A big surprise at 71st Street underpass

A construction worker stands next to a boulder dug out of the ground near the 71st Street work site.
The project to build an underpass at 71st Street in Bridgeview has been marked by many surprises, but none as big as the one that workers found recently.
A boulder the size of a small car was dug up recently 18 feet underground. There were no other rocks found in the vicinity, which was mostly clean dirt.
How did the boulder get there?
Most likely it was left by a retreating glacier from the last Ice Age that covered Illinois from 25,000 to 14,000 years ago. Often, the glaciers carried boulders or rocks called erratics long distances from their source to where they were dropped as the ice receded. This one was covered over by tons of dirt in the ensuing millenniums and remained undisturbed until now.
These same glaciers carved out the rivers, gorges and marshes that mark the southwest suburbs as well as deposited the sediment that became the hills, ridges and moraines that dot the same area.
The boulder was one of the nicer surprises workers have found over the past year. Time consuming surprises included buried cables not being where they were supposed to be, which delayed the project last winter.
Work on the $20 million project is past the halfway point and the weather has continued to mostly cooperate.
Workers recently started pouring the abutment foundation on the south side of 71st Street. That piece of the puzzle will contain about 300 cubic yards of concrete.
Mayor Steve Landek said the project is on target to be completed by late summer.

"We’re tentatively planning the ribbon cutting for the beginning of August," he said.

Work that remains to be done includes support beams and rebuilding 71st Street, which will actually be among the final phases, Landek said.

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