Rock Park: Southern Section

Featuring extrusive igneous rocks (lava flows of basalt) from the Cheapside Quarry, East Deerfield, MA

The Connecticut Valley is famous for the dinosaur footprints and sedimentary rocks that were preserved in this “rift” valley, formed by the splitting of Pangea, the Paleozoic-age “supercontinent” that split drifted apart. As dinosaurs walked in the old Valley’s mud and sand, lava came to the surface and flowed across the valley. Today, these resistant “basalts” form the ridges of the Holyoke Range and much of the Pocumtuck Range in Greenfield and Deerfield.

Almost all these specimens are from Cheapside Quarry, east Deerfield, with a few pillows from other locations in the same basalt formation. They are from the early Jurassic Deerfield Basalt, dated at 194,000,000 years ago by the potassium – argon method.


#1 Lava Flow Bottom — a complicated mixture of basalt and hydrothermal altered material. There are two big and two smaller samples here (1a – 1d). The base of the Deerfield flow is sometimes mineralized by hydrothermal (hot, mineral-rich) fluids that deposit either micro-crystalline quartz or calcite material, and both look very similar until tested for hardness or reactivity to HCl acid (quartz is much harder & resistant to acid dissolution).
#1a – The basalt is dark colored while the mineralized areas (calcite in these samples) is brown.
#1b – The basalt fragments have been altered to a green color.
#1c – This interesting rock has a fossil plant stem (now turned to coal). The lava must have covered the plant, which was probably in wet soil which prevented burning (fragile, do not scratch the coal).
#1d – This flow bottom shows bubbles filled with sand that pushed up into the flow as it moved over wet ground.


#2 Pillow Lava “toes”. There is one big toe and many smaller samples along the wall that represent the outer edges of lava pillows. Pillows are tube shapes that only occur when lava flows under water. Our Jurassic basalt sometimes flowed into shallow lakes.



#3 Pahoehoe (smooth or ropey lava) Flow Top. Note how the bubbles (vesicles) converge at the top of this specimen, and that the top is a rather smooth surface. This represents a pahoehoe lava flow top, as opposed to aa, which is very rough and jagged.

#4 Columnar Basalt, 2 large specimens, central area. The smaller of the two weighed in at 2,250 lbs. The columns are due to shrinkage cracks developed as the lava cools.

#5 Glacial scratched basalt surface. The over-riding glacier commonly scratches rocks. See if you can pick out the dominant flow direction, a secondary flow direction, and some bulldozer scratches. On one of the vertical sides are fault scratches, called slickensides.

#6 Slickensides. Many earthquakes occurred along faults in the valley as Pangea split. The frictional heat and motion of one side against the other, partially melted and scratched the surface of the fault, and that’s what is preserved here! Sometimes minerals filled in and around the broken sections, too.

#7 Breccias, several samples with striking angular patterns with mineralized veins. These are very unusual specimens. The lava has been obviously cracked and mineralized. There are two possible scenarios: earthquakes (faulting) or volcanic explosions could fracture the rock. The Deerfield breccias are probably due to steam explosions that blasted through the basalt creating the zones of broken rock. Groundwater and hydrothermal fluids passing through the cracks would then bring in minerals that would precipitate in the open spaces. The mineralization consists mainly of calcite.




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