Frank’s Beach, Lummi Island: steeply-dipping Chuckanut Formation layers

By Dave Tucker

August 2, 2011

Looking north at Frank's Beach. Click to enlarge any image.

If you are enjoying Lummi Island, stop by the lovely Frank’s Beach to see steeply dipping beds of Chuckanut Formation sandstone and conglomerate. (Aug 5, 2011: I originally called this place ‘West Beach’. I have changed it to the local name. See comment below.) This is a great destination for a bicycle ride with a picnic. The beach is accessed by right-of-way open to the public by the land owners. The beach is mostly nicely sorted cobbles and large pebbles. To the south of the trail access are wave-washed slabs of Chuckanut Formation rocks. The beach is a great place for a picnic or even a swim in the chilly water of Rosario Strait! Then haul yourself out like one of the local seals and warm up. The slabs are a wave-cut bench. I visited them at a 2 foot tide, and there was plenty to see. However, any tide higher than about 4′ would cover most if not all of the rock.

Getting there:

Google map showing the route (yellow line) to Frank's Beach from the ferry dock.

From the Lummi Ferry landing, go south on Nugent 0.4 mi to Legoe Bay Road. Turn west and cross the island to Legoe Bay. Drive along the beach to Village Point- the road now curves very slightly inland. Watch for Skellaham Drive, 2.4 miles from the ferry. Continue on the main road another 0.2 mi. There is a wide shoulder on the water side. The beach access trail begins beside the No Beach Fires sign, crosses a narrow belt of woods, and dumps you out on the glorious beach. Walk south about 200 yards to a large rock exposure poking out from under the cobbles.

For background on Chuckanut Formation geology, visit this page on this website.

Ribbed microtopography at Frank's Beach. Looking south toward Village Point. The cobbles align themselves in the eroded troughs.

The rock beds here are eroded by the waves into a series of low ribs or ridges. Conglomerate and sandstone layers alternate; the conglomerate is more resistant so makes the raised ribs, while the sandstone forms the intervening dips. Both rock types consist of cross-bedded sediment, deposited in the ancient river basin some 50 million years ago. Look closely at the clasts (rock fragments) in the conglomerate layers. Notice that they are mostly all the same sort of rock: dark, with perhaps a few white veins. They aren’t all that rounded, indicating they weren’t carried too very far by that long gone river system. The Chuckanut Formation is not too thick here, and it overlies the Lummi Formation, which consists of rocks that look just like the stones in the conglomerate: metamorphosed sandstones and dark argillite (a low-grade metamorphic rock derived from mudstone or shale). The Chuckanut rivers eroded and picked up clasts from these underlying rocks; the contact between Chuckanut and the Lummi Formation has since been faulted, and is not exposed on the surface. It is just south of here; the east-west shore of Legoe Bay roughly follows the fault.

The right hand rule to determine direction of strike.

The originally horizontal rock layers dip downward to the north quite steeply. The rock beds strike northwest. This is an arbitrary measurement, since you could just as easily say the strike southeast. However, by convention, geologists use the ‘right hand rule’ for consistency. Point your tilted right hand downward so your fingers point down dip. Extend your thumb (right hand, remember?) straight outward along the line of the dipping beds. Your thumb points in the direction of the strike. See another discussion of this concept, and some illustrations of how it works in the field, at the Squires Lake geology hike page.

Looking along strike (dip is steeply down to the right). Matia and Sucia Islands on the western skyline.

Coarse argillite conglomerate bed. We are looking along the strike.

These conglomerate beds cross-cut, or truncate, each other, the result of sequential deposition and erosion in an active river bed. Dip is down to the right of the photo.