GEOLOGY

GEOLOGY. Ordovician rocks in the Dubuque area were formed from deposits made in shallow water that covered this region as much as 475 million years ago. The middle portion of Iowa's Ordovician rocks, the Galena Group, form the prominent palisades along the MISSISSIPPI RIVER. Hills north of Dubuque show shale and limestone rock covered with a thin layer of glacial drift and loess.

Lying just below the surface to depths of hundreds of feet are aquifers that vary from a few feet to hundreds of feet in thickness. Aquifers may be local or reach hundreds of miles. Near surface aquifers, found beneath Dubuque, occur in irregular layers within glacial drift, buried channels in the bedrock, or alluvial deposits near stream beds.

Aquifers located in alluvial deposits found in river valleys are one of Iowa's finest sources of water. Such aquifers found along the Mississippi River range from 100 to 160 feet thick. Surface aquifers yield from several hundred to several thousand gallons per minute. The Cambrian-Ordovician aquifer underlying 66 percent of eastern Iowa and between 400 and 500 feet thick can yield up to two thousand gallons per minute. The quality of this water varies from good to unusable. The Dresbach aquifer has provided wells with as much as three thousand gallons of water per minute with quality again ranging from good to unusable.

The persistent myth that Dubuque had seven hills led to the city being called the "Rome of the Midwest" or Little Rome."

Scientifically, Dubuque has no hills. Geologists as early as 1900 found the hills were actually a line of steep slopes winding back and forth to form rims of valleys. These hills are more properly called "ridgelines" or "crestlines." They were formed when water from melting GLACIERS scoured ravines and valleys, like the one through which Central Avenue passes, into layers of soil. The origin of the seven hills story is believed to be an article written for Harper's magazine between 1870 and 1900. (See:EARTHQUAKES)