Beaches and coasts are excellent places to observe many geological characteristics, such as ria formations, rocky intrusions, honeycomb weathering, stacks, spits and tombolos, beach topography, wave-cut platforms, folding, fault, differential erosion. They are also great places to observe sedimentary rock strata and the different types of sedimentary rocks (mudstone, siltstone, sandstone tuff and mostly andesite and basalt in Izu Peninsula’s case), and fossils.

For this purpose, Izu Peninsula makes a richly rewarding study trip of geography and geology with opportunities to observe how wave and wind forces have transformed the landscape. Last week, we visited different types of coasts and saw a rich variety of coastal formations on the eastern and western coasts of Izu. Below is our account of the things we saw. 

White sand beaches of Izu Peninsula

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We headed for Shimoda City situated south eastern tip of the Izu Peninsula. The Shimoda coast alone has ten white sand bathing beaches, and as we drove past them –  most of them had no one or nearly nobody on them.

Shirahama Beach is the most famous and the most popular – for its white sand, crashing waves and surf and for the sun-worshippers crowd. The beach strip has the best facilities of the beaches, even so, there is just one toilet/bath facility in the centre, so the crowds tend to gather in the central part of the long beach. (Access: Tokai bus to Shirahama Beach bus stop from Shimoda Station (Izu Kyuko Line))

However, the best beach in Izu is considered to be Yumigahama Beach, about 20 minutes south of Shimoda. It is a two-kilometer long beach. Another beach popular with families is the smaller Sotoura Beach because there are no crashing beaches here. As we drove past, we saw few people there though.

Combine your trip to Izu’s beaches with an adapted version of Beach Formation and Types of Beaches and Sand  for a perfectly good homeschooling resource and lesson on beaches.

 

Coastal rias and other formations of Izu Peninsula

 

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The Jogasaki coast consists of deep indentations and towering cliffs and many spectacular promontories. This coastal formation is known as a ria coast. It was formed from the lava of the Amagi Volcano.

Geographical formations defined:  

A ria is a long, narrow coastal inlet (not to be confused with a fjord) whose depth and width gradually and uniformly diminish inland.

A ria coast is a deeply indented or sawtooth coast with several parallel rias extending far inland and alternating with ridgelike promontories.

 

Formation of the Jogasaki coast

The Quaternary volcanoes of the Izu Peninsula (including the Hakone, Fuji, Taga, Yugawara AND the Amagi volcanoes) occur in a northern Izu-Bonin arc – they are aligned into two parallel, eastern and western, chains. The rocks erupted from the zoned magma chambers of the Shimo-Taga group of volcanoes including the Amagi volcano were scattered over a 400 square kilometer area on the eastern side of Izu Peninsula, including the Jogasaki coast. To the east, the volcanic rocks of this chain and the resulting rock formations found on the coast, are composed mainly of calc-alkali dacite or andesite and partially with basalt.

Volcanic rocks can be seen exposed everywhere along the Jogasaki coast. The cliffs of characteristic dark basaltic columns can be seen at the suspension bridge at at Kadowakizaki Point (while the Dogashima coastal rocks and cliffs made of andesite are visibly white in colour).  

The stretch of rugged Jogasaki coast and its marine park runs for about 20 km and a hiking course stretches for 9 km along the coast. However, most people attempt the trail that goes for 3 km which takes them along the rocky cliffs and across the suspension bridge – this trail offers truly spectacular views of the coast.

The cable suspension bridge at Kadowakizaki Point that spans part some of these towering cliffs is a very popular and crowded tourist stopping point – the bridge is 48 m long and 3 m high.

The Lonely Planet online guide describes the spot: “A harrowing 48m-long suspension bridge leads over Kadowakizaki Point, with waves crashing 23m below. It’s a popular location for film and TV shoots, particularly suicide scenes. If you have time, there’s a moderately strenuous cliffside hike with volcanic rock and pine forests, south of the 17m tall lighthouse.”

I didn’t find the bridge particularly harrowing nor the hike strenuous but I can vouch for the crashing waves and described atmospherics. 

Access: 15km from the JR Ito Station and 30 minutes by car

Tsunamis and ria coasts

 Topography is important. A ria coast amplifies the effects of a tsunami. A tsunami a few meters high in the deep ocean increases its height to tens meters at the bottom of a V-shaped bay. At the same time, the coastal topography is also a result of tsunami action. E.g. Habu harbour in the nearby Izu’s Oshima Island was the result of an ancient tsunami.

Dogashima coastal formations – Tensoudo cave and the Tombolo

The Dogashima coastal formations facing Suruga Bay are as dramatic as those of Dogasaki and considered more beautiful by the Japanese, for the coastline is compared to Matsushima in Miyagi Prefecture, one of Japan’s Three Finest Views, and it is called “Matsushima in Izu.”

Walking the trails along Dogashima, one is wowed by the beautifully lined white rock formed out of andesitic lava flow and carved into shape by wave and wind forces.

The highlight of the sightseeing in Dogashima is considered the Tensoudo (Skylight Cave) on Kameshima Island. A network of caves have tunneled into the coastal rock by the sheer power of wave erosion forces at work. At Tensoudo, there is an opening in the ceiling of the tunnel. The sea cave is nationally designated as a Natural Monument.

You can access the Tensoudo cave by boat (a variety boat cruises are available) when the seas are calm or you can see the cave opening from above by walking the coastal walking trail. The seas were too choppy for us to take a cruise when we were there. The cruise take 20 minutes and cost about 920 yen, but you pay more for longer cruises that take more of the coast.

Although Tensoudo is considered the highlight of Dogashima, I think there is another sight that surpasses that of Tensoudo, and it is the “tombolo” leading to Sanshiro Island…because it is a rarer find.  

From Sebama beach at low tide, a 30 meter wide natural rock-and-pebble-connecting bridge emerges and connects the Sanshiro islands with the mainland shore. The three islands (Zojima, Nakanoshima and Takashima) are called “Sanshiro Island.” Sand bars are more usual, this “stone bar” bridge is called a “tombolo” and is designated as a Natural Monument. This kind of natural phenomenon can be rarely seen in Japan (the other tombolo is at Iwo-jima).

Definition of tombolo: “a type of depositional landform” A tombolo is a bar that extends outward from the shore, connecting with an island. (Source: geology.about.com ). Tombolos “form as an offshore hill, or stack, bends incoming waves around it so that their energy sweeps sand together from both sides. Once the stack erodes down to the waterline, the tombolo will disappear. Stacks don’t last long, and that’s why tombolos are uncommon.”

Cautionary tale of tragedy about the tombolo 

Hikers should take care to pay heed to the tide tables so as not to be stranded by the tides – there is a ancient legend from the Heian period (794-1192), surrounding this tombolo.  

Once upon a time in Heian Japan, a young man named Izu no Sanshiro was hiding out on Nakanoshima Island in order to escape from the pursuers of the Taira clan. In the course of things, he fell in love with Koyuki, a daughter of a powerful warrior in a nearby village, who returned his feelings.

However, since it is a tale of tragedy with no happy ending, Sanshiro decided to join Yoritomo’s army. On the day he set out to do so, Koyuki ran to the beach to cross the tombolo bridge. As she did so, she was battered and dragged under and drowned the powerful and rough waves of the high tide.

Access: (Location: Nishina, Nishiizu-cho, Kamo-gun, Shizuoka Prefecture 410-3514)

Dogashima can be reached by Tokai bus from Shimoda (1 hour, 1,360 Yen, 1-2 buses/hour) and Shuzenji (90 minutes, 1970 Yen, 1-2 buses/hour).

 

Notes:

Although we did not go, there are other rocky formations worth visiting, like the Futou-kaigan Futo Coast and precipitous cliffs of Cape Irozaki at the southern tip of Izu Peninsula, faced to the Pacific Ocean – complete with a lovely and romantic lighthouse.

Access: Tokai Bus operates one to two buses per hour between Shimoda Station and Irozaki in each direction. The one way trip takes about 40 minutes and costs 860 Yen. There are also a few buses from Matsuzaki on the western coast to Irozaki. The one way journey takes about 90 minutes and costs 1500 Yen. The closest bus stop to Cape Tarai is Kyukamura, reached about 23 minutes after leaving Shimoda Station by a Irozaki bound bus. The one way fare is 620 Yen. (source)

 

Lesson Plans

 

For older kids, you can: 

Teach your kids the definitions and show diagrams of sea stacks, wave cut cliff tombolo spit and then have them take the online quiz.

Have them learn about coastal processes

•            Coastal Erosion,

•            Coastal Deposition

•            Coastal Management

from this interactive page: Irish Coastal Landscapes and Processes

 

Finally, for more depth knowledge on coastal processes and formations, have your child take this Waves and Shorelines student tutorial.

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