2.2.3 Geology

Two major areas typify District geology.  The uppermost area, called the surficial geologic zone, contains materials deposited by glaciers.  Precambrian, Cambrian and Ordovician rock formations make up the lower or bedrock geology.  This section describes each major geologic area.

Glaciers crafted the watershed's surface geology when they retreated and dropped "drift" on the land.  The glacial drift refers to any of the unconsolidated sand, clay and gravel materials deposited by glaciers.  This material forms Lake Minnetonka's morainic hills and lake basins, and the sand and gravel deposits next to Minnehaha Creek and the Minneapolis Chain of Lakes.  It generally covers and obscures the underlying bedrock.  All of the known glacial drift came from "the Pleistocene Epoch's Wisconsin Age".

The Superior glacial lobe that flowed into the area from the Lake Superior basin to the northeast dumped the sandy glacial drift that lies under the entire District.  Crystalline igneous and metamorphic rocks primarily make up the glacial drift's sandy matrix.  In the western watershed, the sandy drift lies buried beneath a veneer of gray, clay-like glacial drift, laid down by the Grantsburg sublobe which moved northeastward through the area as an offshoot of the Des Moines glacial lobe.  The Des Moines lobe came from the Keewatin District west of Hudson Bay in Canada.  This highly calcareous glacial drift contains limestone and shale fragments typical of northwestern Minnesota and Manitoba rock types.

The glacial deposits run up to 300 feet thick in the buried bedrock valley under the area near Lakes Calhoun and Harriet.  At Minnehaha Falls and along the Mississippi River, the glacial drift has completely eroded away exposing the bedrock.  Figure 4 shows bedrock geology.

Although all of the major geologic units shown in Figure 4 underlie the watershed, only the Platteville limestone and St. Peter sandstones outcrop in this area.  At Minnehaha Falls, the Platteville limestone unit forms the resistant rock layer that slows weathering of the underlying St. Peter sandstone.  This geologic form creates an escarpment at the Mississippi River gorge.

Basin structure and a buried erosion surface or disconformity helped distribute various bedrock formations throughout the area.  Preglacial and glacial streams deeply cut the bedrock and, in the eastern or lower watershed, completely removed all rock down to the top of the Jordan sandstone in several north-south valleys.  The glacial drift has buried these valleys.

From Grays Bay to the watershed's western limit, the disconformity has leveled the bedrock formations and successively older rocks directly contact the glacial drift.  Such contact areas serve as major recharge points to exposed bedrock aquifers.