Hitchhikers and Rolling Rocks
In this NASA image (left) there are black erratic boulders found in the Nylosyrtis-Mensae area of Mars. Scientists are unsure of what brought them there. Rocks that seem to be out of place because they are made from different materials are called erratics.
The large boulder pictured at left is 59 feet wide by 26 feet high. It can be found near Soap Lake, Washington, in a large floodplain that was used as a testing area for the Mars Pathfinder rover. The boulder was washed down in one of the many giant floods that swept over eastern Washington forming the Channeled Scablands. In this area for as far as you can see there are hundreds of boulders of all shapes and sizes. Most are medium sized at about 2 feet across. It has not been farmed and so the rocks are in the same place they have been for thousands of years.
As the ice age glaciers melted and retreated they left behind the sand, gravel and rocks they were carrying. The material is piled into mounds called a till. Deposits of rocks, boulders, gravel and sand (called glacial drift) left behind by flowing water from glaciers as they melted is called glacial outwash.
In the photo at right you can see that the glacial till is made from a variety of rocks, gravel and sand. At the top are smaller cobbles(larger rocks) with midsized rocks in the middle. At the bottom is dark basalt gravel and rocks. Mixed in with everything is sand and gravel.
Some deposits of glacial drift left behind by glaciers are called moraines. Terminal moraines are at the end of a glacier showing how far the glacier traveled before it stopped. Other moraines are formed from the debris being pushed along by the glacier.
Eskers on Mars
On the Waterville Plateau are eskers, sinuous (snake-like) ridges formed by glacial action. These narrow, winding ridges are made of gravel and are usually formed by streams flowing on a glacier or in a tunnel below the glacier or ice sheet. In the image at left you can see these narrow, winding ridges on the floor of Argyre Basin on Mars. This indicates that at some time glaciers moved over this area on Mars and running water left these formations. See more about glaciers on the Glaciers page.
Here are some basic terms used in the tour. Find more geology terms in the Glossary.
- A mound of gravel and sand deposited by flowing water. Bretz and other geologists identified many large bars in the Channeled Scablands.
- Volcanic rock caused by partial melting of the Earth's crust.
- The deepest part of a river or bay.
- Channeled Scabland
- Area in Washington state where huge floods made channels in a large, deep basalt flow. Named by J. Harlan Bretz during the 1920's in various publications. See also Channeled Scablands.
- Lifting and removal of rock, dirt, sand and the like caused by wind, water, or glacial ice.
- Large rock or boulder carried by water or glaciers and left behind.
- A narrow, winding ridge made of gravel usually formed by streams flowing on a glacier or in a tunnel below the glacier or ice sheet.
- Rounded rock fragments larger than sand.
- Ice Age
- A period in Earth's history when much of the continents are covered with ice sheets and glaciers.
- Deposits of rocks, boulders, gravel and sand (called glacial drift) left behind by glaciers as they melted. Terminal moraines are at the end of a glacier.
- Deposits of rocks, boulders, gravel and sand (called glacial drift) left behind by flowing water from glaciers as they melted.
- Outwash Plain
- A smooth plain covered by deposits from water flowing from glaciers.
- The period of geologic time that began about two or three million years ago and ended approximately 8,000 years ago.
- Deposit from a glacier of unsorted rocks, boulders, gravel and sand (glacial drift).
- Layers of light and dark sediment on the floor of a glacial lake. Each light and dark pair indicates a year since light deposits are from rock dust from melting ice in summer and dark layers are from organic matter.
Kids' Cosmos… Expanding Minds Beyond the Limits of the Universe
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