2. Major geologic terranes and belts of Massachusetts 1

3. The Boston Basin is in the Precambrian Avalon terrane. 2

4. 3

5. Geologic Section for the Boston Basin. Note the boundary fault at the left of the section. 4

6. Stop 1 – Chestnut Hill Mall Roxbury Formation (Brookline Member) Outcrop sequence (from west to east) 5

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16. Types of Rocks found in the Chestnut Hill Mall Outcrops 15

17. Conglomerate – large pebbles in sand-sized matrix 16

18. The types of pebbles in the conglomerate tell us what rock(s) were being eroded. These rock fragments were deposited in the basin of deposition. 17

19. Basalt clast Rhyolite clast 18

20. Quartzite clast 19

21. 20 Sandstone – sand sized grains cemented together

22. 21 Siltstone – silt-sized pinkish color in center of picture

23. 22 Basalt – fine-grained mafic igneous rock

24. 23 Contact between conglomerate and sandstone

25. 24 Asymmetric ripple marks show current direction. Pencil points in direction of current. N

26. 25 Sill – a tabular intrusion injected parallel to existing layering. We can tell this is a sill, not a lava flow, because it has chilled margins. Close-up view of contact between sill and country rock. Note the fine- grained margin indicating rapid cooling.

27. 26 Direction of movement Slickenslides show sense and direction of movement on a fault plane.

28. 27 Glacial grooves and striations. Pencil shows sense of motion. N

29. 28 Physical weathering – growing tree wedging rock apart

30. 34 Stop 2- Beacon Street

31. 35 Along Beacon Street – conglomerate overlies siltstone

32. 36 Corner of Beacon Street and Bishopgate Road – Siltstone

33. 37 Soft sediment deformation

34. 38 Soft sediment deformation

35. 39 Soft sediment deformation

36. 40 Soft sediment deformation

37. 29 Stop 3 – Newton Conservation Area

38. 30 Roxbury Formation – conglomerate and sandstone Lava flow

39. 31 Roxbury conglomerate Note change in pebble (clast) size and density

40. 32 Channel fill in conglomerate Cross-bedding in channel fill Current direction

41. 33 Lava flow Vesicular basalt at top of lava flow