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Schoolyard Geology Home Lesson 1 | 2 | 3 Download
2 Rock Stories Intro << 2.1 Rock Descriptions << 2.1.1 Blank Table & Instructions

Example Rock Descriptions

Here are some examples of "rocks" that we found on schoolyards and sidewalks near schools. For information about how to fill out the table, click here.
Photograph of asphalt grains
Location outside Art Studio, UC Berkeley
Colors Dark Grey.
.....All the same color? Yes. Almost no difference between grains.
Grain Size
.....Minimum grain size 0.3 cm
.....Maximum grain size 2 cm by 1 cm
.....All the same size? Most grains are similar in size: about 1 cm.
Grain shapes Medium Angular
Strength Very strong.
Other Comments
Interpretation: All these grains are the exact same color, as is the area in between each grain. In the schoolyard, we know that the color comes because the grains are coated with tar to make them stick together. All sedimentary rocks also have something that makes the grains stay together -- the "cement." Commonly, calcite or quartz form the cement in sedimentary rocks. However, even a few natural rocks are cemented by tar in places where oil and tar naturally seep to the surface (common along the California coast, especially at beaches near Santa Barbara). <Example photo of this environment>

Image of angular asphalt grains
Location east of South Hall, UC Berkeley
Colors Most grey with a slight brownish tint.
Larger grains are darker grey with a slight bluish tint.
Very few white, milky looking grains.
.....All the same color? No.
Grain Size
.....Minimum grain size 0.2 cm
.....Maximum grain size 1 cm
.....All the same size? A pretty wide range in sizes.
Grain shapes Angular (most grains, especially the biggest ones)
Strength Very strong.
Other Comments Very rough surface.

Interpretation: In natural environments, landslides can produce this combination of angular fragments spanning a wide range of small size grains. Landslides are quick events that break the rocks apart but are not steady or long enough to round the grains. <Example photo of this environment>

Image of rounded cobbles
Location east side of McGlaughlin Hall, UC Berkeley
Colors Mostly light grey, with some bluish grey and even whitish grey
.....All the same color? No.
Grain Size
.....Minimum grain size 2 cm
.....Maximum grain size 6 cm
.....All the same size? Mostly. Most Grains are about 4 cm.
Grain shapes Rounded
Strength Very strong.
Other Comments Each of the little rocks within this picture are rounded and smooth.
Interpretation: The large size of these grains means that something with a lot of energy moved them -- a rapidly moving river is a good bet. The fact that they are so smooth and rounded indicates that sat in the river for quite a long time. <Example photo of this environment>

Image of rounded, fine grains
Location corner of Parker and College, near Emerson School, Berkeley
Colors Overall, the rock is fairly light colored.
Majority of grains are light grey (>60%).
~20% dark grey. ~10% white.
A few rusty red-brown grains.
.....All the same color? A very wide range of colors.
Grain Size
.....Minimum grain size 0.1 cm
.....Maximum grain size 0.5 cm
.....All the same size? Mostly. Most Grains are about 0.2 cm.
Grain shapes Some grains Medium Rounded
Some Medium Angular
Strength Very strong.
Other Comments This section of the concrete had a stamp stating that it was poured in 1962.
Interpretation: The small size of these grains means that they could have been in an environment with relatively low energy, but the medium rounded shape tell us that they sat there a long time and were reworked over and over again. In nature, we might find this combination at a beach. <Example photo of this environment>

Image of rounded medium grains
Location outside Art Studio building, UC Berkeley
Colors About 50% milky white, 20% light grey, 25% dark grey. Some reddish grains (5%).
.....All the same color? No.
Grain Size
.....Minimum grain size 0.5 cm
.....Maximum grain size 1.5 cm
.....All the same size? Mostly. Most Grains are about 0.75 cm.
Grain shapes Rounded, with a few medium rounded grains.
Strength Very strong.
Other Comments Some of the grains have fallen out, leaving behind rounded holes in the pavement.
Interpretation: These grains are about the size of a fingernail. Try to imagine how fast water would have to be flowing to move a pebble that size (think of a playing with a hose or sink faucet). It couldn't be too slow, but wouldn't have to be too fast either. A small creek would fit the bill. The round grains again indicate that it sat in the bed for a very long time. <Example photo of this environment>

Image of rounded grains
Location corner of Parker and College, near Emerson School, Berkeley
Colors Brownish-red, light orange, very light grey, greenish grey, turquoise, dark grey with a purple tint.
.....All the same color? No! Multi-colored.
Grain Size
.....Minimum grain size 0.5 cm
.....Maximum grain size 1.5 cm
.....All the same size? Mostly. Most Grains are about 1 cm.
Grain shapes Rounded
Strength Very strong.
Other Comments Some of the grains have fallen out, leaving behind rounded holes in the pavement.
Interpretation: The size and shape of the grains in this rock are similar to the one above, so we can assume that it came from a similar river environment. The wide range of colors is the most notable part of this rock, indicating that it is made up of a wide range of rock types. In nature, a river moves pieces of rock from all of the area upstream of it within its watershed. To get so many different types of grains, this rock needs to have come from a river with a large drainage basin having a wide range of rock types. <Example photo of this environment>

 

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Page Last Modified: Wednesday, 14-Oct-2009 18:41:07 EDT