As they so often do, the past and the future are colliding.

Workers building what should end up as the state's tallest skyscraper are drilling into the foundation of what sat before on the site: the J.L. Hudson's department store downtown, more than two decades after it was imploded and more than a century after it opened.

The foundation pieces are breaking drill bit teeth, delaying work a little, but not enough to cause concern.

"Hitting obstructions is a very normal part of drilling caissons," said Joe Guziewicz, vice president of construction for Dan Gilbert's Detroit-based Bedrock LLC real estate development, management, leasing and ownership company, which is developing the $909 million project.

Caissons are part of the foundation that will ultimately support the tower that could rise up to 912 feet in the air, and the so-called "block" — a smaller multiuse building to the north with an activated alley separating the two.

Pieces of the old foundation as well as other materials have caused more than 300 hours of what's known in the industry as delayed obstruction time so far — but that's common for a large new development in a major downtown environment, Guziewicz said. He declined to say how many obstruction hours were projected when construction first began.

"This is normal," he said. "But it's kind of cool, some of the old foundations that we have dug through and cut through."