroe Landing (UTM E0523813 N5342849) is the site of a Skagit village according to the interpretive sign. The town of Coupeville is across Penn Cove. There is a prominent bluff to the right (west) which I didn’t visit, and is beyond a beach signed ” private “. The beach walk to Blowers Bluff goes east; stay on the beach, as the uplands are private. You will shortly need to step across a small stream. In less than 1/2 mile, Scenic Heights Road will be right above the beach; there is access to the beach with some cement steps at the intersection with Park Road. This place, with a bench, is the tiny Penn Cove Park, though there is no place to park. If you are riding a bicycle, this would be the place to start the trip.
For a little terminology review, ‘drift’ is material deposited either directly from ice or from meltwater issuing directly from ice. Drift includes ’till’, which is laid down directly by ice without any intervening water transport, but drift also includes outwash gravel and sand deposited by streams flowing outward from the front of the glacier, and glaciomarine drift (gmd), which is sediment that rains down from floating ice to the floor of the sea or a lake. To be recognized unequivocally as gmd, fossil shells of clams or other molluscs in growth position, meaning intact with hinges closed, and not arguably remobilized from some other deposit, need to be present. These are often pretty scarce and require persistent, diligent searching. All till is drift; not all drift is till.
Take a look at the wave-eroded bank just to the west of these steps (UTM E0524355 N5342829). Note a layer of concentrated broken shells below the top of the bank. I wondered if this could be an old shell midden. The diamict ( i.e. an unsorted mixture of fine and coarse-grained sediment, in this case silty clay and rounded pebbles) on top is only a foot thick or so, but it is reminiscent of drift. Do you think that this diamict is in place, that is, ‘natural’? What accounts for the shell layer? The little park with the bench on top of this bank is sure flat. Perhaps the upper foot or so of the bank here is fill, and the shell layer is indeed a midden or an earlier beach surface prior to post-glacial rebound. Well, this is an insignificant point.
Now, look carefully for scattered shells in the 10′ high bluff a little east of the steps; they are there. The presence of shells in growth position in a diamict like this is a hallmark of glaciomarine drift (gmd), deposited on the seafloor beneath floating ice. As discussed on the Useless Bay field trip, gmd is associated with recessional stages of the Pleistocene ice ages. As the ice sheets melted, sea water was able to flood the lowlands via the Straits of Juan de Fuca and float the thinning ice. The final stages of deglaciation in the lowlands around the Salish Sea usually resulted in gmd deposition. The shells I saw were not obviously intact and in growth position, so this is not undeniable gmd. We’ll return to, and discuss, this deposit again at the end of the trip.
The bank gradually rises, but is mostly covered with ivy and other nasty invasives. There are bedded deposits visible in the base of the bluff, but you’ll see much nicer exposures further along, so don’t get too excited yet. Continue past several private beach access ladders and some rotting pilings, put in to retard beach erosion of the bluffs the houses are built on. Along this stretch, I was followed by two friendly old yellow dogs, but not for long. The bluff has gotten much higher now. When you reach a 10′ high green erratic boulder about 100′ out from the bluff (UTM E0525152 N5343148), note that the layered Whidbey Formation (~100-150,000) makes up the brownish bottom half of the bluff; but what is going on about midway up? There is a sharp, horizontal contact with gray, coarse gravel, which is mixed with large, angular, buff-colored blocks of something that is much
finer grained. This might be a good time to take a beer break. This is, after all, interpretive geology, and as you saw in an earlier post here, beer is an integral part of geological thought. Got it figured out? Continue on east and all will be revealed. The gravel with its mixed-in silt blocks suddenly ends, in vertical contact against a thicker unit of the same massive silty sediment, which is continuous and conformable with the rest of the Whidbey beneath it. Some event eroded away the western portion of the silty Whidbey unit, leaving an abrupt vertical scarp (which we see in cross section) and the gravel and enclosed Whidbey chunks are butted against it. Continue yet further; now you will see that the gravel is now only near the top of the bluff, with a lot of other geology lying between it and that Whidbey layer. Now we have a nigh on complete late-Pleistocene stratigraphy (realizing that the lowermost Double Bluff drift seen at Double Bluff is not present here):
1) the top, or nearly so, of the Whidbey is the prominent 15-20′ thick layer of well sorted, buff silty sand ( or maybe it is sandy silt, I couldn’t get to it). This is overlain
2) by a different looking, 15-20′ gray glacial till or gmd, studded with boulders; this is the ~70,000 year-old Possession Drift, the penultimate major advance of ice out of BC into the Puget lowlands. With binoculars, you can maybe see that there are two units; probably gmd overlying till.
3) Above that, a prominent 20′ unit of fine – grained dark to medium gray brown silt that occasionally protrudes slightly from the cliff face is part of the Olympia interglacial sediment, which has dates in the 22-28,000 year range. The Olympia interglacial is the climatic event that lasted from the close of the Possession until the glacial readvance during the Fraser glaciation. Pollen in peat shows that this interglacial period had a climate and vegetation very similar to today’s.
4) The coarse gravel above is the Esperance ‘sand’, the advance outwash of Fraser glacial times, and at the very top of the bluff is the Vashon drift of the Fraser ice sheet.
The mixed gravel with the big blocks of Whidbey we saw to the west represents some profound erosion- the Possession and Olympia were entirely eroded, probably by the ‘Esperance’ advance outwash rivers fanning out from the much later Vashon ice; one of these powerful streams with a load of gravel cut into a bank of Whidbey silt, which collapsed as large blocks into the shallow river, to be surrounded by the alluvial gravel.
The Whidbey stratigraphy is more accessible further east, and gets more and more interesting to
look at in detail. This is particulary noticeable after you climb over a few small logs protruding out onto the beach from a little patch of trees. I can think of few other places in the lowlands where such a thickness of thin beds can be observed in detail. The beds are varied in color; there is a striking red sandy layer, and at least two buried soils, or paleosols, around 6 and 15 feet above the beach. These are black paleosol (‘ancient soil‘) layers that pinch and swell in thickness, and represent depositional hiatuses, lasting long enough for an organic mat to form. Watch for erosional unconformities in the Whidbey. Technically, the term ‘unconformity’ refers to much larger gaps in deposition, or longer intervals of erosion separating rock strata. A better term is ‘diastem’, a brief depositional hiatus, or a minor, localized eroded contact between sedimentary beds.
There is a vertical, textbook example of a clastic dike at about the point I turned around (UTM E0525605 E5343592). This 6-inch-wide structure runs straight up from beach level into the vertical bluff of Whidbey formation sand beds. If you are watching for it, you can’t miss it. Note that the sand filling the ‘dike’ is vertically-bedded, while the sand and silt beds it cuts are horizontally bedded. The ‘dike’ has vertical, parallel walls. The clastic dike ends abruptly about 25′ up the bluff face, where it appears to widen somewhat at the top. Is this related to seismic disturbance? Did an earthquake fracture the Whidbey silts here, with fluidization in a lower layer sending a gush of sand upwards in the crack? This is the classic explanation for these things. Note that this clastic dike looks quite different from the ones described on the Double Bluff field trip, which are swirled and anastomozing. Somewhere around this spot, a thermoluminescence date of 98 ± 16 thousand years was obtained from clay near the base of the Whidbey.
The bluffs continue for a long way further to the east and north. They look more vegetated, although there are more nice bluff exposures further along. I ran out of time; perhaps you will continue and report back on interesting geology that you see!
On the way back, look again at the shelly diamict in the 10 foot bank a little to the east of the concrete steps at Penn Cove Park. What glacial unit is this? We are well west of the high walls of Blowers Bluff, and there has been an intervening section of houses and vegetated bluffs and generally poor exposure, so we can’t trace this diamict back to the complete sections to the east. This appears to be a natural exposure, as opposed to the thin, probable anthropogenic, fill above the shells beneath the little park. Consider all the glacial and interglacial sediments seen so far. If this is something other than the youngest Fraser unit, which is very widely and nearly ubiquitious in the lowlands, then there has been really profound erosion, and something has even removed the Everson gmd, the last Fraser glaciation deposit. It seems reasonable to assume contiguous lateral deposition from the huge Vashon glacier- this is the case in all the other units at Blowers Bluff except where obviously eroded by later glaciations.
Turns out this is Everson gmd, filling a depression eroded in the earlier Olympia, Possession, and Whidbey Formations. The Vashon ice was thick here, 4500′ by Don Easterbrooks’s reckoning. That provided plenty of erosive power to remove lots of soft sediment. Turns out there is a 14C date from shells here: 11,850 ± 240 years, right in the middle of the range for Everson gmd. So, it was the Fraser ice that stripped out the lower units here, leaving this behind at a relatively low elevation as it melted.
Next stop: Swantown and West Beaches, for a look at Vashon stratigraphy, perhaps a tsunami deposit, and efforts to retard beach erosion. Subscribe to this blog so you’ll note when the trip has been posted- use the subscription widgett at the upper left of any page on the blog.
Easterbrook, D.J., 1994, Stratigraphy and chronology of early to late Pleistocene glacial and interglacial sediments in the Puget Lowland, Washington in Swanson and Haugerud, editors, Geologic Field Trips in the Pacific Northwest, volume 1: Geological Society of America, p. 1J 1- 38