Friday, September 23

Sticks and stones


Petrified wood is an amazing phenomenon. It’s the fossilized remains of ancient trees that retain the shape and texture of real wood despite being made entirely of stone. It’s created when the tree’s organic material is replaced over thousands of years by minerals, often quart and other silicates. The word petrified comes from the Greek root petro meaning "rock" or "stone"; literally "wood turned into stone".

Conditions have to be just right for petrifaction to occur. The wood must first be buried by sediment that inhibits aerobic decomposition. Mineral-laden water must then flow through the sediment depositing minerals in the tree’s cells as passes, forming a permanent stone mould. Erosion of the surrounding sediment then exposes the rock logs for all to see.


I remember learning about petrified wood at school. At the time my Science textbook included fascinating images of the Petrified Forest in Eastern Arizona. I was totally captivated by the concept of logs made from stone and longed to see them for myself. My childhood dream came true this week after Garry and I spent a day driving through the Petrified Forest National Park.

First designated as a national monument in 1906, the 94,000-acre park is scattered with fossilized vegetation dating from 225 million years ago. Petrified logs are literally lying everywhere you look. They’re a fascinating shade of red, often lying in jumbled line of truncated segments. Up close each log really does look like a tree trunk. The trunk’s rough, crevassed bark surface is clearly visible, occasionally broken by classic branch knots. It cross-section reveals hundreds of narrow growth rings, each preserved by colourful minerals. It’s amazing to see.


Despite the heat, we ventured out to explore several walking tracks that took visitors passed the park’s best preserved logs. Some were more than ten metres long, others were as wide as we were tall. Interestingly, every log was always found in short sections. We discovered the reason for these segments on one of our walks. Basically, the hard mineral logs are buried in layers of comparatively soft sedimentary sandstone. Over time these sediments erode. Flash floods form deep crevasses in the soft rock, occasionally exposing the end of a log. These exposed ends eventually collapse under their own weight. A new length of log is then exposed and the process repeats until the entire log has been excavated from the ground.


The process of erosion has resulted in some incredible sights. At one location we saw exposed logs resting precariously on the crest of narrow ridges, tens of metres above the surrounding valley. Elsewhere the erosion has simply exposed colourful layers of sediment that ring the hills for miles around. In fact, just a few miles north lies the equally spectacular Painted Desert.


You reach it by crossing over Interstate 40. This highway bisects the entire state of Arizona. It replaced the famous two-lane highway, Route 66, became immortalized in American pop culture. Today, the road that once passed through the park is gone. However, its route is still traced across the desert by a decaying row of old wooden telegraph poles. They’re a haunted memorial of another, more simple time. The route is more formally memorialized by a rusting old car mounted in stone on the park roadside.


I was rather disappointed by the Painted Desert. Prior to our visit I’d seen images of a dry, red and eroded valley floor. The undulating, multilayer rock formations looked stunning. However, our visit came at the tail end of weeks of heavy rain. As a result the entire desert floor was awash with grass and plant life. Instead of seeing classic images of a harsh baren desert we encountered something that looked remarkably like a local sheep farmer’s paddock.  As a result, I've enhanced colours in the images posted here to bring the spectacle to life.


The entire experience was made all the more amusing by Garry’s constant comments about the amazing desert colours. I simply couldn’t understand why Garry thought such grey and green landscape was so spectacular. It wasn’t until we were leaving our final outlook stop that we suddenly realized his polarized sunglasses had helpfully tinting the entire scene a vivid, desert red. I tried them on and instantly entire desertscape was transformed. We both had a chuckle at the misunderstanding that had dominated our afternoon. Judge the differnce for youself below..

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