We have all heard it. “There are two seasons in North Dakota, winter and road construction.” Of course, that is a tongue in cheek proclamation. But however you define the road construction season, one thing is certain, gravel and sand pits are going to be busy places. Many of us see this activity, but I suspect that few of us have given much thought as to how these sand and gravel pits were formed.
Sand and gravel deposits are mainly found in the glaciated region of North Dakota north and east of the Missouri River. South and west of the Missouri River they are much less frequently encountered. In that part of the state scoria and crushed rock largely replace sand and gravel.
Glacial deposits may generally be considered mixtures of sand, silt, clay, gravel, and boulders in various amounts. A question one might ask is why are these deposits mainly sand and gravel with little silt or clay? In a word, the answer is water. Sand or gravel pits are typically deposits formed in association with shorelines (e.g. beach deposits) or associated with rivers and streams (e.g. sandbars, gravelbars) often times associated with melting glaciers at the end of the last ice age.
Many sand and gravel deposits are associated with the shorelines of glacial lakes such as glacial Lake Souris in north central North Dakota, glacial Lake Minnewaukan in the Devils Lake area, and of course glacial Lake Agassiz along the eastern border of North Dakota and adjacent Minnesota.
It is worth noting that the beach deposits on the Minnesota side of glacial Lake Agassiz generally are much more pronounced than on the North Dakota side. That is because the prevailing winds out of the west resulted in increased wave action on the downwind side, depositing coarser materials and also flushing out the finer silts and clays.
Other sand and gravel deposits were also formed in association with meltwater channels and rivers toward the end of the last ice age. Some of this water flow was even on or in the glacier. Think of sand and gravel bars forming in rivers and how the energy of the flowage influences the size of the material. And as some of you know, the delta of the Sheyenne River when it flowed into glacial Lake Agassiz is what we now call the “sandhills” of Richland and Ransom counties.
So the next time you pass a gravel or sand pit, give consideration to its formation.
~Chuck Lura
