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«A Thesis Presented by Soma Das to The Graduate School in Partial Fulfilment of the Requirements for the Degree of Master of Science in Geosciences ...»

-- [ Page 1 ] --

ORIGIN AND EVOLUTION OF DRY VALLEYS SOUTH

OF RONKONKOMA MORAINE

A Thesis Presented

by

Soma Das

to

The Graduate School

in Partial Fulfilment of the

Requirements

for the Degree of

Master of Science

in

Geosciences

Stony Brook University

August 2007

Copyright by

Soma Das

August 2007

Stony Brook University

The Graduate School

Soma Das

We, the thesis committee for the above candidate for the

Master of Science degree, hereby recommend acceptance of this thesis.

Gilbert N. Hanson, Thesis Advisor Distinguished Service Professor, Department of Geosciences Troy Rasbury, Chairperson of Defense Assistant Professor, Department of Geosciences Henry Bokuniewicz Professor, Department of Marine and Atmospheric Science This thesis is accepted by the Graduate School Dean of the Graduate School ii

Abstract

of the Thesis

Origin and Evolution of Dry Valleys south of Ronkonkoma Moraine by Soma Das Master of Science in Geosciences Stony Brook University Long Island hosts a network of straight, parallel Dry Valleys with few tributaries along its southern fork, south of Ronkonkoma Moraine.

In this work I shall examine the characteristics of these Dry Valleys with the help of Digital Elevation Maps. It shall be shown that the distinguishing features of these Dry Valleys- their straight drainage patterns, few tributaries, rectangular watersheds, steep walls and flat floors at some places- are similar to the valleys formed on perennially frozen ground (permafrost) and latter sapped by groundwater associated with perched water table.

I shall propose four models towards interpreting the formation of these Dry Valleysoutwash plain valleys, groundwater sapping valleys, tunnel valleys and surface runoff valleys.

I shall also focus on the East Hampton region of Long Island, and discuss the special features observed here, Scuttlehole Ponds, where tributary extensions of a Dry Valley are iii preserved across depressions that were formerly filled with buried ice.

The event of streams flowing across depressions filled with buried ice will suggest the timing of the retreat of the Laurentide Ice Sheet during the Last Glacial Maximum in Long Island, around 18,000 to 20,000 before present (10Be age from boulders).

Certain characteristic features of the Dry Valleys, coupled with the evidences of permafrost on Long Island and surrounding regions of New Jersey and Connecticut in the form of fossil ice-wedge, sand-wedge and thermokarst involution, suggest that these Dry Valleys developed after the glacier retreated from Long Island - when there was desert tundra and the environment was periglacial.

iv Acknowledgements

I record my sincere thanks to my advisor Professor Gilbert N. Hanson who provided constant guidance through his wise suggestions and directions in the right track in the completion of the project. I am grateful to him for his support and help from the first day I walked into the Stony Brook campus till the last day I worked on this thesis. Special thanks are due to Mark Demitroff for acquainting me with the permafrost in the field around New Jersey region and also for providing me related valuable study materials. I am greatly thankful to Professor William Nieter for his unique gesture in lending his thesis, helping me thereby to know more about the glacial history of Long Island. My lab colleagues whose cooperation was of immense value to me deserve my heartiest thanks. I am also grateful to my parents living in India, for providing me moral encouragement. Last, but not the least, the contribution of my husband Kundan Sen, but for whose constant inspiration and encouragement and the help in giving final shape to my thesis was not possible, merits special acknowledgement.

Table of Contents

–  –  –

2.6. Stream Order Classification by Ojakangas,1982. The figure shows a 4thorder valley with finger tip tributary labeled as 1. Two finger tip tributaries combine to form 2nd order tributary, labeled as number 2 in the figure. Two 2nd order then combines to form a 3rd order tributary. Two 3rdorder tributaries finally form the main trunk channel which is the highest order and is labeled as number 4, Ojakangas1982............................ 16

2.7. The figure shows the Cannan Dry Valley Patchogue, Central Long Island. It has small number of tributaries and the highest order is 3, which represents the trunk channel, DEM.............................. 18

–  –  –

2.9. An example of coarse drainage density. The dry valley here represents East Meadow Dry Valley Freeport, Western Long Island, NY with only 2orders of tributaries, DEM.

–  –  –

2.12. Angulated Drainage of Cannan Dry Valley, Patchogue. The tributary valleys meet the trunk valley at steep angles of 40◦, hence the name. The yellow line is the line of cross-section of the tributary valley. The profile indicates a V-shaped, steep walled valley, DEM...................... 23





2.13. The figure represents the flat-floored profile of Cannan Dry valley in Patchogue, Central Long Island, NY. The yellow line shows the cross section path.... 24

–  –  –

2.15. Steep V-shaped transverse profile of tributary valley of Setuck Dry Valley, East Moriches, Eastern Long Island. The yellow line shows the cross section path......................................... 26

2.16. The figure illustrates the sudden change in slope along the thalweg which is the floor of the basin. The yellow line shows the cross section path, DEM. 27

2.17. Seismic study of the Atlantic Coastal Shelf. The figure on the top shows the southern part of Long Island. Seismic study of the Atlantic coastal shelf reveals that the southward extension of the Dry Valleys join the dendritic network of the paleochannels (Foster and Scwab, 1999)............ 28

–  –  –

2.21. Three elliptical ponds are shown in the DEM, with the tributaries of the Hayground Cove Dry Valley system cutting through.............. 32

–  –  –

2.24. Thermokarst involution in Stony Brook Campus, (Kundic and Hanson 2005) 37

2.25. Tributaries of runoff valleys preserved as extensions of groundwater sapping valleys in Swan Dry Valley in Medford..................... 38

2.26. Process of groundwater sapping and formation of streams, (Dunne, 1980). 39

3.1. Permafrost thaws due to two sources of heat, Szewczyk, 2005......... 46

3.2. Models describing formation of Dry Valleys in Long Island: (a) At the time of Ronkonkoma Glacier advance(60-20 kyr), (b)Ronkonkoma Glacier retreat(20-18 kyr), (c)During time when of permafrost until about 15kyr.. 47

3.3. Models describing formation of Dry Valleys in Long Island: (d) During time of perchedwater table on melting permafrost (15-10kyr), (e)After permafrost melted and before water table rose with post-glacial sea level rise (10kyr to ?) 48

3.4. Scuttlehole Ponds alligned in the nw-se direction, Sag Harbour, Long Island. 49

–  –  –

Long Island hosts a network of straight, parallel Dry Valleys with few tributaries, and small drainage areas south of Ronkonkoma Moraine. These valleys are Dry Valleys because they do not have water flow at present and are therefore dry mostly at the upstream ends which is at the northern ends of these valley system. However as we move further south along the valley lengths on the DEM, we can see water present at their lower ends, where the bottom of the valley intersects the water table. The Dry Valleys are named by the ponds and lakes that they cut through for example the Swan Dry Valley cuts through Swan Lake in Medford. There are two valleys- the Connetquot River Valley and the Carmans River Valley, which were not studied because these are broad, occupy large areas and originate north of Ronkonkoma Moraine unlike the other dry valleys of this study all of which originate south of Ronkonkoma Moraine. The valleys are visible clearly on a digital elevation map of Long Island (Figure 1.1). A larger version of the DEM of Long Island with the names of the Dry Valleys is enclosed in the back cover of the thesis.

According to Fuller (1914) these valleys are outwash channels that were formed due to the collection of outwash waters through the agency of an older topography instead of by normal conditions pertaining to outwash accumulation. He described these valleys as well-defined channels with flat bottoms, terraced borders and steep banks. The valleys as observed in the DEM are aligned in the North-South direction.

Such Dry Valleys are also found in England, The Netherlands, Siberia on the previously formed permafrost. Here the Dry Valleys have a subdued cross-section, and are distinct from the valleys which are currently occupied by streams. According to Schmitthenner (1926) these Dry Valleys are also called dells or dellen. He described dellen as flat, elongated, Figure 1.1: DEM of Long Island, from eastern Nassau in the left to East Hampton in the right.

often branched depressions, with uniform gradients, the walls of which merge into each other in a gently rounded manner without a break at the bottom. Penck (1924) referred to them as corrasional valleys based on their mode of origin. Dry Valleys with flat floors have been termed cradle valleys. Shallow valleys at the head of the present streams are called rain valleys (Greenwood, 1877). Dry Valleys have been referred from several places in the world including Britain where these are preserved in the chalk of Southern England, they are also known as coombes or escarpment valleys (Reid, 1887). In Britain, Chittern Hills preserve such a network of asymmetrical Chalk Dry Valleys. Ollier and Thomasson (1957), argued that the asymmetry of these valleys are the result of frequent freeze thaw activity on the south and west facing slopes during the fluctuation of climates with colder east facing slope remaining inert (perennially frozen). Ollier and Thomasson (1957) commented that ‘it is worth stressing that by becoming dry the valleys have been left in stage of youth, relics of an early period of erosion, and their features are ”fossil” and not related to the present day conditions ’. Many of these kinds of Dry Valleys having a general asymmetry have been discovered from several other places in the world including Pleistocene gravels in Southern Germany, glacial outwash sands in the Netherlands (Gregory and Walling, 1973). Other places where these kinds of Dry Valleys are present include Wyoming (USA)(Walker, 1948) and in permanently frozen subsoil in Siberia (Schostakowitsch, 1927). Some of these terms refer to the geological conditions and environments that give rise to the Dry Valleys.

There are three proposed areas of origination of Dry Valleys, as follows:

1. Areas of permeable rock and particularly limestone. However, studies in Britain revealed that these valleys can also form on other rock outcrops (Gregory, 1966).

2. Areas which experienced periglacial conditions during Pleistocene times. In western, eastern and central Europe they are called Dellen and in North America the same term applies to rounded depressions in Piedmont plains (Sharpe, 1941).

3. Areas characterized by an arid to semi-arid environment often have wadi-systems.

The wadi-systems include systems which do not function during the occasional present storms and thus resemble Dry Valleys (Gregory and Walling, 1973).

All geomorphic features reflect the role of some geological processes and in many cases like Dry Valleys, may reflect the paleoclimate or the conditions in which these features developed. Thus the main interest in studying Dry Valleys of Long Island is to evaluate the role of the geological processes and also the climate in forming and shaping these Dry Valleys. To understand all these aspects, it is necessary to know the following aspects of Dry Valleys.

1. What are the characteristics of these Dry Valleys?

2. What geological agents and factors were important for their origin?

3. When and under what climatic conditions did these dry Valleys develop?

4. Why are the valleys dry at present?

Certain characteristics of Dry Valleys such as their shape (straight, meandering and braided), area occupied (watershed), number of drainage tributary valleys (order) and the cross-section of the valleys at separate elevations (transverse profile) should enable us to understand the processes that were important for their evolution and development. I will

test four hypotheses for the formation of these Dry Valleys, as follows:

Figure 1.2: Two types of Dry Valleys in Long Island.

The Cannan Dry Valley has low order of tributaries and the Setuck Dry Valley on the right has higher order of tributaries.

1. Melting water from glacier forming outwash plain valleys

2. Sub-glacial meltwater forming tunnel valleys

3. Surface runoff forming dendritic drainage network

4. Groundwater sapping process leading to the development of straight, steeply sloping, low order stream valleys.

Dry Valleys of Long Island mainly exhibit two types of features. At some places the valleys are straight, parallel to each other and have few tributary valleys. The Cannan Dry Valley and Swan Dry Valleys in Patchogue and Medford exhibit these features. Other Dry Valleys have dendritic drainage networks and a large number of tributary valleys, as in Setuck Dry Valley of East Moriches (Figure 1.2).

Analyzing these characteristics, along with other identifying features of Dry Valleys discussed in the next chapter, I shall discuss the role of surface runoff process and groundwater sapping process in forming these particular valleys of Long Island. Impervious frozen ground or permafrost and a very cold, dry climate were also important for the formation of the Dry Valleys.



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