CTgeo

Sleeping Giant State Park

Investigating the Giant’s Bedrock, Till and Stratified Drift

 

Today you will become familiar with the geology of Sleeping Giant State Park, one of the favorite hiking sites of many Connecticut residents. Sleeping Giant State Park is in the central part of Connecticut, in the northern part of the town of Hamden, which is sometimes referred to as Mount Carmel. Several maps of this area that you will use contain different kinds of information about the Giant: 

1.     Trails on the Sleeping Giant: This simple flier made available by the Sleeping Giant Park Association at the park entrance shows all of the established trails in the park. Each trail is coded by color or by the shape of its blaze. A blaze is a symbol (often a rectangle) painted on trees at eyes’ height along a trail. 

2.     Mount Carmel Quadrangle Map: This map is a United States Geological Survey topographic map. This topographic map shows contour lines at intervals of ten feet that can be used to visualize the relief or shape of the surface of the land. Topographic maps are highly detailed and show not only roads and streams but even the locations of individual buildings that were present at the time the map was made. 

3.     Bedrock Geology of the Mount Carmel Quadrangle, Connecticut: This quadrangle map, published by the United States Geological Survey, has three parts: 

                 a.  The map itself, which is color-coded according to rock type. 

                 b. An extensive explanation of the characteristics of each of the rock types shown on the map. 

                 c. Several cross-sections diagrammed in color to show the kind of rock and the structure or shape of the rock layers below the          surface. The kind of rock present and its structure together help geologist’s interpret the geologichistory of the Sleeping Giant and its surroundings. 

The Mount Carmel Quadrangle map includes a description of the rock types on the map sheet. 

4.          Depth to Bedrock, Mount Carmel Quadrangle Map: This black-and-       white map uses various kinds of shading to symbolize the depth to bedrock. Depth to bedrock is really another way of saying the thickness of the loose deposits, like sand and gravel, over the bedrock.  

5.     Surficial Geologic Map of the Mount Carmel Quadrangle, Connecticut: This color-coded map indicates the different kinds of deposits lying over the bedrock. The type of deposit indicates whether the deposit was made by a river (alluvium), melted water within or beyond a glacier (stratified drift) or simply dumped in place by glacial ice (till). Since water sorts sediments by size, alluvium and stratified drift are sorted, but till is a mixture of many sizes of materials. The map is accompanied by a booklet explaining the glacial geology in detail.  

Preparing for the Sleeping Giant Geologic Hike 

(Each group should select a set of maps to work with before beginning the hike.)

1.  Find the Gorge Cascade on the Sleeping Giant trail map. The hike will be along this stream. Note where the stream is relative to nearby landmarks such as roads or other obvious features:

2.  Use the above features to find the Gorge Cascade on the Mount Carmel topographic map.

  1. What is the general shape and size of this stream? How does it compare with the shape and size of the Mill River? 
  2. What does the map symbol used for this stream indicate about the stream?
  3. In what ways is this stream different from the Mill River?
  4. Observe the shape of the contour lines near the Gorge Cascade. What do you expect the terrain will look like?
  5. What direction will you travel if you hike to the Giant’s left leg?
  6. Describe the area found at the head (beginning) of the stream.

3.  Locate the Sleeping Giant on the geologic bedrock map.  

  1. What kind of rock(s) is the Sleeping Giant made of?  
  2. Now that you know what the Giant is made of, what geological terrane (Proto-North America, Iapetos, Newark, Avalonia) do you think it is part of and how did that terrane form?

 4.  Locate the Sleeping Giant on the surficial geologic map. Do the following on the outline map of the Sleeping Giant below:  

  1. Sketch in the locations where the bedrock is covered by deposits.
  2.  Make a key to color code (or code by different kinds of shading) the names of the different kinds of deposits.
  3.  Key (and color or shade in) the remaining surface as bedrock.

 

Sleeping Giant State Park

  

  1. Make a separate surficial map that indicates how deep the deposits are in Sleeping Giant State Park.

 

 Background Information to Study the Rocks of Sleeping Giant

 

Rock Composition: 

You already know from the map study you did prior to this hike that a variety of materials is found on the Sleeping Giant:  

1.     Arkose: Arkose is a poorly sorted sandstone containing feldspar that formed from sediments that were once part of the ancient mountains of Connecticut. When the central part of Connecticut sank as Connecticut was stretched 200 million years ago, mountain streams brought these sediments into the young Central Valley. Only the eroded cores of these mountains still remain. Arkose has a reddish color because of the iron oxide cement that holds its grains together. (Generations ago arkose was a popular building stone which was commercially known as brownstone. Some buildings today are still referred to as brownstone houses, brownstone churches, etc.) 

2.   Conglomerate: Conglomerate is similar in origin and composition to arkose, but it contains pebbles as well as sands and clays.  

3.   Diabase: This is igneous rock that is commercially known as traprock and which formed from magma. The magma squeezed into cracks that opened up in rock as Connecticut was stretched. This occurred when Pangaea separated into today’s continents 200 million years ago. Some cracks reached the surface, causing fissure eruptions of lava that flowed over Connecticut's landscape, forming basalt.     

      Diabase has the same composition as basalt, but its crystals are larger and more noticeable. The magma that produced diabase was farther from Earth’s surface, so it cooled more slowly giving the crystals time to grow. The crystals in basalt formed near the surface where they quickly cooled to microscopic size. (Another rock found in Connecticut with the same composition as basalt and diabase is gabbro. Gabbro formed from a magma that cooled very slowly far below the surface; like granites, gabbro has crystals that are large enough to identify easily and are called coarse-grained. The crystals in diabase, which are fine-grained, are not easy to identify.) 

 

Characteristics of Deposits Found at Sleeping Giant State Park

 

1.     Alluvium: Alluvium refers to deposits that streams have carried and sorted by size. The size of the particles that were moved and deposited by a stream is related to how fast the stream traveled.  Steeper streams travel faster. Likewise, the more water there is in a particular stream, the faster the stream travels. This is one of the reasons that streams can move larger objects during floods. 

2.     Till: Till is unsorted material that was embedded in a glacier or that the glacier bulldozed at its front. The materials remained in place, unsorted when the glacier retreated, and are of many sizes. 

Alluvium Deposits

 

Figure 1. Alluvium deposits beside the Mill River in Sleeping Giant State Park.

 

 

 

 

     3.   Stratified Drift: During warm periods of a glacial episode much melting occurs causing       streams to form in cracks inside the glacier as well as beyond it. These streams sort the          sediment just as ordinary rivers do. The deposits found today that result from moving water in glacial streams is called stratified drift. Some of these deposits are large and are of commercial value. For example, deltas that formed at the end of glacial streams are quarried for their well-sorted sands.  

4.   Swamp deposits: Lakes and ponds are basins in which water is trapped and cannot flow as it does in streams. Lakes and ponds slowly fill up with sands and clays brought in by water and wind, as well as decayed vegetation. Eventually lakes become swamps, which are mixtures of all these materials with a high water content. During rainy periods swamps can become temporary ponds.

 

Terminology for Groundwater Studies

      1.     Aquifer: Rock or loose earth material that is highly porous and contains large quantities of water underground, which can be pumped

2.     Groundwater: Water that is stored in rock or loose earth materials below Earth’s surface. 

3.     Permeability: The rate (speed) at which water can move through rock or loose earth materials. 

4.     Porosity: The percent of a rock or earth material that is filled with air or water (the space between the particles or the space in the cracks)

 Ready to Check Up on the Sleeping Giant? 

Now that you have some background on the geology of the Sleeping Giant, you are ready to do some real field work! You should bring a notebook with you for collecting data, a simple calculator, a ruler and a camera, if possible. Group activity equipment will be provided, but each group will carry and be responsible for its own equipment. This will include plastic beakers, sieve sets, and maps. 

On this hike you will investigate a variety of geologic features. Read the outline and the activities that follow before the hike begins so you have some idea of what you should be observing and doing as you hike: 

1.     How the rock composition changes as you hike the Gorge Cascades trail 

2.     Whether the sizes of particles in the sedimentary rocks change with their location in Sleeping Giant State Park 

3.     Erosional and depositional features of the Gorge Cascades stream: 

a.      The stream’s overall shape 

b.     Evidence (and location of) of sedimentation

c.      Types of erosional features within and alongside the stream

 

4.     Particle size analysis and comparison of alluvium, ice-contact stratified drift and till deposits in the park. 

5.     What kind of deposit can hold the most water? 

6.     What kind of deposit makes the best well site for water supplies? 

7.     Features of diabase  

8.     Looking for contacts (boundaries) between rock types 

Procedure:  

1.   Collect a few sample rocks at the start of your hike. (Do not use rocks close to the road or the parking lot since they may have come from  another location.) Describe the rocks (include color and size of particles that make up the rocks) and identify them by name. Record every significant feature you can see. Follow this same procedure periodically to note whether the rocks change in any way as you progress up the trail. 

(In the following table, fill in igneous, sedimentary or metamorphic in the Family column. In the Location column record whether the rock was found in a stream bed, at the bottom of a cliff, etc. When you complete the table, write the number of each rock on your Sleeping Giant map at the approximate location it was found. Identify the type of rock (Name) if possible.)

 

Rocks Identified in Sleeping Giant State Park

 

Name

Family

Description (include grain size in mm if the rock is sedimentary)

Location Found

1

 

 

 

 

 

2

 

 

 

 

 

3

 

 

 

 

 

4

 

 

 

 

 

5

 

 

 

 

 

6

 

 

 

 

 

 

Observation: Did you hike over any area(s) where there was an abrupt change from one kind of rock to another? The boundary between two different kinds of rock is called a contact. If you found a contact, describe its setting and the two kinds of rock you found in contact with each other. Then mark its location on your Sleeping Giant map. 

Hypothesis (Include illustrations):  

Try to imagine what happened to produce the contact and explain your hypothesis.  

2.   Streams constantly change the land over which they flow. Look for evidence of any changes this stream has made. Record and illustrate the features you see. Note that in some cases the stream erodes (removes materials from) its streambed and its banks, and in other cases the stream deposits (dumps) materials. Using this stream as a example, try to figure out why streams sometimes deposit materials, but other times do exactly the opposite. Explain your hypothesis as thoroughly as you can.  

 PROBLEM:  

Where is the best spot in Sleeping Giant State Park to drill a well for water? (You will work in groups for this part of the field trip. Each group will do one activity. All groups will share results and will participate in a full class discussion to arrive at a final conclusion on the above question.)

A.    The alluvium deposits of the Mill River 

B.    The ice-contact stratified drift deposits near the Mill River 

C.    The till deposits on the north side of the park 

D.    Swamp deposits 

Use the surficial map of Mount Carmel to determine where the alluvium, the  stratified drift, the swamp deposits and the till are located. Collect a small sample of each to use to do one of the following three activities. After doing these activities, each group will share its results with the class so the entire class will have access to all the information gathered. Then each group will reconvene and make a final decision about the best location for a well. 

Experiment 1: Design and perform an experiment to compare the percent composition (by size of particle) of the following: Alluvium alongside the Mill River, the ice-contact stratified drift near the Mill River, the sediment alongside the Gorge Cascade and the swamp sediment. (See Table 1) Then write a short, but complete lab report detailing your work. The lab report should include the following components: Problem statement, hypothesis, procedure, results (with all data collected and any math you did), conclusion, and validity. Be sure to write the lab report in third person just like a scientist! Do this on a separate sheet of paper.  

In the conclusion of your report, record the percent composition by sizes measured with the sieve set and indicate which material you think would make the best aquifer.

Table 1. Table 1

 

 

 

 

 

 

 

 

 

 

Experiment 2: Design and perform an experiment to compare the percent porosities of the alluvium, the ice-contact stratified drift, the swamp deposits, and the till in Sleeping Giant State Park. Percent porosity is determined by dividing a total measured volume of the drift (or till) into the space between the particles of sand, and them multiplying by 100%. Write a short, but complete lab report detailing all of your work. The lab report should include the following components: Problem statement, hypothesis, procedure, results (with all data collected along with any math you did), conclusion, and validity. 

 Be sure to write the lab report in third person just like a scientist would do! 

Write your report on a separate sheet of paper. In your conclusion, discuss whether you think the ice-contact stratified drift or the till would make a better aquifer.  

Experiment 3: Design and perform an experiment to determine which would be the best location to collect water easily and quickly: In the alluvium, the ice-contact stratified drift, the swamp deposits or the till. The rate at which water can move through earth materials is referred to as permeability. Use the depth-to-bedrock map to choose a particular site for the well after you have selected the earth material with the highest permeability.  

Write a short, but thorough lab report detailing your procedure and results. The lab report should include the following components: Problem statement, hypothesis, procedure, results (with all data collected along with any math you did), conclusion, and validity. 

Be sure to write the lab report in third person just like a scientist would do!

Write your report on a separate sheet of paper. In your conclusion discuss the speed with which water could be moved from each of the three materials tested. 

Follow-Up: Collect information from all three previous experiments to determine which material would probably make the best well site.

 Learning to recognize basalt and diabase in the field

 Basalts, diabases, and gabbros all contain a significant amount of iron. In the presence of water the iron combines with oxygen to form iron oxide or rust. So when these rocks are weathered they lose their dark color and look reddish. Sometimes the reddish color is the same as the reddish arkose. So how can you tell (without cracking the rock to look at a fresh surface) if you are walking over bedrock that is arkose or diabase?  

Basalt and diabase show some unusual features that can be used for identification even when the rocks have weathered surfaces that do not show the rocks’ real textures and colors: 

1.     Rock in steep locations weathers and weakens over time; chunks of rock break off, fall and form a pile at the base of the cliff. Basalts and diabases tend to break off in the shape of columns with polygonal ends (often pentagons or hexagons). If you see a pile of columns with several flat sides, you can be reasonably sure you are looking at basalt or diabase. Diabase talus does not show these shapes as well as basalts, but still tend to be very angular with straight sides. The diabase and basalt is also more of an orange-brown color than the arkose, which is more maroon or reddish. 

 

akrose 

Figure 2. Sample of arkose. Notice its reddish color. Basalt and diabase tend to be a more rusty color. Basalt and diabase are also harder than arkose. Sometimes arkose contains visible pebbles.

 

2.     In some places you can observe polygonal shapes on the surface of basalt and diabase that represent jointing (cracking) that occurred as the young rock cooled and contracted. Look for surfaces like this on large pieces of diabase. If you are lucky to find a good example of a pattern of polygons (like a mosaic) on a rock’s surface, take a picture of the feature or illustrate it. Indicate where you found it on your map. (Note: Mudcracks, which form when a very fine-grained sediment in a lake bed dries out can also look like this. But the rock will then be a dark maroon or dark gray color, rather than a rusty brown color.)

column tops on basal outcrop

 Figure 3. Column tops on basalt outcrop. The filling between the columns is more resistant to weathering than the basalt, so the filling stands out as ridges.

 

 

 

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