Wednesday, March 30, 2011

Turbid Conglomerates And The Point Reyes Lighthouse

My early–March geology adventure at Point Reyes National Seashore was slowly winding down, ebbing like high tide turning to low. Earlier in the week, I had walked the Earthquake Trail at the Bear Valley Visitor Center, discovered the granites at Point Reyes headlands, unmasked the Salinian Block, and found the Franciscan Mélange at Shell Beach. Next on my travel agenda was a visit to Point Reyes Lighthouse.
By the time I got there the fog was so thick it seemed I could barely see thirty feet in front of me. The 20-mile drive along the peninsula had been fairly clear but as I neared the parking lot a coastal cloud crept in to shroud the headlands. No wonder they put a lighthouse here! 
Foggy Path to Point Reyes lighthouse

There were not many other visitors as I ambled up the path. This paucity of people is one reason I enjoy traveling in the off-season. I like chatting with the odd person here or there, but I am not a fan of crowds.
IMG_5849 PointReyes LighthouseVisitor Center
Visitor center at lighthouse


IMG_5856 PointReyes LighthouseVC
Point Reyes conglomerate and whale remains near visitor center

The lovely rocks peeking out from behind the whale remains are the Point Reyes conglomerate of Early Tertiary age, around 65–55 million years old. It is the oldest marine sediment found on top of the granite bedrock of Point Reyes. Quartzite and feldspar cobbles are found in a matrix of sandstone and granitic rocks. Marine fossils indicate an origin deep in the ocean. 

The similarity of these rocks with some found 100 miles to the south on the Monterey Peninsula suggests that the Point Reyes conglomerate was formed while it was adjacent to Monterey in Early Tertiary time. As the Salinian Block traveled northwest along the western edge of North America, so did Point Reyes and its conglomerate.

IMG_5855Point Reyes Conglomerate
Graded bedding in Point Reyes conglomerate

IMG_5857 PointReyes Conglomerate
Point Reyes conglomerate

The cobbles and sandstones most likely were deposited in a submarine canyon during the slurry of water and sediment of a submarine landslide or turbidity current. As the current flowed down the canyon slope towards the ocean floor, larger coarse cobbles and boulders settled out first followed by finer sediments of sand and then mud or silt.  This formed what it known as graded bedding and is a good indication of submarine landslide activity. If the turbidity deposits are composed mostly of sandstones, the submarine canyon was closer to shore. If the deposits are mostly shale (mud or silt deposits), the canyon was farther out in the ocean.

The time span between landslides flowing down these submarine canyons could be tens, hundreds, or thousands of years; most likely earthquakes were responsible for them.  These outcrops appear to represent several episodes of landslide deposits closer to shore, with coarse cobbles of a later landslide sitting on top of finer-grained sandstones of an earlier deposit.

The fog was not going anywhere as I started down the first of 300 steps. I could barely see the lighthouse so I did not expect any great views of gray whales today.
300 steps to lighthouse

 I watched through my binoculars as a peregrine falcon searched for lunch on the cliffs…
Peregrine falcon

…and later wondered if any surf scoters were likely to be its next meal.
IMG_5890 SurfScoters
Surf scoters enjoying the Pacific Ocean

It took me probably an hour to walk down the steps. Although it was chilly and a bit windy, I was in no hurry.  The fog seemed to be lifting, and there were quite a few little “balconies” where I could pause, out of the path of what little visitor traffic there was. I thought these balconies would surely come in handy on the trek back up those 300 steps!
IMG_5872Windy HairDay
Yowza!  I hoped my toupee would not fly off in the wind

IMG_5869Point ReyesLighthouse
Steps descend the Point Reyes conglomerate

IMG_5879Point ReyesLighthouse
Point Reyes lighthouse
For some lighthouse workers, it apparently was quite a desolate, unforgiving place at which to live and work.  Early logbook entries tell of men going insane while staying out here for weeks at a time.
Remnants of the historic lighthouse operation are still on display.

IMG_5885Point ReyesLighthouse

IMG_5892Point ReyesLighthouse


The upper part of the lighthouse with the lamps and lenses is closed to visitors but anyone can walk around outside. 

The Point Reyes conglomerate outcrops nicely underneath the lighthouse structures…
IMG_5899Point ReyesLighthouse OnConglomerate

IMG_5898Point ReyesHeadlands

 …and southeastward along the headlands for a bit.

It was soon time to scamper back up those 300 steps.
IMG_5905Point ReyesLighthouse Steps

I paused at each balcony and watched scoters surf the waves, buzzards glide on thermals, and gulls gossip on the sea rocks, but I never again saw the peregrine falcon I had watched earlier.
IMG_5908GullsOn RocksAtPointReyes Lighthouse
Hundreds of gulls can be seen on the rocks below

Although the sun did not come out all day, it was nice that the fog lifted and the chill dissipated. I walked slowly back to my car as a few more visitors appeared, embraced in the beauty that is Point Reyes.
IMG_5876 CaliforniaPoppy
California poppy

IMG_5917DeerOn PointReyes Headlands
Deer grazing on the headlands

IMG_5901Point Reyes Conglomerate
Life as Art - Point Reyes conglomerate


Evens, J.G., 2008, Natural History of the Point Reyes Peninsula, second edition, California Natural History Guides: University of California Press.
Sloan, D., 2006, Geology of the San Francisco Bay Region, California Natural History Guides: University of California Press.


  1. I'm lovin these geology posts and learning a lot. 300 steps, hmmm. Wondering, as you scampered back up, if you overheard anyone comment on why the light house is so far from the road. ;)

  2. Yes! The day we met... ah :) You know, it was not too foggy, you could see the lighthouse from the top of the steps! That's saying something! Gosh I miss that narrow strip of rock and those peregrines! <3

  3. The lighthouse was built at ~300 ft. above sea level to be underneath the layer of coastal fog. The top of the headland is ~600 ft.