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As a delta forms most of the sedimentary material is carried across the relatively flat top surface, then deposited over the edge in steeply sloping foreset beds. Finer material is carried further out and deposited as relatively flat bottomset beds. While the delta grows the flat top aggrades and some additional material is deposited as topset beds. Over time the delta develops a cross section such as the one shown below.
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A look at the bathymetric map of the area around the mouth of the Boquet River clearly shows the delta.
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The extent of the topset beds is approximately shown by the 10-foot contour line. You can see that the topset beds of the delta extend about 1000 meters (about 3000 feet) into the lake. Compare this distance with how close to the shoreline the 10-foot contour is just north or south of the delta. Also, note that the delta is not completely symmetrical; it is skewed to the south. This is probably due to the movement of delta sediments by longshore currents tending from north to south.
The foreset beds abruptly plunge downward at about a 20 degree slope and extend down to a depth of about 250 feet. It is difficult to define exactly the bottom of these beds since they seem to sit right on the edge of the scarp of a north-south fault that defines this side of the Lake Champlain basin. (Notice how the contour lines from 60 to 180 feet indicate the steep slope of this scarp extending south beyond the delta.)
The bottomset beds lie beyond the 240 foot contour and are relatively flat.
In the early 1980's the Boquet River delta was studied by T. Curtin as part of his degree in geology. His studies of the delta confirm the information derived from maps and aerial photographs. In addition his sonograms show the steep foreset slopes to be riddled with small slumps where the poorly consolidated materials deposited by the river have subsequently given way.
A major portion of Curtin's work was to gather and analyze sediment samples from the delta. As you would expect the bigger and heavier particles dropped out of the water near the shore while the smaller and lighter particles were carried farther into the lake. Superimposed on the bathymetric map above is the line where sediments change from being more than 50% larger sand particles to more than 50% small clay and silt particles.
Most of the sands are derived from the metamorphic rocks that underlie or river's watershed (see bedrock geology of the Boquet). The sands in the topset beds are well sorted after their deposition. This is probably due to wave action and the longshore current that distorts the shape of the delta.
Most of the silts and clays in the bottomset beds do not come from the bedrock. They were deposited in the Boquet watershed during the last ice age. These deposits are now being eroded by the river and are the source of the sedimentary material being carried into Lake Champlain. The Boquet River is said to be one of the main suppliers of silts and clays to the Central Basin of Lake Champlain.
Curtin pointed out some interesting anomalies in the sediments of the Boquet River delta:
| Return to the material on the aboveground delta. |
References:
Curtin, T. 1983. Investigation of the Boquet River delta: morphological, compositional, and textural analysis. Master' Thesis, Department of Geology, Middlebury College, Middlebury VT, 94p.
Strahler, A. 1981. Physical geology. Harper & Row, New York. 612p.
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