Mt. Perry Rocks

We received this interesting email concerning the large rocks in Hopewell Township

Hi Tim:

Tom Johnson forwarded your query to me regarding the oft discussed slump blocks of Hopewell Township.  This part of the county was influenced 2 glacial periods.  The Illinoian Glacier, about 300,000 years ago and the Wisconsin about 12,000 years ago. The earlier of the two leveled ridge tops, filled valleys and undercut the existing bedrock in places, while the more resistant rock formation above the cuts was largely undamaged.  Eventually the undercuts could not support the upper elements and the blocks cracked loose from the adjoining surface and slumped off.  The blocks have been migrating down the hills for the last 300,000 years.  The Wisconsin only penetrated the county was far as central Thorn Township but its out wash as it receded north would have assisted in moving the blocks down the hill.  Most of the blocks strata or layers are roughly parallel to the land form they occupy, any that tend toward perpendicular show influence from the Wisconsin out wash event.

Folks used to think that the glacier transported and deposited the blocks on the slopes, but as we now know that is not true.  They undercut the bedrock and caused them to slump off.  As you drive around Hopewell you’ll find them in lots up places.  The slumps higher in the landscape were not influenced by the Wisconsin.  The lower ones were definitely assisted in their movement down the slope.

I hope this helps.

Best regards,

David Snider

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