In the photo below right part of the mine has been reclaimed with the
downtown buildings of Maple Grove, parking lots and ponds. The
elevation of the city steets is about 20 to 60 feet less than before
the mining started. All gravel is not created equal. In the photos you
should be able to see different layers identified both by color and by
size. Much of the color difference is due to the composition of the
gravel. Glaciers deposited different layers of gravel with the
composition depending on the source of the glaciers. Glaciers that
traveled over Lake Superior and eastern MN will deposit gravel
consisting mostly of basalt, rhyolite and granite (and of course the
treasured agate). Glaciers that traveled over western MN will deposit
gravel containing limestone and shale. Shale is an unacceptable rock
for concrete, asphalt mixtures or as a base for buildings because it
will absorb water and then swell. As it increases in size it will cause
"popouts" in roads or buildings.
Sand and Gravel
The largest mining industry in Minnesota
is Aggregate Mining. Getting the quantities and quality of sand and
gravel (or crushed stone) necessary for construction have resulted in a
statewide effort to locate all of the deposits that are readily
available in Minnesota. The Metro counties have been mapped for likely
resources and the process is spreading to outstate Minnesota. In the
photos below sand and gravel in Maple Grove is mined for construction
projects in the Twin Cities area.
The quarry in the photos just above and below is for crushed limestone.
The quarry is located in Burnsville, just off Cliff Road and I35W. Sand
used in construction often does not have large enough pieces to provide
the correct mixture for road bases or asphalt mixes. In these
situations adding crushed limestone can provide the necessary mixture
of sizes. In addition the "fines", the powder from the crushed stone,
packs very hard when mixed with the larger rock, providing an excellent
base for roads. This base drains water very well and is very stable.
The limestone in this quarry is not used for dimension stone - stone
used directly in building structures - because there are too many
joints. The limestone here is mined in a fashion very similar to
the taconite mining of northern Minnesota. The rock is blasted and then
loaded into large haul trucks and taken to a crusher and sorter. Of
course the further work of processing ore. The machines are smaller
than those currently used in the iron mines.