Mansfield Hollow State Park:  After the Ice Age

This EarthCache will observe post-glacial sediments that were deposited in Mansfield Hollow State Park at the end of the last Ice Age.  You will observe the shape they give to the land and explore how and in what sequence the sediments were formed. The interested cacher may wish to access the Park’s website₁ which includes a short description of the geology. 

1.  http://www.ct.gov/dep/cwp/view.asp?a=2716&q=325236&depNav_GID=1650

A trail map₂ for Mansfield Hollow State Park will be useful and we recommend that you download it before starting this EarthCache.  We also recommend that you pre-read this EarthCache (at least up to Activity 1) prior to starting so that you will notice the lay of the land as you approach the park.

Purpose: This EarthCache is produced by the Connecticut Geological and Natural History Survey of the Department of Environmental Protection.  It is one in a series of EarthCache sites designed to promote an understanding of the geological and biological wealth of the State of Connecticut.

Supplies: Download  2 and bring it with this write-up, a gps unit and water. Spoilers may be included in the descriptions or links.

Location:        N41^o45.867’, - 072^o10.722’,  PARKING LOT

A PART OF THIS EARTHCACHE (ACTIVITY 3) IS IN A PERMITTED HUNTING AREA AND APPROPRIATE HIGH-VISIBILITY CLOTHING (ORANGE) SHOULD BE WORN DURING HUNTING SEASONS.

Directions:  From Hartford: take I-84 east to Exit 68 (Route 195).  Follow Route 195 south 12+ miles to Bassett Bridge Road. On the way you will cross Route 44, go past the UCONN campus and the junction of Route 89 before seeing Bassett Bridge Road. Bassett Bridge Road is the first light after Route 89. Take a left on Bassett Bridge Road. The Park is on your left, off Bassett Bridge Road.  Drive past the main entrance and turn into the boat launch parking area about a third of a mile further on Bassett Bridge Road (if you cross the reservoir you went too far).

From Willimantic: Route 195 north to Bassett Bridge Road. Take a right onto Bassett Bridge Road. Park is on your left, off Bassett Bridge Road.

Introduction:  20,000 years ago Connecticut was in the grips of the last Ice Age.  Glacial ice about a mile thick covered the state; indeed, it covered most of the northern third of North America. Glacial ice is a powerful agent of erosion and creates enormous amounts of sand, gravel and mud. This EarthCache is about what happens to all that sand, gravel and mud when the ice melts at the end of an ice age.

 When the ice melts two things happen. First, the meltwater collects into streams and rivers.   Second, all the debris, large and small alike, on and in the ice gradually gets deposited on top of the ledge that was under the ice, forming a glacial soil.  Glacial soils, in effect get plastered on top of the bed-rock.  It is referred to as glacial till and covers the adjacent uplands that surround Mansfield Hollow.

Water from the melting ice collects into streams and rivers.  Many streams flow on top of the ice toward the edge of the glacier. Many streams on top of the glacier get swallowed into cracks and crevasses and flow in tunnels beneath the ice. Waterpower carries away some of the debris left by the glacier.  The amount and maximum size of material that water can transport is related to the velocity of the flow.  High velocity currents can move large particles.  During the summers at the end of the ice age a flood of water accumulated in high velocity streams which redistributed the debris of the glaciers.

Water is interesting because it has the ability to sort the sediment by size.  High water velocities are able to move both large rocks and small particles.  When the velocity decreases the largest fragments come to rest on the stream bed and only smaller particles continue moving.  The tiniest particles (mud) are carried in suspension and only settle out when the velocity slows to a stop.  This happens in ponds or the ocean.  Former stream beds, like their modern counterparts, consist of sand and gravel without mud.

Furthermore, during water transport, the moving sand and gravel grains bump into each other.  This bumping causes small chips to break off individual grains, rounding of corners and sharp edges.  This can be confirmed by observing any modern stream.  The rock fragments are all rounded.  We call them river-rock.

In addition, streams deposit sand and gravel in beds, or layers.  Each layer is generally deposited parallel to the ground-surface.  Most layers are roughly parallel to one another and usually taper gradually in thickness.  They only end abruptly if they are eroded after deposition.

ACTIVITY 1.  Notice the shape of the land when you turn east onto Bassett Bridge Road.  In general it is a flat plain that has a generally hummocky surface (see map below).  As you approach the park you will notice several depressions on the surface.  Some have a small pond in their bottom; they are deep enough to intersect the water table.  If you look at the map you will notice there are several “holes’ with small ponds on the plain surface.  The map shows that the plain surface has a maximum elevation about 275+ feet.  What underlies the plain will be the focus of our first activity.

After you park the car, find the following location: N41^o46.092’, -072^o10.490’  This should place you near the edge of the boat-launch parking area.  Notice the composition of the materials along the shore of the reservoir and any place where erosion has exposed them (just south of the boat-launch parking area is a small kettle-pond that fills up when the reservoir is flooded; erosion has occurred along the channel through which flood waters enter that pond;  this has exposed sand and gravel).  Actually the parking lot is composed of the same material, but we cannot prove that it was locally derived.  It most likely came from a small abandoned gravel pit located around the corner to the southwest.  This material is notable because it is composed of rounded fragments of sand and gravel….i.e., river rocks.  Several other gravel pits (one that is actively removing gravel for construction purposes) in the vicinity suggest that the entire area with the plain like surface is underlain by sand and gravel.  If you have looked into or been in any gravel pit you may remember that all are

Topographic map showing part of the plain over which Bassett Bridge Road travels.  The parking area is near the area on the map designated “Picnic Area”.  The plain top (far west of map) has an elevation in excess of 270 feet.  Notice two small ponds filling the bottoms of depressions (holes) just to east of Picnic Area.  Mansfield Hollow Lake is maintained at an elevation of about 210 feet, more than 60 feet lower than the plain surface.

layered, all are composed of rounded grains of sand and gravel, and none have much mud.  This suggests they were deposited in the beds of former rivers.  This, in turn, suggests that the entire plain area was similarly deposited by some former river.

The question is, “What river?”   The modern Fenton and Mount Hope rivers flow (or at least they did before the reservoir was impounded) at the bottom of a valley that is 50 to 70 feet lower than the top of the plain.  It is hard to imagine that they ever carried enough water to flow over the top of the plain.  In addition, the modern streams do not carry very much sediment and certainly are not depositing thick layers of coarse sand and gravel along their course.  Besides, if there were enough water to cover the top of the plain, it is more likely that the rivers would erode their banks rather than deposit sand and gravel.  Rivers in the Fenton and Mount Hope valleys, full with glacial melt-water at the end of the last ice age, carried an abundant sediment load and could have deposited much of that sand and gravel that they carried.  It is reasonable to hypothesize that glacial melt-water streams deposited sand and gravel on the plain.  The problem, however, is how the melt-water streams could have flowed at an elevation 60 feet higher than the valley bottom.  We will address that question after another observation in Activity 2.

ACTIVITY 2.  Follow the “Kettle-hole” trail up the hill toward the picnic area of the park and find the following location: N41^o45.903’, -072^o10.625’.

You should be standing on the north side of a large depression (50-60’ deep hole in the ground).  It has steep sides (about a 35+^o slope).   Perhaps you can see another depression to the east that lies just northeast of the large depression next to which you are standing (tree leaves will block your view during the summer and early fall).  A low area between the two depressions has been eroded, allowing us to see that the land here is composed of stratified sand and gravel.  Note that the gravel-sized grains are rounded.  This helps confirm our interpretation that river deposits underlie the entire area.

But, how were the large depressions (holes) formed?”  The topographic map shows several similar holes, mostly filled with water on the large plain of river-deposited (interpretation) sand and gravel.  Modern rivers can erode pools (water-filled holes) that are usually elongate and sinuous.  They usually fill in with sediment shortly after they form (rivers are dynamic and continually changing).  They usually are only a fraction as deep as they are wide.  The holes we see here are roughly circular and have a much greater depth to width ratio than pools in a modern river. It is hard to imagine that any ancient stream would erode such a hole and even harder to explain why any eroded hole was not refilled soon after its excavation, as in modern streams. Thus, it is unlikely that these depressions resulted formed by erosion in a river.

Remember, we interpret that the sand and gravel plain was deposited by streams and rivers fed by glacial melt-water at the end of the last ice age.  Modern glacial melt-water streams deposit extensive outwash plains of sand and gravel in front of the glaciers that feed them. They flow around and in some cases bury blocks of detached ice in front of the glacier.  What if a large block(s) of leftover glacial ice remained in the ancestral Fenton/Mount Hope valley?  The ancestral stream would flow around it and deposit sand and gravel surrounding the ice block, perhaps eventually covering it.  Later when the ice melted, the sand and gravel would collapse into the void creating the hole  Such depressions are called kettles.  There are several kettles on the sand and gravel plain in the Mansfield Hollow area.

If blocks of ice could get in the way of the streams, could a longer more continuous tongue of ice be the last to melt in the very bottom of the valley?  Perhaps this is why the melt-water streams flowed on top of the plains.  They were prevented from flowing in the valley bottom by a long tongue of leftover ice.  Instead the streams flowed along the side of the leftover ice, depositing their sand and gravel on the valley sides instead of on the valley bottom.  Later when the ice in the middle of the valley melted, the streams assumed their valley bottom position that we see today.  Hence, the area occupied by the reservoir today was occupied by ice when the melt-water streams deposited the gravel.

Such ice-contact deposits are called kames.  They are found in many Connecticut valleys.  Many are mined as a source of sand and gravel for construction.  The surface of the deposits is usually 20-60 feet above the valley floor.  They were deposited by glacial melt-water streams that flowed along the side of a valley while leftover ice filled the middle of the valley.  If the deposit is narrow it is referred to as a kame terrace; if it is broad it is referred to as a kame plain.  All are rather hummocky (characterized by depressions and short irregular ridges and hills) and may be pitted with kettles.

ACTIVITY 3.  Follow the trail back to the boat-launch parking area. Get back in your car and drive (left out of parking area) on Bassett Bridge Road about 2 miles to North Windham Road.  Go right on North Windham Road just before the bridge that crosses the Natchaug River.  A parking area is at the end of the road (about a quarter mile;  N41^o45.121’.               –072^o09.675’).

 Follow the abandoned continuation of North Windham road west and north to N41^o45.215’,             -072^o09.912’.

Alternatively, you may cross the reservoir (Bassett Bridge) on foot and hike about 2 miles south along a blue-dot trail to the following location:   N41^o45.215’,  -072^o09.912’. You will encounter several low ridges along the way, eventually coming to the abandoned North Windham Road and the above referenced location.

THIS IS IN A PERMITTED HUNTING AREA AND APPROPRIATE HIGH-VISIBILITY CLOTHING (ORANGE) SHOULD BE WORN DURING HUNTING SEASONS.

Map showing surface deposits surrounding the south end

of the Mansfield Hollow Reservoir (after Stone et al, 2005).

The road goes along the sides and on top of a low ridge, perhaps 30 feet high and several hundreds of feet long. You can tell by the rounded cobbles (i.e., river rocks) on the surface that the ridge is made of sand and gravel.  The largest rocks are a foot or two in diameter and must have been deposited by high velocity streams.  This land form is called an esker.  The question here is why the melt-water stream that deposited the gravel flowed on top of a ridge rather than in the valley bottom?  By now you know the answer is that ice constrained the stream to flow in a long narrow space.  Today we know that melt-water streams can flow on top of glaciers, in cracks and crevasses within a glacier, or within a tunnel beneath the glacier.  The size of the largest boulders suggest that the velocity of the stream was forced by hydraulic pressure in a tunnel beneath thick glacial ice instead of by gravity in an open crack or channel on top of the ice.  The ice may have filled much more of the valley when the esker formed than when the kame plain and kettles formed.

The question for you to ponder:  did the esker form at the same time, before or after the kame.  Did the kettle form at the same time, before or after the esker?  Why? 

How people respond to this EarthCache.

1.  Post a picture showing you or your companions at the edge of a kettle in such a way as to

show the depth of the kettle.

2. Provide and answer to the “Question for you to ponder”.

Difficulty: 3

Terrain: 1.5 or 2 depending on whether your drive or hike.

Type of land:  State Park                   

EarthCache category:  Glacial feature

References:

Stone, J.R., Schafer, J.P., London, E.H., DiGiacomo-Cohen, M., Lewis, R.S., and Thompson,

W.D., 2005, Quaternary Geologic Map of Connecticut and Long Island Sound Basin, U. S. Geol. Survey, Sci. Inv. Map 2784, 2 sheets.  Also see U.S. Geological Survey Open File Rept. 98-371.