Geology of the Lakeshore Preserve  - continued

Geology of Eagle Heights Woods

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When the glacier retreated, it left behind sediment called till. It is a mixture of particles ranging from clay size to boulders.    
Many of the boulders, like many of those seen in the wall at the entrance to Picnic Point, were carried southward hundreds of miles by the glacier. These are called erratics.    
Erratics are common on the ground surface on Eagle Heights Hill. This rock is gneiss, a metamorphic rock. The light gray bands are quartz veins.     
Here is another erratic that can be seen from the trail through Eagle Heights Woods. The pink to orange rock beneath the green lichens is rhyolite, a volcanic rock. It likely was carried by the glacier from northeast of Lake Superior.   
This slide shows how the glaciers shaped the landscape of much of Dane County. The hill in Eagle Heights (EHH) is one of many elongate hills formed by the sliding motion of the glacier. It is composed of till over bedrock that is fairly close to the surface. (Slide #11 shows an outcrop of this ancient bedrock). These hills are called drumlins, and there are thousands of them in southern Wisconsin. 

Bascom Hill, Observatory Hill, and the hill the Capitol sits on are also drumlins, but they are composed of till over sand and gravel instead of rock. There are no drumlins in southwestern Dane County where the glacier did not reach.    
Drumlins are oriented parallel to ice flow direction and they are often steeper on the up-glacier end and less steep on the down glacier end, but that is not always the case.    
Drumlins do not form near the edge of the glacier but further back under thicker ice. The glacier was likely about 800 feet thick over this spot when the glacier was at its maximum extent.    
This glacial geology map shows drumlins in brown. Light and dark green are till surfaces, and yellow and orange are sand and gravel. The light blue stippled pattern is the former bed of glacial Lake Yahara.    
When the glacier began to retreat about 18,000 years ago a lake was damned in the low areas in the landscape.    ere to edit.
The shoreline of this former lake can be seen where Observatory Drive rises near Allen Centennial Gardens. 
This map shows the elevation of the bedrock surface beneath glacial deposits in the Madison area. The red indicates where the top of the bedrock is less than 500 feet above sea level. It is the bottom of the preglacial Yahara River Valley. Glaciation partly filled this valley with till, and where the valley was not completely filled, we now have the Madison lakes and the Yahara River. The drop from the top of the hill in Eagle Heights Woods ((EHW) 1010 feet) to Lake Mendota (850 feet) today is about 160 feet. Imagine standing on top of this hill in Eagle Heights Woods before glaciation and looking toward the northeast into a valley that was over 500 feet deep!