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Geomorphology of Franciscan bimrocks

Geomorphology of Franciscan bimrocks
1. Franciscan melange hillside
1. Franciscan melange hillside
erosion resistant blocks protrude from weaker matrix; Arizona Inn, Oregon (E Medley, 2007)
2. Franciscan blocks appear as brown areas in springtime
2. Franciscan blocks appear as brown areas in springtime
the majority of blocks in Franciscan bimrocks are graywacke sandstone; the matrix is sheared shale. In spring warm weather, sandy soils around near-surface buried blocks dry faster than the clay soils above matrix. Grass dries on the sandy soils above buried blocks, while grass remains green longer on matrix clayey soils; near Nicasio, California (E. Medley, 1993)
3. 1993 Franciscan blocks embrace Jenner
3. 1993 Franciscan blocks embrace Jenner
Franciscan blocks can be hundreds of meters in largest dimensions. Here, the blocks surround the town of Jenner, California (E. Medley, 1993)
4. Resistant Franciscan blocks
4. Resistant Franciscan blocks
Blocks are common in the Franciscan melanges of Northern California (E. Medley, 2008)
5. Patriotic block (knocker)
5. Patriotic block (knocker)
Blocks are often called "knockers" by earthwork contractors because their bulldozers and scrapers unexpectedly and unwelcomingly "knock" into them during excavation; Nicasio area, California
6. Franciscan melange earthflow
6. Franciscan melange earthflow
a large earthflow showing the "melting ice-cream topography" of unstable slopes in Franciscan bimrocks; Hwy 101, near Cloverdale, California (E. Medley, 1993)
7.  block-rich toe of a landslide
7. block-rich toe of a landslide
The 2002 Polhemus Road landslide blocked a major rural road for several months. The slide initiated at the head; the toe, composed of block-rich melange, resisted movement; Polhemus Road, San Mateo County, California
8. buttress effect of blocks
8. buttress effect of blocks
The toe of the Polhemus Road slide was buttressed by large blocks. Some were composed of extremely strong greenstone (metamorphosed basalt) which complicated earthwork excavation and repair; Polhemus Road, San Mateo Country, California (E. Medley, 2002)
9.  back-facing compressional scarps in Franciscan melange landslide
9. back-facing compressional scarps in Franciscan melange landslide
9. Collapse of the landslide head was resisted by the block-rich toe: deformation was accommodated by compression resulting in back-facing scarps; Polhemus Road landslide, Sant Mateo County, California
10. Actively eroding shoreline in Franciscan melange
10. Actively eroding shoreline in Franciscan melange
Large blocks form cliffs and headlands, and surrounding block-poor melange is eroded and unstable' near Coleman, California (E. Medley, 1993)
11. Coastal retreat in block-poor Franciscan bimrock
11. Coastal retreat in block-poor Franciscan bimrock
Block-poor melange actively failing at shore leading to the undermining and loss of homes, Coleman. California (E Medley, 1993)
12. Large block protects a ridge of block-poor bimrock
12. Large block protects a ridge of block-poor bimrock
Large blocks originally surrounded by block-poor matrix become isolated and eventually split and survive as relict blocks; Coleman area, California (E Medley, 2015)
13. House on large block
13. House on large block
The actively eroding shoreline in Franciscan bimrock is prone to major landsliding and loss of personal property; This house has a better prospect of surviving because it is based on a large block. The road is more vulnerable; Coleman area, California (E Medley. 2021)
14. relict blocks in eroded Franciscan bimrock
14. relict blocks in eroded Franciscan bimrock
resistant blocks (relicts) remain after weak matrix is eroded; Arizona Inn area, Oregon (E Medley)

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