Archive for March, 2007

Glacial Lake Concord

Many geologists believe that about 18,000years ago, the Wisconsinan glacier was retreating north and leaving behind two massive blocks of ice that as they melted created Glacial Lakes Concord and Sudbury.

Near Concord, Glacial Lake Sudbury occupied the lowland generally south of Massachusetts 2 (the Sudbury/Assabet watershed) and Glacial Lake Concord occupied the area mainly north of Massachusetts 2 (the Mill Brook watershed).

Walden Pond and Sandy Pond are kettle-ponds created from blocks of glacial ice large and deep enough to extend to the underlying ground water located at the bedrock interface.

As the glacier continued to melt Walden Pond and Sandy Pond water could and still do flow north through the sand and gravel kame (Brister’s Ridge) created when the glacier melted on to the north, pausing to deposit another such ridge a mile further now known as Author’s Ridge. The Great Meadows and Bedford Levels sand plains were created from fluvial outflows towards today’s Concord River.

Today, the Mill Brook drains the bed of the original Lake Concord and is fed by numerous springs that flow from the kames formed on the norh side of Walden and Sandy Ponds located immediately south and at higher elevations.

The most obvious examples include:

Walden Pond (elev. 158′) appearing at Brister’s Spring (elev. 135′) flowing into Fairyland Pond (elev. 127′) and merging at Cambridge Tpke (elev. 125) with a branch of the Mill Brook flowing south from near the Hawthorne Lane Bridge before flowing west to the Mill Dam and then the Concord River (elev.112).

Sandy Pond (elev. 221′) flows north into Crosby’s Pond (elev. 131′) and thence the Saw Mill Brook before joining outflow from Bedford Levels near the Hawthorne Lane Bridge and thence flowing south to join the branch of the Mill Brook from Fairyland Pond at the Cambridge Tpke bridge 2.

Sandy Pond outflow also appears as the Elm Brook and flows into the east side of the Bedford Levels. To the west of the Bedford Levels, a new branch of the Mill Brook begins flowing south passing Miriam’s Corner (elev. 137′) before joining the Saw Mill Brook near the Hawthorne Lane Bridge.

Illustrations below are from USGS, ESRI.com, Roadside Geology of Massachusetts by James W. Skehan copyright 2001 and H.W. Gleason 1906

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