Sands of Time

San Diego Geology, part 2 of 3

In addition to large and dramatic events such as earthquakes and volcanic eruptions, subtle, ongoing processes of geologic change are at work around us — continually, but with imperceptible slowness, altering the face of the land.

Over the years, massive boulders are broken apart by plant roots, by the expansion and contractions of heat and cold, and by ice freezing in small cracks and crevices. Water flowing down a canyon or lapping as waves against the shore wears away the surfaces of rocks, leaving them carved and polished to a slippery smoothness and carrying them off as fine particles of erosion, depositing them as sediments at the base of a mountain, along streambeds, on the bottoms of lakes and seas.

In areas where these sediments build up, layer upon layer, the underneath layers become compressed as particles shift and settle together. Minute changes take place in the compacted sediments: as the particles press against each other, chemical bonds separate and reform, welding adjacent particles together. Minerals, dissolved from the sediments into the surrounding water, begin to recrystallize, cementing the bits and pieces into a solid mass.

Gradually, these processes of compaction, welding, and cementation form new rock from fragments of the old. Sediments consisting of fine, silty particles become claystone, shale, or mudstone; sand becomes sandstone, and sediments mixed with smooth, rounded pebbles form a type of rock called conglomerate. Rocks formed in this way are called sedimentary rocks.

A wide variety of sedimentary rocks are found in San Diego County, including the erosion-sculpted sandstone bluffs at Torrey Pines State Reserve, mud hills of the desert badlands, and the cobbly conglomerate beds exposed in many canyons and road cuts of urban San Diego.

If you've ever stopped in at Aqua Caliente to soak in the hot mineral waters, you've probably noticed the deeply eroded masses of white rock, streaked and splotched in places with patches of red, that give the Tierra Blanca mountains their name. This is breccia, another sedimentary structure, similar to conglomerate, except that the rock fragments from which it was formed were jagged, angular pieces, such as the debris that breaks off and piles up at the foot of a cliff, rather than the smooth, water-rounded pebbles of stream and shore. The breccia of the Tierra Blancas was formed in the fault zone that runs along the base of these mountains. Rock fragments caught between the moving land masses were crushed and ground like flour, then welded together under the influence of this almost unimaginable force.

Sediments and sedimentary rocks are tremendous storehouses of information about the distant past. Fine-grained rocks such as shale or mudstone may bear intricately detailed fossil imprints: leaf patterns, outlines of skeletons, or the shapes of insects or shells — images of ancient plants and animals which became buried and entombed in the sediments. Subtle patterns of surface texture sometimes show the movements of water: streamflow, tidal wash, even rain splatters can leave traces in sedimentary rocks, offering clues to the climate and landscape of the past.

The Borrego Badlands and other badlands areas in Anza Borrego Desert State Park consist of deep sediments that once lay at the bottoms of ancient seas and lakes. These waters have long since vanished, and the land has been uplifted by movement along earthquake faults. Amidst this contorted landscape of eroded sedimentary material are the fossil remnants of mammoths, sabertooth cats, ground sloths, and giant vultures — creatures of a bygone era.

Even when fossil remnants are lacking, sedimentary rocks speak of time and of change. Their mere presence tells us that in the far distant past, the land was different from what it is now. The pebbles of San Diego's conglomerate beds came from volcanos which are no longer active and were carried to their present location by streams that no longer flow. Badlands formations show that the desert was once a place of abundant waters. And sandstone bluffs that rise above the wash of the waves once lay at the bottom of the sea.

The processes that form sedimentary rock have not ceased. Braided desert washes, wind-blown dunes, silt-laden rivers, and sandy shores are the stuff of a landscape in a tomorrow beyond our reckoning. It has often been said that the Hand of Time is the great undoer — unraveling the strands of existence, eroding, defacing, bringing all things to nought. Yet time is also a builder and a creator — fashioning new forms, weaving and reweaving life's intricate patterns, passing the shuttle back and forth among strands of infinite possibility.

Robin Hewitt, 1990