Sedimentary Rocks--Cool Stuff

Earth Science Essentials

by Russ Colson

 

 

Vanished Mountains of Ice

 

In my region in Northern Minnesota, a typical farm-kid activity is rock-picking.   Large stones come up in the fields and can damage farm implements if they are not removed.

 

Question 1: Why are there so many rounded boulders in the fields here and not in places farther south like Kansas?

 

Question 2: Also, the landscape in Minnesota is rolling (again like Kansas), but un-drained (not like Kansas). Why would this be so? 'Undrained' means that a drop of water can't find its way toward ever-lower elevations as it can where a system of gullies, streams, and rivers drains the landscape. This produces many small lakes in the land of 10000 lakes!

Comparison between a poorly drained "lumpy" glaciated landscape from NE South Dakota and a drained river-formed landscape from NC Kansas.

 

Questons 1 and 2 recap: Why is the landscape here in Northern Minnesota un-drained and marked by many small rocks, unlike the rolling terrain in Kansas?

 

Question 3: Most of the land contains a mixture of boulders, sand, and fine clay. But sometimes there are cone-shaped sandy hills called kames. What could cause the hills to be sandy when the land around them contains a lot more clay (people often build cemeteries on the sandy hills because they are a lot easier to dig through in winter than the frozen clay).

 

An Explanation: Former glaciers in our area can explain all these features.   Boulders, sand, and mud all become caught up in the ice, and when the ice melts it leaves this mixture, called glacial till, behind.   Because the distribution of sediment in the ice is uneven, some places get more sediment than others, making for a lumpy, un-drained landscape.   Sometimes, rivers flowing over or within the ice cascade down cracks in the ice, depositing sand at the base (the finer mud being washed away by the running water).   When the glacier melts, a cone shaped hill is left behind.   The washing of sediment by rivers within the mountains of ice also influence where people drill wells (which need sandy sediment) and how pollutants migrate (sometimes material from holding ponds for hog-barn runoff can escape along a hidden underground sand channel left by an ancient glacial river).

 

Thus, we have the story of ancient mountains of ice, with rivers, waterfalls, and perhaps even forests established on the melting ice sheet!

 

 

Lost Rivers from Ancient Mars

 

In 2012, the Mars rover Curiosity, discovered a conglomerate on Mars (see picture below).   Although we don't find oceans or rivers on Mars today, what does this discovery tell us about the Martian past?   Are we talking about a

a) Martian swamp

b) Martian beach

c) Swift mountain river

d) Salty ocean

e) Extensive coral reef?

 

Image credit: NASA/JPL-Caltech/MSSS and PSI

 

 

 

Here is part of the text written by Guy Webster and Dwayne Brown that NASA posted for public consumption.   Take note of the connection between evidence and conclusion that is present in the article (Making this type of connection is a key science skill).   For example, what is the evidence cited in the article that the stream was "vigorous"?   What is the evidence that the sediment was transported a considerable distance from the crater rim?

 

NASA's Curiosity rover mission has found evidence a stream once ran vigorously across the area on Mars where the rover is driving. There is earlier evidence for the presence of water on Mars, but this evidence -- images of rocks containing ancient streambed gravels -- is the first of its kind.

 

Scientists are studying the images of stones cemented into a layer of conglomerate rock. The sizes and shapes of stones offer clues to the speed and distance of a long-ago stream's flow.

 

"From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep," said Curiosity science co-investigator William Dietrich of the University of California, Berkeley. "Plenty of papers have been written about channels on Mars with many different hypotheses about the flows in them. This is the first time we're actually seeing water-transported gravel on Mars. This is a transition from speculation about the size of streambed material to direct observation of it."

 

The finding site lies between the north rim of Gale Crater and the base of Mount Sharp, a mountain inside the crater. Earlier imaging of the region from Mars orbit allows for additional interpretation of the gravel-bearing conglomerate. The imagery shows an alluvial fan of material washed down from the rim, streaked by many apparent channels, sitting uphill of the new finds.

 

The rounded shape of some stones in the conglomerate indicates long-distance transport from above the rim, where a channel named Peace Vallis feeds into the alluvial fan. The abundance of channels in the fan between the rim and conglomerate suggests flows continued or repeated over a long time, not just once or for a few years.

 

The discovery comes from examining two outcrops, called "Hottah" and "Link," with the telephoto capability of Curiosity's mast camera during the first 40 days after landing. Those observations followed up on earlier hints from another outcrop, which was exposed by thruster exhaust as Curiosity, the Mars Science Laboratory Project's rover, touched down.

 

"Hottah looks like someone jack-hammered up a slab of city sidewalk, but it's really a tilted block of an ancient streambed," said Mars Science Laboratory Project Scientist John Grotzinger of the California Institute of Technology in Pasadena.

 

The gravels in conglomerates at both outcrops range in size from a grain of sand to a golf ball. Some are angular, but many are rounded.

 

"The shapes tell you they were transported and the sizes tell you they couldn't be transported by wind. They were transported by water flow," said Curiosity science co-investigator Rebecca Williams of the Planetary Science Institute in Tucson, Ariz.

 

Guy Webster / D.C. Agle 818-354-5011

Jet Propulsion Laboratory, Pasadena,Calif.

guy.webster@jpl.nasa.gov / agle@jpl.nasa.gov

 

Dwayne Brown 202-358-1726

NASA Headquarters, Washington

dwayne.c.brown@nasa.gov

 

 

 

 

 

Last updated March 10, 2017.   Pictures and text property of Russ Colson except for the noted NASA image and text.