Apostle Islands National Seashore

Transcription

1 Apostle Islands National Seashore David Speer & Phillip Larson October 2 nd Fieldtrip Report

2 Table of Contents Introduction 1 Stop 1: Apostle Island Boat Cruise 1 Stop 2: Coastal Geomorphology 5 Stop 3: Apostle Islands National Seashore Headquarters 12 Stop 4: Northern Great Lakes Visitors Center 13 Stop 5: Montreal River 16 Stop 6: Conclusion 17 Bibliography 18

3 Introduction: On October 2 nd, 2006, I and the other Geomorphology students took a field excursion to the Apostle Islands and the south shore of Lake Superior. On this trip we made five stops along our journey where we noted various geomorphic features and landscapes of the Lake Superior region. We reached the south coast of Lake Superior in Bayfield, Wisconsin at around 10:00am. Here we made our first stop which included the entire boat trip we took which navigated through the various islands in the Apostle Island National Lakeshore. Our second stop was a beach on the shore of Lake Superior where we noted active anthropogenic and environmental changes to a shoreline. Stop three was at the visitor center for the Apostle Island National Lakeshore where we heard a short talk by the resident park ranger as he explained some of the unique features of the islands and why they should be protected. After that we proceeded to our fourth stop which was at the Northern Great Lakes Visitor Center. Here we learned, through various displays and videos, how the region had been impacted by various events over time. Finally our fifth stop was at the point where the Montreal River emptied into Lake Superior (Figure 1). The purpose of this field trip was to build awareness of this unique landscape and the geomorphic processes that created the features we see today, not only in a local scale but a regional scale as well. Stop 1: Apostle Island Boat Cruise: Our first stop of the trip was a cruise through the Apostle Islands. We arrived at Bayfield at 10:00am and boarded the Isle Royal Queen on Lake Superior to begin our voyage. It was fairly warm at 21 degrees Celsius. It was a clear day for the most part with some light cirrus clouds appearing throughout the voyage. The first thing we learned was that the Apostle Islands were formed during the last ice age which ended anywhere from thousand years ago.

4 Figure 1 Map of the Lake Superior Region toured on October 2 nd, any future reference to islands and stops can be referred back to this map (TerraServer.com).

5 When the glaciers grinded through the area they shaped the landscape we see today. The force of the glaciers was able to scour out the landscape down to the sandstone bedrock, which we see today. As the glaciers melted away large amounts of glacial outwash and debris were left behind and created the unique landscape we also see today. So what you see is sand and till deposits on top of the islands from the receding glaciers and a bedrock sandstone formation at the base of the islands. The islands structure was composed mainly of bedrock sandstone which was known as Brownstone of the Bayfield group. This formation was created roughly one billion years ago as ancient fluvial systems carried sand into the area. Figure 2 is a prime example of some of the bedrock sandstone we saw on our voyage. Figure 2 Brown bedrock sandstone (brownstone) exposed to the coastal processes of Lake Superior. The same sandstone can be found making up the bricks of buildings in Chicago and Milwaukee. These sands eventually solidified with time and pressure into the sandstone we see. These rocks were clearly visible along the shoreline of each island as it had been eroded away by the lake s waves. This brownstone, as the name indicates, was brown to reddish-brown in color. The

6 layers of brownstone had been deposited in horizontal layers that dip slightly from north to south. Knowing this then the conclusion can be made that the Devil s Island sandstone is the youngest and most elevated layer of sandstone in the area. The journey itself started at the port of Bayfield and proceeded northeastward along the northern shore of Madeline Island, the largest of the Apostle Islands. As we continued in this path we passed both Basswood Island and Hermit Island to the southeast. Madeline Island is, by far, the largest of the Apostle Islands and is the only Island here to have a permanent settlement on it. The population uses a ferry system to transport themselves and their necessities back and forth between the mainlands. Basswood Island is an average sized island here. It is approximately 1,917 acres with very rocky shores (sandstone). Hermit Island was very small at only 778 acres but there was accumulation of sand along its southwestern shore creating a small beach. After passing Hermit Island we turned northwest and passed between Stockton Island and Oak Island. Stockton Island is the second largest of the Apostle Islands. At 10,054 acres, Stockton Island has become one of the premier camping and recreation islands in the park and is also known for its world famous tombolo. The formation of a tombolo occurs when nearby islands are situated in such a way that sediment becomes deposited between the islands. The angle and exposure the two islands have with open water and longshore drift cause wave refraction and sediment deposition. Figure 3 shows how wave refraction from the formerly offshore island causes the convergence of longshore drift sediment. Tombolos are also a unique environment composed of dunes, swales, ridges and wetlands. Theses features represent the coastal processes from the past. A second depositional feature the Apostle Islands are known for is a sand spit. Sand spits are long stretches of sand that are deposited through longshore transport. The land ward side of a

7 sand spit is well protected, while the lakeward sides are exposed to wave action. Long island is not actually an island but rather a well developed sand spit that has been connected to the mainland for over 30 years. Figure 3 Formation of Stockton Island s tombolo by wave refraction and longshore drift (Mapquest and

8 Oak Island is the tallest of the Apostle s, as well as being quite large. At 5,078 acres it has long, Oak Island served as a landmark for sailors nearing the port of Bayfield. We continued north and passed a few of the smaller islands including, Manitou, Otter, Rocky, and Bear Islands. We finally reached our farthest extent at Devils Island, which was very impressive from a geomorphic and geologic standpoint. Devils Island is unique and spectacular because of its sea caves which have been cut into the sandstone. There are also sandstone cliffs which drop into the water on the northern half of the island (Figure 4). The sandstone here on Devils Island is the youngest of all the bedrock seen on the islands. After circling Devils Island we began our journey back to port. Figure 4 Sea caves and sandstone cliffs along the northern part of Devils Island. Devils Island has the most prominent sea caves because it has a more intense exposure to open water. This time we hugged the shoreline as we traveled and the only new island we passed by was Raspberry Island. Raspberry Island is a very small island at 296 acres but is known mainly for

9 its lighthouse, which was the second lighthouse built on the Apostle Islands. Recently it has undergone major shoreline rejuvenation to prevent further shoreline erosion (Figure 5). Figure 5 Raspberry Island and its prominent lighthouse, rocks placed along the shore absorb waves that would otherwise erode the shoreline at a quicker rate. Stop 2 Coastal Geomorphology At around 2:20 pm the class visited Bayfield s most popular beach to look at coastal geomorphology. The beach was located directly north, adjacent to the pier where our boat was harbored. The conditions were pleasant with a temperature of 18º C and winds out of the north/northeast 8 km/hr. There were several features on and around the beach that made this particular spot relevant to coastal geomorphology. The formation of the beach can be attributed to the jetty put into place on the south end of the beach. Jetties are a common way of artificially shaping coastal landscape by trapping sediment carried in longshore drift. Longshore drift is the process by which waves hit shorelines in an oblique manner, resulting in a net movement from one direction to another. The direction the waves hit the shoreline is primarily determined by the water current, or wind direction. Sediment such as sand and silt is carried by the waves as they break along the shore. When waves recede