Sunday, February 18, 2024

New Plates

 Just a short post today...

The Hudson Valley Geologist's wife bought me new customized license plates for Valentine's Day. I love them!

Sunday, February 11, 2024


If you frequent certain areas of social media, or visit new age mineral shops, you may have seen stuff touted as obsidian that is nothing but colored glass.

Real obsidian is volcanic glass. It's formed from volcanic eruptions when lava (molten rock) cools so quickly minerals don't have time to nucleate and grow. Most obsidian is black and glassy with distinctive conchoidal fracture.

Conchoidal fracture can be seen in the sample above and is the curved, shell-like way obsidian (and other glass) breaks leaving sharp edges. Native people around the world took advantage of this fracture pattern to use obsidian to make knives and spear tips.

Some obsidian has inclusions of a white mineral called cristobalite (a form of quartz) leading it to be called snowflake obsidian which is very pretty when tumbled or carved.

Mahogany obsidian has a brown coloration from some trapped iron oxide in the rock.

There are also some rare varieties of obsidian (sheen, rainbow, or fire obsidian) that show a metallic sheen or colorful iridescence. This is usually from microscopic gas bubbles or different mineral inclusions that reflect various wavelengths of light.

When visiting metaphysical shops that sell crystals, as I'm wont to do out of curiosity (I usually don't buy from them because I can go to rock and mineral shows and buy genuine samples at a fraction of the price), I have started seeing a lot of colored glass being sold as "obsidian". Here are some examples from Etsy.

While marketed as obsidian, and sometimes priced at hundreds of dollars, these are really just hunks of colored glass. They're usually formed as a by-product of steel manufacturing and found in industrial dumps. One variety that I know of is artificially manufactured from Mt St Helens ash (Helenite) and sold as natural. It about as natural as your kitchen window.

These are often marketed as Andara crystal, blue, green, or red obsidian, Gaia obsidian, Aqua Lemuria, and other made-up names with sometimes outrageous magical claims associated. Break a green beer bottle if you want some green glass - it will have about the same healing energy as these "crystals" and you can drink the beer beforehand.

Sunday, February 4, 2024

Wallace Creek and the San Andreas Fault

 This post is about an area a little far afield from the Hudson Valley but about a geologically significant area - it's featured in the lab manual I use for my Physical Geology course at SUNY Ulster.  It's about a place I visited on vacation a couple of years ago.

Wallace Creek is on the Carizzo Plain halfway between Los Angeles and San Francisco, east of the Coast Range, and in a dry, arid desert environment. I drove 7 miles or so down a dirt road to get there. (why I own an all-wheel drive vehicle with high clearance!).

What makes Wallace Creek so interesting is that it crosses the San Andreas Fault - the boundary between the North American tectonic plate and the Pacific tectonic plate. The Pacific Plate is trying to move northwards with respect to the North American Plate and, after stresses build up for a while, it eventually slips along one segment or another generating an earthquake (sometimes a large one).

On January 9, 1857, this segment of the fault actually moved 6 meters (20 feet or so) during the 7.9 magnitude Fort Tejon earthquake. As the fault moves over the years (centuries), the creek has become offset where it crosses the fault clearly showing the direction and magnitude of movement.

Here's a nice aerial view of the offset.

This view is looking south. On the left is the North American Plate and on the right is the Pacific Plate. You can clearly see the offset of Wallace Creek in the center of the photo with the Pacific Plate moving north (toward the bottom of the photo). The fault scarp of the San Andreas is clearly visible.

What's it look like on the ground? Below is the interpretive sign at the parking area.

Here's a look back to the parking area (two black dots) from the creek. My wife and stepson stayed in the car with the AC running because it was around 100 degrees F (the Hudson Valley Geologist is made of sterner stuff and will suffer for geology). The white in the distance is a soda (alkaline) lake bed. The path to the left follows the fault scarp of the San Andreas Fault.

The feature is a bit difficult to see from a ground picture, but the creek is running away from me here (it's dry but marked by the greenery in the ditch). In the distance it bends left (onto the Pacific Plate) and in the foreground where I'm standing it turns right onto the North American Plate (off the picture). Looking straight ahead (north) is looking along the axis of the San Andreas Plate.

After visiting Wallace Creek, we stopped at the Parkfield-Coalinga Bridge across Little Cholame Creek for a photo op. The bridge literally crosses the San Andreas Fault.

 I was so excited I couldn't resist a little pole dancing here (I'll keep by day job).

Sunday, January 28, 2024

Cohoes Mastodon

 If you visit the New York State Museum in Albany, you will see the Cohoes Mastodon.

The mastodon was discovered in September, 1866 during the construction of Harmony Mill #3 (one of the largest cotton mills in the world) near Cohoes Falls on the Mohawk River. Here's the sign on North Mohawk Street, at the northwest end of the massive Harmony Mill building (now apartments).

The bones were found buried in a deep pothole at the base of the falls. The falls themselves can be viewed from the appropriately named Falls View Park in Cohoes.

The potholes formed during higher water flow just after the last ice age. As the glaciers still sat up in Canada, the massive volumes of fresh meltwater created Glacial Lake Iroquois where the smaller Lake Ontario sits today. This water ripped through the Mohawk Valley into the Hudson Valley with 100 times the water flow of today's river.

Based on growth rings in the mastodon's tusk, we can determine he was around 32 years old when he died some 13,000 years ago. He led a hard life, almost starving to death at 11 from a wound to his lower jaw (probably from the tusk of another mastodon) and dying at 32 from another tusk wound to the temple. Here's a neat video from Dr. Robert Feranec at the New York State Museum talking about the mastodon.

The Cohoes Mastodon display at the State Museum is well worth visiting with a lot of information presented about this mastodon and mastodons in general. Around the corner is also the famous mastodon diorama recreating a mother and baby mastodon in the mid-Hudson Valley with Storm King Mountain in the distance.

These were real animals literally walking around our backyards a few thousand years ago.