Thursday, August 30, 2007

The Ecology of Otter Creek

Otter Creek
N 44 10.754 W 073 18.457

Otter Creek is the nicest, most nature filled, fresh water river we know. In the past hour we saw 4 eagles, numerous blue herons, an owl, turtles, a sturgeon, gulls, hawks, a turkey vulture, and another one of those chocolate brown weasel-like animals that might be an otter.

The land on either side of the creek is mostly marshland. It is separated from the creek by banks, or dikes, or levies. Where we get a clear glimpse of the marshes, they appear to be packed with some type of sawgrass. The sawgrass grows so tightly packed that it seems that even a mosquito could not fit between the adjacent stalks.

In several places, the marshland adjacent to the creek seems to lie below the elevation of the creek yet it is dry. At least it is dry at the moment. I'm sure that the spring flood causes the creek to spill over and flood that low lying land.

The overall appearance of Otter Creek is similar to the salt marshes one finds in northern Florida and southern Georgia. Miles wide areas of marsh are bisected by a few narrow channels where the water is deep enough to discourage grass and to allow navigation by boats. I know that the salt marshes are well known to be the incubators of marine life in those areas. Everything from the shrimp to the birds give birth and raise their young in the shelters of the marsh. It may be very much the same
story for Otter creek.

I've been struggling to determine if the river banks are natural or man-made. Clearly some stretches are man made and were made to support a narrow road that leads to a few houses near the mouth of the creek. Still, most of the river bank appears to be natural. However, if it was all natural I'm sure that there would be gaps that would allow the creek water to flood the low-lying adjacent land. Therefore, I conclude that man made supplements, patching and repair must maintain the banks water
tight. That will drastically alter the ecology of those low lands. Instead of nearly perpetual submersion, they will no be flooded only in spring.

The creek is eminently navigable up to the waterfalls in Vergennes. Water depth is 12-15 feet deep almost all the way to the banks.

A few, very few, houses and boat ramps, and docks line the creek on either side. My guess is only one man-made structure per 1/3 mile. That's very sparse population by modern standards. I'm sure that contributes to the continued presence of wild life. I wonder what fraction of the wildlife in this region is dependent on the marshes near Otter creek.

Elsewhere in Vermont and upstate New York marshes are very rare. The closest similar landscape that Libby and I are aware of is Montezuma Swamp in New York that lies West of Syracuse and East of Rochester.

As I write this, we are sitting out a thunderstorm and hail. I has been several years since I saw real hail. There are three fishermen in a 14 foot motor boat right in the middle of it. I give them credit, they just continued to fish and ignored the storm. The only nearby shelter for them would have been under a tree, but that is a very bad place to take shelter in a thunderstorm.

Tuesday, August 28, 2007

Two Nice Shots

Here is a fun shot I took while camping last night. The camp fire is in the foreground. The full moon, just hours before an eclipse in the sky and the reflection of the moon on the water. Peculularly, the reflection on the water appeared to me and to Libby to be curved. It even looks curved in the picture. The bright light on shore is in Colchester. Behind it is the Burlington Airport. Look closely and you can see Mount Mansfield and Camels Hump on the horizon. Not bad for a night photo.

Here's a good shot of Pete from when he came for a day sailing with us last week. Not a bad looking guy huh? He'd probably look even better if he opened his eyes while sailing :)
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Valcour Shores

It's no secret that I love Valcour Island. Its shores and its trails keep drawing me like a magnet. One of the mysteries of Valcour are the vertical holes that appear in the limestone. I recently wrote about them in a blog article. I received a reply from a reader who suggested that they might be bore holes drilled for blasting. They are not drilled and the reader's comment revealed the inadequacy of writing on this topic without pictures. Now I returned to Valcour once again and this time I did take pictures.

The following is a long illustrated report on my expedition.

Above is a typical section of the Valcour shoreline. It is made of bare limestone cliffs, with rock falls and shelfs at the base. At the top of the cliffs tenacious trees manage to grow from the very sparse soil. Most of the trees are red cedar. This stretch of shoreline faces the prevailing winds.

Limestone is sedimentary rock. Accordingly, this limestone is laid down in prominent horizontal layers. In places the thin layers separate as they erode. The rocks are also fractured by vertical cracks which tends to break them into block-like structures. The holes that I refer to are vertical holes that are aligned with the vertical cracks.

The above picture shows two of the holes and also apparently reveals part of their secret. We are looking straight down. As the limestone was subjected to great pressure and temperature underground, its crystal structure was broken down in places allowing metamorphosis to quartz. In the above picture you can see one of the thin lines of quartz embedded in the limestone. You can also see how the holes are perfectly aligned with the quartz vein.

Above is a close up of two of the holes, plus a shot of the interior of one. Many of these holes extend all the way down through the vertical cracks to water below. Others, like these two, have become plugged by rubble.

It seems that the primary mechanism is for the slabs of rock to become undermined, with cavities extending from the water line inland. The holes connect the surface to the undermined sections below. Sometimes, the undermined sections collapse. The above picture shows a clear example of that., again looking down from above. I left my shoes in the picture to show the scale. Note that this collapsed section preserved the holes.

In other instances, the rubble from the collapse seems to have been completely removed somehow, leaving behind a v-shaped gash.

I couldn't help admiring the lichen growing on the rock surface.

In this section of shoreline, you can clearly see collapsed sections, some with the rubble intact and others with the rubble vanished.

A wonderful fossil track of a nautilus.

A number of indented pock marks on the rock surface, each filled with lichen. It is not clear whether the lichen grows in the indentations, or whether the dark lichen patches cause the indentations.

Above top is a specimen of a much larger hole. In this case you can see all the way down to the water, 15 feet below. Also, in this case, the rock has collapsed below leaving the surface layer penetrated by the hole. Next is a shot of the collapsed cleft with the hole just discussed at the top.

I couldn't help notice the sailboat passing by. The scenery and the weather were idyllic. Also, looking backward, I could see Tarwathie in the cove illuminated by beams of sunlight from the heavens. There is a specific name for these sunlight beams but I can't recall it just now. It's all just so pretty.

I took the dinghy off shore a bit and got some of these underwater shots. The shore line structure extends under water at least 100 feet out from the shore. Some of the same effects seen on shore are visible under water. Also clearly visible in the shot on the right are large pock marks. Hmmm, these underwater structures undermine some of my speculations as to the origin of the holes and rock structures. There can be no dripping water or running streams under water. Were these structures once above the water line? If so, that might rescue my theories. I know that this area was recently pushed down under the weight of glaciers and should be rebounding upward. However, Lake Champlain is 96 feet above sea level so absolute altitude and lake level should not be related.

Also note the zebra mussels visible in the top-most shot. These critters are a non-native invasive species that are reputed to cause a lot of trouble. On the other hand, they have filtered out the turbidity in the water making it very clear.

Talk about undermining. Here I found a section of shore spectacularly undermined by what can only be called shallow caverns. One of them was big enough for the dinghy to fit inside. This was an enchanting scene. It reminds me of the famous place called The Baths on Virgin Gorda in the British Virgin Islands. I'll name it Valcour Baths.

On top is the ultimate view of a hole or cavern. It is seen from below. Sunlight and trees are visible beyond the opening. Below is the foot of this cavern. Clearly it is formed by a block of rock broken off and turned 90 degrees as it fell. The horizontal layers of sediment are now vertical and some of them eroded away leaving vertical slits to see through. I love it.

Here I found what must be the ultimate proof of the nature and origin of the holes. In the cleaved face of this rock you can plainly see bisected sections of several such holes. Very significant I think is the fact that they are not all vertical. Certainly these holes were formed by water dripping down.

Here I found another seemingly clear example of another phenomenon. In this case it seems unmistakable that a stream of running water passing over these quartz veins have eroded horizontal channels in the rock face. Those channels extended 50 feet under water.

Another diversion. The golden lichen is beautiful. There is also a rust colored variety. Nearby was a beautiful fossil specimen.

Also nearby was this beautiful cedar tree.

In the evening, we built a fire on the shore and enjoyed the twilight scenery. Mount Mansfield, Burlington, and Camel's Hump are all visible across the lake.

Now, what about the true nature and origin of the holes? If I were a scientist I would have to say that the evidence I have so far is inconclusive and no conclusions are warranted. I'm not a scientist so I'll indulge in speculation.

In an earlier blog I theorized that the holes were formed from below by wave action, perhaps accelerated by the ice skirts surrounding the island in winter. I now believe that to be wrong. I believe that the holes were formed by water dripping down from above.

I changed my mind because of three things. I noticed that many of the holes were funnel shaped with the widest part at the top. That is inconsistent with growth from the bottom. Second, I found a number of deep pocks, several inches deep that appeared to be holes in the making. Third, I found holes in the rocks well inland and more that 100 feet above water level.

My theory is that everything starts with the horizontal layers of sedimentary rock and the vertical veins of quartz rock. Some of the horizontal layers are more soluble than others. Quartz is definitely not soluble but it is brittle and the quartz face may not bind strongly to the limestone face. I speculate that the quartz and limestone faces started to separate allowing water in to the cracks. Freezing in the winter caused the cracks to widen. Eventually, freezing water also shattered the quartz into small pieces and those washed away. Below is a close up of a quartz vein.

Undermining starts with erosion at the shore line. Lake level varies over the years so the elevation of the shore varies up and down several feet. Erosion of weak horizontal layers undermines the rock and creates cavities (caverns) under the rock.

Rain water, or splashes from waves flow across the surface of the rock. When the surface water encounters a crack, such as at a quartz vein, some of it percolates downward. The percolating water dissolves more rock, especially the quartz. Over time, the quartz is entirely washed away leaving a vertical crack in the rock.

In some places, a pock mark in the surface happens to lie on top of a quartz vein. Water pools in the pock and thus accelerates the percolating action at that spot. Therefore, pock marks on veins evolved into vertical holes extending from the surface to the caverns below.

Eventually, the undermining causes collapse. In some cases the rubble from the collapse is carried away erasing evidence of the holes, the cracks and the original veins.

I'm unclear on the origin of the pocks. Could they be the remains after a soft fossil washed away? Could they be caused by a round patch of lichen that changes the albedo and retains moisture?

In other places, I've seen pocks in rock caused by other sources. In Canajoharie NY, and near Syracuse, NY there are pocks caused by waterfalls falling off the faces of retreating glaciers. Those pocks vary in size from one or two inches to half a mile in diameter. Green Lake near Syracuse is one of the larger pocks. It is about ½ mile in diameter and reputed to be 1,000 feet deep.

In Sweden, there are pocks in the rock 6-12 inches in diameter and 6-12 inches deep. They are called “Devils Bowls” by the Swedes. They are said to be created by the glaciers. The glaciers are said to have picked up boulders, and drag them along underneath. In some places the boulder encounters softer underlying bed rock. The moving glacier spins the boulder in an eddy of flowing ice and it grinds it's way in to the bed rock. In some of these “Devils Bowls” the original boulder is still enclosed in the hole, and the mouth of the hole is inexplicably narrower than the diameter of the enclosed boulder.

In still other places, notably limestone caverns, dripping water creates stalactites and stalagmites. We've all seen them before. Reasonably, for every stalactite, there must be a cavity (a hole) where the stone was dissolved and washed away.

In the case of Valcour, I believe that it was not dripping water from above, not glaciers, not waterfalls, but rather the seepage of surface water that created these holes.

There you go. I really enjoyed myself today prowling the island, gathering evidence, then writing this report. It whets my appetite to go ashore and to explore the nature and geology of other places we visit by boat.