Dating Deglaciation

Laurentide Retreat

Bulk Sediment Data Calculation:

Francis Lake (NJ) and Sterling Pond (VT)

431.22 km

Difference in Age: 22200 - 15200 = 7000 years

431.22 km / 7000 years or 6.16 km / 100 years

Calculated Rate: 6.16 km / 100 years

Macrofossils Data Calculation:

Dutchess Quarry Cave (NY) and East Barnet (VT)

382.44 km

Difference in Age: 14700 - 13100 = 1600 years

382.44 km / 1600 years or 23.9 km / 100 years

Calculated Rate: 23.9 km / 100 years

Cosmogenic Nuclide Data Calculation:

Androscoggin Moraine (ME) and Terminal Moraine (NJ):

478.84 km

Difference in Age: 25200 - 13200 = 12000 years

478.84 / 12000 years or 3.99 km / 100 years

Calculated Rate: 3.99 km / 100 years

The data shown above are the three ways in testing for the time it took the Laurentide ice sheet to retreat to various points in New England. The first calculations were done for bulk sediment, then macrofossils and cosmogenic nuclides. The calculated data predicted that the slowest retreat of the Laurentide ice sheet was 3.9 km / 100 years which was dated using cosmogenic nuclides. This is followed by the bulk sediment at 6.16 km / 100 years, and macrofossils at the quickest retreat at 23.9 km / 100 years.

The differences between the three techniques can be accounted for by how each dating technique is used. When dating bulk sediment there is always going to be difficulty in discerning whether the sediment is from a glacial moraine or sediment that was moved there after the moraine. Same goes for macrofossils. The fossils that are carbon dated in bottoms of lakes or ponds can be evidence of a non glaciated time but there is a lag for when there is a glaciation and the regrowth of plants. The most promising technique used is cosmogenic dating, especially for lowland areas where there is no chance for continued glaciation after the major ice sheet has retreated. The evidence coincidences with Lambeck's data concerning the observed sea level over the past 35 thousand years showing a start to a rise around 25,000 years ago. This is similar to the oldest age of 27,500 years, using cosmogenic dating, of glacial retreat. Lambeck's graph is shown below.