The Sanbagawa Metamorphic Belt

The Ara River flows down from the Chichibu Mountains in Saitama, Japan and works its way across the Kanto Plains to Tokyo Bay where it empties into the Pacific Ocean. During its journey through the mountains, Arakawa (its proper name in Japanese) incises its way through various ancient layers of rock, and at Nagatoro Town, the river has worked itself through a fault and carved out an impressive gorge in some very beautiful crystalline schist. The site is a popular scenic attraction, and the so-called Iwadatami (rock tatami) is a natural treasure.

The schist present at Nagatoro is part of Japan’s largest metamorphic belt. Known as the Sanbagawa Metamorphic Belt, this continuous strip of rock continues from the western shores of Kyushu Island, across the Island of Shikoku, past the Nanki Peninsula and along Honshu until it bends deep inland, arcs around and approaches the coast again. The metamorphic rocks here were once sediments on the ocean floor and located near the subduction zone of the oceanic plate. These sediments accreted over the period of about 130 to 120 Ma and the time of subduction is estimated at about 116 Ma. The sediments underwent metamorphism under high pressure and temperatures but were subsequently thrust toward the surface again at a relatively high speed at first, then gradually their ascent slowed down until they reached the surface about 50 Ma.

In more recent history, the Izu Peninsula, growing from the Higashi Izu monogenetic volcano field and colliding with the Island of Honshu, pushed its way into the island and bent the various belts of metamorphic rock northward. Thus the waters of Arakawa flowing north and far from the ocean cut through the metamorphosed sediments of Jurassic times. Depending on the type of protolith, the schist may appear a metallic blue/grey, smoky blue, light grey, greenish grey, or brown. Many of the rocks have crystalline bands running through them, giving them beautiful patterns.

Because of the abundance of exposed rocks here, Nagatoro is known as the birthplace of Japanese geological study.  

The Hopewell Rocks

The famous “flower pots” of the Hopewell Rocks are a natural treasure of Canada. Located in the Province of New Brunswick, on the shores of the Bay of Fundy, visitors can easily see how the erosion of the tides has worn away the sandstone and conglomerate bases of these sea stacks, creating slender columns supporting blocky-looking masses of rock. Trees and other vegetation sprout from the tops of many of these, making the rocks appear as giant potted gardens.

The history of the sediments present here goes back to over 600 million years ago when a colossal mountain range known as the Caledonia Mountains had formed in the region due to the collision of what are now the European and North American Plates. Though the mountain range stood higher than today’s Rocky Mountains, there were no eyes to view the lofty crags. The earth was in the Ediacaran Period, which lasted from 635-542 million years ago, and life had only just begun evolving complex multicellular organisms. Imagine that this grand mountain scenery was devoid of green vegetation and consisted solely of rock, snow, ice, and meltwater streams and rivers. As the mountains were eroded, sediments accumulated in the valleys. Further tectonic movement uplifted and tilted the sedimentary layers, causing them to fissure and split into blocks.

In the more recent geologic past, the glaciation periods of the ice age covered the region in glaciers that carved out a valley leading to the sea. When the last of the ice sheet retreated about 13,000 years ago, sea levels rose and flooded the valley, leaving us with the Bay of Fundy, famous for having the highest tides in the world. Rainwater and ice have worked through the fissures, widening the cracks in the ancient sedimentary layers.

I enjoyed a stroll between the towers of rocks at low tide in September of 2005. Many otherwise interesting landscape shots were spoiled by the yellow signs warning visitors to stay out of dangerous areas where unstable rocks might fall loose. I found most of my best photographs were of detailed sections of the ancient sedimentary rocks.

Painted Hills, Oregon

Photographers and painters cannot resist the sight of the colourful Painted Hills in Oregon, U.S.A. Located along Highway 26 near Mitchell, the hills are a part of the John Day Fossil Beds national monument and provide a visual record of nearly 40 million years of climate and ecological change in the area and are a rich library of knowledge regarding prehistoric plant and animal life.

The reddish layers of clay are laterite soil which are rich in iron oxides and typically form in tropical climates. The black parts are lignite, also known as brown coal. It is a very low grade of coal and formed from the remains of plants. The greenish and ochre layers formed later as the climate became drier. There are also layers of mudstone, siltstone, and shale.

All layers are mixed with volcanic ash from different eruption periods of different volcanoes, most notably volcanoes of the growing Coast Range Mountains. Prior to their formation, the region maintained a warm, humid climate. However, as the Coast Mountains grew in height, they created a rain shadow over the region and the climate gradually became drier.

I visited the Painted Hills in March of 2006. The day was overcast with light snow falling in the mountain pass. In spite of the flat light, I was enjoying exploring the area by myself as there were no other visitors while I was there. Then the clouds shifted and for about 20 minutes beautiful afternoon sunlight bathed the scene in warm light. How the hills came alive! A little later in the spring, yellow flowers bloom in the folds of the red slopes, adding one more dimension of colour and beauty to the landscape.

For a detailed report of the area's natural and human history check out this link.