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Home > Explore! > Bedrock Geology > Sedimentary Structures

Primary Sedimentary Structures in Some Metamorphic Rocks

In most introductory geology classes, be it in an elementary school or at a college, students learn that there are three fundamental rock types: sedimentary, igneous, and metamorphic rocks. Sedimentary rocks are formed by the deposition of sediment by several processes, either by settling of sediment particles in a body of water, by chemical precipitation in water, or by transportation of the particles by water such as streams and rivers, or by wind (Figure 1, Figure 2, and Figure 3).

glacial marine sand beds
Figure 1		 fluvial trough cross bedding
Figure 2		 coarse cobble gravel
Figure 3

Over time, the sediment eventually undergoes a conversion from a loose material into a solid rock by natural processes such as cementation, compaction, desiccation, or crystallization (Figure 4, Figure 5 and Figure 6).

basal conglomerate
Figure 4		 closeup of conglomerate
Figure 5		 Haskell Rock
Figure 6

Here in Maine, and in particular in southwestern Maine, many of the rocks we find today formed originally as sedimentary rocks in an ancient ocean, but over time the rocks were subjected to processes that altered their original composition, making them what we call metamorphic rocks. Figure 7 is a generalized map of the regional metamorphic facies (zones) in Maine. At the highest degree of metamorphism, the minerals in the metamorphic rock are completely different from the minerals in the original non-metamorphic rock. Also, any physical features of the sedimentary rock, such as layering or current structures, usually can no longer be seen. Sometimes the original rock layering can be represented by the new metamorphic minerals, and based on the mineralogy a determination of bedding top, or which way is up in the layering, can be made (Figure 8). In other instances, the new metamorphic minerals can help determine what the original rock type may have been and what the geologic setting was when it formed. (Figure 9).

metamorphic facies map
Figure 7		 layered schist and granofels
Figure 8
Figure 9

In lower grade metamorphic rocks, sometimes it is possible to find primary sedimentary features that can assist the geologist in an interpretation of the origin of the rock and its original geologic environment where it was formed. Some examples of these rocks can be seen at Two Lights State Park in Cape Elizabeth (Figure 10 and Figure 11) where the rocks are weakly metamorphosed. Another location where primary sedimentary structures are present in rocks at a higher metamorphic grade than at Two Lights is in Kingfield, Maine, behind the Kingfield Elementary School off of Route 142 heading west out of Kingfield (Figure 12, Figure 13, and Figure 14). The rocks at both of these localities were formed originally as sediment deposits in an ocean. Over time they became deformed and metamorphosed and now are found on dry land. They may have been deposited horizontally originally, but now they can be found dipping at very steep angles up on end.

quartzite and phyllite beds
Figure 10		 graded beds
Figure 11		 cross-bedded metamorphic sandstone
Figure 12		 sandstone separated by siltstone
Figure 13		 undulatory features
Figure 14

Additional reading

A Geological Traverse along South Branch Ponds Brook, Baxter State Park

Haskell Rock, East Branch Penobscot River, T5 R8 WELS

A Geological Tour of Tumbledown Mountain

Stretched Metamorphic Rocks, Friendship Boat Launch

The Geology of Two Lights State Park, Cape Elizabeth

Site by Thomas K. Weddle

Originally published on the web as the November 2006 Site of the Month.

Last updated on November 13, 2006