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Double Bluff

DOUBLE BLUFF - SOUTH WHIDBEY ¾ ACRE

6325 Double Bluff Rd., Freeland

Directions: Turn South off Highway 525 onto Double Bluff Road. Follow to road end for access.

This access has close neighbors.  Please respect private property.  Follow off leash guidelines and rules.  PLEASE - NO AGGRESSIVE DOGS ALLOWED OFF-LEASH.  

Parking is very limited, and it can be difficult to find parking especially during peak periods like busy summer weekends.

Amenities: Beach Access, Cl​amming, Vault Restrooms, Interpretive Signage, Parking, Picnic Area, Off Leash Area​.  Vista Views of South Puget Sound, Seattle, Mount Rainier, Cascade and Olympic Mountains, Kitsap Peninsula and Shipping Lanes.  

Bluffs are on Private Property!

They are unstable and can move without warning!

Please stay off and a safe distance away.

DOUBLE BLUFF GEOLOGY

Courtesy of Grant Heiken

The bluffs at Double Bluff may be the best exposures (outcrops) on Whidbey Island where you can get an idea of the late glacial history of Puget Sound. This sequence is made up of glacial, glacial-marine, and non-glacial sediments that were deposited between 290,000 years ago and 11,000 years ago.

Let's break for a minute for some definitions (From Tucker, 2015, Geology Underfoot in Western Washington, Mountain Press Publishing Company):

Glacial till—A poorly sorted mixture of clay and rock fragments that melted directly out of glacial ice without any further transport. It is massive and rarely stratified.

Outwash gravel and sands—These were deposited by meltwater streams flowing outward from the glacier.

Glaciomarine drift—Looks like till but was deposited from floating ice onto the floor of the sea or a lake from floating ice. It may be crudely stratified. The presence of fossil shells in the sediment provides another clue as to its origin.

The terrain of south Whidbey Island is dominated by north-south ridges called drumlins. Drumlins were left by 4000- to 5000-foot thick glacial ice masses moving south. The deep basins between island like Saratoga Passage and Admiralty Inlet were also carved by the glaciers.  West-southwest-trending Double Bluff cuts across this terrain and the drumlin interiors are exposed above the beach in the 20 to 350 foot cliffs. 

IslandCountyShadedRelief_Markup.jpg

LIDAR shaded relief image of Island County, which highlights the north-south trending drumlins that are most of the county's hills.

Let's take a walk (at low tide):

Once leaving the parking lot (there is a toilet at the lot), pick your way through the driftwood logs and go SSW along the beach. The bluff here is covered with trees and shrubs, so there is little exposure of the sediments.

Thickest Stratigraphic Section and an erratic boulder

At about 0.6 mile cliffs rise about 300 feet above the beach. On the beach at this point is a 10-foot-diameter sandstone boulder (a glacial erratic), which was left about 13,000 years ago as the last glaciers melted and retreated to the north. This fossil-bearing boulder may have been transported by the glacier from a rock exposure south of Sedro-Wooley (28 miles north of here).

The cliff above the erratic boulder is composed of sediments accumulated over the last 30,000 years. It is a little more complex than a simple accumulation in that there were also periods of erosion. 


DoubleBluffCliff.jpg



4. At the top of the cliff is 10 to 20 feet of glacial till left during the last glaciation, 15,000 to 11,000 years ago.

3. Most of the remaining cliff consists of sands and silts, deposited by melted water from the glacier.


2. The well-bedded grey unit, which is about 50 feet thick, consists of silt and clay.  These beds were deposited within a large lake formed when southward-moving ice dammed streams flowing from the east and south. The deposit today acts as an aquitard, a poorly-permeable unit that blocks rain water seeping into the ground and allows accumulation in overlying sands and gravels (the aquifers from which we get our well water). With your binoculars you can see water seeping out from the cliff at the top of these clays. This unit has been dated at about 18,000 years.


1. The lower half of the cliff consists of non-glacial fine sands, silts, and fragments of peat. The sediments were left by streams crossing a floodplain and perhaps a few swamps. This ice-free episode between glacial advances lasted between 39,000 and 18,000 years ago.

On the beach is a glacial erratic boulder (circled)



Contorted "Flame Structures"—How did they get here?

Continue your beach walk WSW for another half mile. The massive sand beds within the non-marine sediments have been intruded by 25 foot high, contorted fine-grained sand dikes. These peculiar deposits were water-saturated sands that "liquified" and rose into overlying deposits. One hypothesis is that this liquefaction occurred during a large earthquake and the lower-density, water-saturated sand rose to the surface. After large modern earthquakes, "sand boils" at the ground surface are the tops of these intrusions. Another hypothesis is that the dikes formed because of irregular distribution of overlying sediment and/or ice.

Continuing West along the Beach

Well-bedded sands are frequently cross-bedded; cross-bedding is typical of sediments deposited by streams.  Again, these deposits are part of a depositional sequence left during an interglacial period.

The last 2000 feet of bluff along the beach before reaching South Bluff is much lower (25 to 100 feet) and consists of an older sequence of non-glacial sediments (Dated at more than 40,000 years old). What is cool about this sequence of rocks is the presence of peat (yes, peat—the same organic sediment used in Scotland to fire the boilers at scotch distilleries). There is a bed of black peat about 50 feet above the beach. It apparently was deposited in a marsh and consists of carbonized plants (mostly horsetails). Slabs that have broken off of this unit are scattered on the beach and look like slabs of wood. Pick up a piece and look closely for plant fossils.

From South Bluff the beach turns north and continues along the shore of Mutiny Bay. I recommend that you turn back here, in that the north-south shoreline is lined with waterfront homes and their private beaches.

Returning to the Double Bluff Parking Lot.

If the tide is very low you can get a great overview of the cliffs above Double Bluff beach. You can also relax with some beachcombing or a summertime swim.

Near where you began at the large glacial erratic boulder there is a scree slope that is popular with those who want to have a scramble. I recommend that you steer clear of this landslide. A couple of years ago, on the Fourth of July, the cliff collapsed and covered the beach with large blocks of sandstone and siltstone. Luckily there was no one present at that time. The landslide actually generated a shock wave heard by people on the beach. Landslides are frequent along the cliffs that line Puget Sound. For people with cliff top property they are a continuing hazard. For the beaches landslides intermittently supply sand to maintain those beaches by longshore drift.


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