Surficial Geology - Frequently Asked Questions
- Q1. What is "surficial geology"?
- Q2. What's the difference between a surficial geology map and a soils map?
- Q3. How do glaciers form?
- Q4. How many times was Maine covered by glacial ice during the Ice Age?
- Q5. What is the "Desert of Maine", and is it similar to other so-called deserts in Maine?
- Q6. Rocks keep coming to the surface in my garden. How do they get there?
- Q7. How do we know that parts of southern Maine were covered by the ocean when the last glacial ice sheet melted away?
- Q8. Can we determine the ages of glacial events from the fossil record?
- Q9. Where can sand and gravel be found?
- Q10. Are we out of the "Ice Age" yet?
- Q11. What is the biggest glacial boulder in Maine?
Surficial geology concerns the loose sedimentary materials that overlie bedrock and which are found at or near the Earth's surface. It includes most materials that can be dug with a shovel or mechanical excavator. Common examples of surficial sediments are sand, gravel, silt, clay, till, and peat. Many of these deposits were formed by glacial ice or meltwater issuing from the ice. Others are the products of nonglacial streams, wind, organic action, landslides and other mass movements, or waves and currents on shores of lakes and the ocean.
A soils map shows the different kinds of soil formed near the land surface. This information is especially important for agricultural purposes. Many types of soil form by surface weathering of underlying sediments (parent material), though other factors such as slope and drainage also determine soil type. Maine soils usually extend no deeper than about 3 feet. A surficial geology map shows various types of glacial and postglacial sediments such as sand, gravel, and clay. These are the parent materials on which most Maine soils have developed, and in many cases they extend deeper than the zone of soil formation. The surficial sediments, in turn, overlie the solid bedrock that occurs everywhere beneath Maine.
Glaciers develop over a period of centuries to thousands of years, when a combination of cold climate and abundant snowfall causes long-term snow accumulation to exceed summer melting. As the snowpack thickens, the deeper portions become compacted into glacial ice. The ice mass is plastic and reaches a point where it spreads (flows) outward under its own weight. Glaciers range from small alpine glaciers (which are mostly confined to valleys) to giant continental ice sheets such as those found today in Antarctica and Greenland.
There is much evidence of the most recent continental ice sheet (the Laurentide Ice Sheet) but this latest glaciation eroded away, or left sedimentary deposits that covered, most traces of previous glacial episodes. Geologists have found numerous places in New England that show remnants of deeply weathered glacial till that was deposited by an earlier ice sheet. There were several glaciations in other parts of the country during the last 2-3 million years, but we don't know how many of them covered Maine.
Most of these "deserts" originated in late-glacial time, when strong winds swept across the barren landscape that was freshly exposed by recession of the ice sheet. These winds eroded sandy marine deposits near the coast, as well as river valleys farther inland. The sand was blown downwind, usually to the east or southeast from its source, and deposited in dunes like those seen in deserts around the world. Most of the sand dunes eventually became vegetated and stabilized, but in some areas of Maine, grazing by farm animals exposed them to renewed wind erosion during historical times. The well-known deserts in Freeport and Wayne were reactivated in this fashion.
Frost action heaves the rocks upward during the winter. When the ground thaws in springtime, soil material tends to migrate into the space beneath each rock, due to the action of gravity and groundwater seepage. So the rock can't sink back to where it was the previous year, and there's a net upward movement. Eventually it works its way to the ground surface.
Q7. How do we know that parts of southern Maine were covered by the ocean when the last glacial ice sheet melted away? Back
Many low-lying areas of southern Maine are blanketed by clay deposits. These clays are know to be marine, because they contain fossil shells of clams, mussels, scallops, barnacles, and other marine life found in the ocean today. In some places where the clay is not weathered, the actual shells are fully preserved. Elsewhere they have dissolved away, leaving only their impressions.
Most geologically recent fossils in Maine are found in the marine clays deposited shortly after recession of the Laurentide Ice Sheet. If they are well preserved, samples of shells, wood, etc. can be sent to a laboratory for radiocarbon dating (based on their content of the carbon-14 isotope). The ages obtained in this manner are approximate and must be adjusted to determine their equivalents in actual calendar years. In most cases, ages of fossils from marine clays only tell us that the glacier was gone by a certain time. However, fossils are occasionally found in sediments formed right at the glacier margin, and then we can get a more definite timing of glacial retreat from different parts of Maine.
Much of the sand and gravel used for construction purposes in Maine was deposited by glacial meltwater. Water runs downhill (unless under pressure in subglacial tunnels!), so most gravel deposits are found in valleys and other lowlands where they were laid down in glacial outwash streams, glacial lakes, or the ocean. Ice-tunnel deposits (eskers) are notable exceptions. Some of these ridges of sand and gravel climb up hillsides and pass through elevated gaps in the hills. Surficial geologic maps are a useful guide to where geologists have found sand and gravel resources.
Not necessarily. It has only been a little over 10,000 years since Maine was covered by glacial ice. Studies of annual layers of ice from the Greenland ice cores show that the climate is very fickle and can abruptly become much warmer or colder. Future trends are difficult to predict, especially given the human influence of "global warming."
The largest known glacially transported boulder in Maine is Daggett Rock (PDF, 0.6Mb) in Phillips. It is about 80 ft long, 30 ft wide, and 25 ft high, with an estimated weight of 8000 tons! Daggett Rock is a true glacial "erratic," meaning that glacial ice carried it far enough that it came to rest in a place where the underlying bedrock is different. This giant boulder is believed to have traveled many miles from its source in the Saddleback Mountain area.
Last updated on January 20, 2011