20th Massachusetts and the Roxbury Conglomerate

This post is going to be a combination post,first we will discuss the 20th Massachusetts Infantry and then I will mention a little bit about the geology of their monument and why they chose this material.

The 20th Massachusetts Infantry brought 301 men to the fight on the 2nd and 3rd days of the battle out of a total of 5800 Massachusetts soldiers at the battle of Gettysburg. In this time they will lose two commanders and suffer 127 casualties.

The 20th Massachusetts Monument. The tree line in the background is West Confederate Ave.

The Regiment was commanded at the beginning of the battle by Colonel Paul Joseph Revere. He was the grandson of the famous Paul Revere of the revolutionary war. He was mortally wounded on the 2nd and would later die on the 4th. He was replaced by Lieutenant Colonel George N. Macy until he was wounded, ultimately losing his hand on the 3rd day. At this point Captain Henry L. Abbott assumed command of the regiment.

According to the official report written by Captain Henry L. Abbott on July 16 1863, his regiment did not fire a shot as they were in the second line. They did suffer casualties from artillery shelling that penetrated the first line, the regiment commander was among the 11 Casualties on the 2nd.

Abbott writes of a skirmish line set up late in the day, his words are below.

“Two companies had been previously sent out as skirmishers, some distance in front of our lines, under Captain Patten. I wish to mention this officer particularly for the meet distinguished gallantry with which he held his position after losing a third of his command (10 men), remaining on the field after he himself had been severely wounded, only retiring his command when our own advance had been driven back completely routed and the rebel line was close upon him. Second Lieutenant Cowgill was also wounded on this picket.”


At about 2 pm the Confederates opened “a terrific cannonade”, but the regiment lost few men since they were sheltering in a small depression which they had created the previous day. After the confederates ceased their artillery barrage Colonel Hall, brigade commander, gave the order to open fire as the Confederate charge began. Abbott wrote:

“the regiment rose up and delivered two or three volleys, which broke the rebel regiment opposite us entirely to pieces, leaving only scattered groups. When the enemy’s advance was first checked by our fire, they tried to return it, but with little effect, hitting only 4 or 5 men.”

When the Confederates broke through the Union line at the “clump of trees” the regiment turned to face and provided support to a “very thin” line holding back the Confederates. It was at this point the Confederates returned fire both by infantry and artillery causing heavy losses in the 20th Massachusetts regiment.

Abbott writes in recollection of the chaotic events the “clump of trees”:

“It seems to me that great praise is due the enlisted men of this regiment for the speed with which they reorganized, for the discipline and esprit de corps which made them stick together in such a scene of confusion, where organization had been so completely broken up for the time. All the officers of the regiment behaved with the greatest gallantry, but I am enabled to select two, as their position or occupation made them more conspicuous than the rest. One of these (Captain Patten) I have already mentioned. The other is First Lieut. Henry Ropes, who was shot dead…He was in battle absolutely cool and collected, apparently unconscious of the existence of such a feeling as personal danger, the slight impetuosity and excitability natural to him at ordinary times being sobered down into the utmost self-possession, giving him an eye that noticed every circumstance, no matter how thick the shot and shell; a judgment that suggested in every case the proper measures, and a decision that made the application instantaneous.”

I feel I should mention the son of a poet who’s work I will mention later in this blog regarding the geology section. The sons name is Oliver Wendell Holmes JR. Many may know him as the supreme court justice who brought about legal realism after the Civil War. A Boston native Holmes JR joined the 20th Massachusetts volunteers after his senior year at Harvard. He fought at several battles getting wounded 3 times and when his service was up he entered Harvard Law. Some years late he would be nominated by Teddy Roosevelt for Supreme Court Justice.

Oliver Wendell Holmes JR Image from National Archives Arlington

The 20th Massachusetts along with all of the other Union regiments helped repulse Pickett’s Charge and create what we now call the High Water Mark of the Confederacy.

The Monument to the 20th Massachusetts can be found on the left side of the Copse of Trees at the angle. It is approximately 30 tons of “pudding stone” or conglomerate. This material was a very important and influential building material in the areas around Boston in the areas of Roxbury Massachusetts. The scientific name of this rock is the Roxbury Conglomerate although the common name in the New England area was “Pudding Stone”.

Conglomerate simply means that it is made of small to large pebbles, cobbles and boulders. These clasts are held together by a matrix of silt or sand that cements everything in place. If you look at the clast you can get an idea of how far the clasts traveled from their source area. The more rounded a clast the more it was  bounced around and eroded and therefore the longer its travel period. 

View showing the upper portion of the monument. Here you can see the clasts of different sizes bound together by fine sandstone.

The clasts are generally metamorphic and igneous in origin with rock types of granite rhyolite and quartzite in abundance. the pinkish rhyolite was formed during volcanic eruptions and the “speckled” granite formed from cooling magma in the earth. 

rock samples

Close up picture of a section of the Monument showing rhyolite, quartzite and sandstone matrix.

The conglomerate itself is about 570-595 million years old. Because of how it forms the smaller clasts must be older than the conglomerate as a whole. It forms the bedrock for much up the areas surrounding Boston including Dorchester. The current hypothesis on the formation of the Roxbury Conglomerate is that it was formed in a deep marine basin as a network of deep water alluvial fans. These fans would have relatively little lateral transport and would have a similar effect as turbidites (look them up or wait for a later post!). This is obvious in the images above, notice how the clasts are not well rounded and in some cases angular. Compare them to the image below which shows a well rounded conglomerate that underwent a lot of transport. 

1285 conglomerate cyprus
Image of a conglomerate with well rounded clasts (gabbro in this case). This image is from sandatlas.org

It is said that the idea to use this material came from soldier of the regiment who, as kids, would climb on large boulders of Roxbury conglomerate. The Roxbury Conglomerate occurs in outcrops in the Boston area so frequently that it really is no wonder why they chose this to represent the troops of the Boston area. 

 I  mentioned it was influential as a building material. It would also seems that it was a recognizable land mark around the Boston area. Evidence of this can be found in literature from 1830. The following poem written by Oliver Wendell Holmes Sr is inspired by the Roxbury Conglomerate, then known as pudding stone. It is called The Dorchester Giant.

THERE was a giant in time of old,

A mighty one was he;

He had a wife, but she was a scold,

So he kept her shut in his mammoth fold;

And he had children three.

It happened to be an election day,

And the giants were choosing a king;

The people were not democrats then,

They did not talk of the rights of men,

And all that sort of thing.

Then the giant took his children three,

And fastened them in the pen;

The children roared; quoth the giant, “Be still!”

And Dorchester Heights and Milton Hill

Rolled back the sound again.

Then he brought them a pudding stuffed with plums,

As big as the State-House dome;

Quoth he, “There’s something for you to eat;

So stop your mouths with your ‘lection treat,

And wait till your dad comes home.”

So the giant pulled him a chestnut stout,

And whittled the boughs away;

The boys and their mother set up a shout.

Said he, “You’re in, and you can’t get out,

Bellow as loud as you may.”

Off he went, and he growled a tune

As he strode the fields along

‘Tis said a buffalo fainted away,

And fell as cold as a lump of clay,

When he heard the giant’s song.

But whether the story’s true or not,

It isn’t for me to show;

There’s many a thing that’s twice as queer

In somebody’s lectures that we hear,

And those are true, you know.

. . . . . .

What are those lone ones doing now,

The wife and the children sad?

Oh, they are in a terrible rout,

Screaming, and throwing their pudding about,

Acting as they were mad.

They flung it over to Roxbury hills,

They flung it over the plain,

And all over Milton and Dorchester too

Great lumps of pudding the giants threw;

They tumbled as thick as rain.

. . . . .

Giant and mammoth have passed away,

For ages have floated by;

The suet is hard as a marrow-bone,

And every plum is turned to a stone,

But there the puddings lie.

And if, some pleasant afternoon,

You’ll ask me out to ride,

The whole of the story I will tell,

And you shall see where the puddings fell,

And pay for the punch beside.

I know this post is getting long (thanks for reading this far) but there is one more quote from Oliver Wendell Holmes I would like to Share. It sums up the questions that geologist thinks pretty well when looking at a new rock and what you should think as well!

“I wonder whether the boys (and girls) who live in Roxbury and Dorchester are ever moved to tears or filled with silent awe as they look upon the rocks and fragments of the “puddingstone” abounding in those localities… Look at that pebble in it. From what cliff was it broken. On what beach rolled by the waves of what ocean. How and when imbedded in soft ooze, which itself became stone, and by and by was lifted into bald summits and steep cliffs…”

Oliver Wendell Holmes, The Professor at the Breakfast Table, 1859

As always thank you for reading! If you have questions please ask!






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