PEAT-SWAMPS ON THE SHORES OF THE BAY AND GOLDEN GATE. 159 
ALLUVIAL AND DRIFT DEPOSITS. 
Around the shores of the Mission Bay, and near the road leading from the city to the Mission, 
there are extensive flats of swampy land of alluvial origin. The surface consists of a very thick 
turf, which, when cut out and dried in the sun, is suitable for fuel. "This is underlaid by thick 
strata of clay, which have been penetrated to the depth of 50 feet or more in several places by 
boring for Artesian wells. This clay has a dark bluish black color, is very fine, and when first 
removed exhales a putrescent odor, probably due to the decomposition of leaves and vegetation, 
which are sometimes found in the borings. Between Fort Point and Point San José there is a 
narrow, swampy tract, but little elevated above tidewater, which appears to be like that near the 
Mission, having also a thick layer of turf. This tract is separated from the water of the channel 
by a narrow dyke, or beach, formed of sand and rounded stones. This has evidently been thrown 
up by the waves, and piled upon the alluvium. This can be easily seen at low tide, for on the 
outer side of the sand-beach, and just below the surface of the water at low tide, there is a shore 
of turf, and an abrupt descent from its margin to deep water. Large masses of the turf are 
dislodged and thrown up by the waves upon the overlying sand. It is not easy to account for - 
the formation of this peat swamp, which certainly appears to have been much more extensive, 
and to have required a permanent barrier on its seaward side, as ome of the conditions of its 
formation. A great change in the position of the sea relatively to the land has undoubtedly 
taken place, for the present action of the sea is directly adverse to the deposition of alluvium, or 
the formation of a marsh. It is encroaching rapidly upon the margin of this deposit of fine 
alluvial clay, and it is evident that a formerly existing barrier to the action of the sea has been 
broken away. What the nature of this barrier was it is difficult to decide. It may not have 
been a wall-like barrier, like a bar or reef, but the water may have been shoal, and free from 
swift currents, so that a gently sloping beach of mud, or mud-flats, were formed. 
The point of serpentine rock (Fort Point) projects out into the channel, and acts as a natural 
breakwater to the shore immediately east of it, thus preventing the entire destruction of the 
alluvial tract. This point is fully exposed to the heavy surf of the Pacific and the violent cur- 
rents of the channel. It has been much broken down and abraded under these continued 
influences, 'consequently the channel has become widened, and the currents have acted with 
more and more force upon the adjoining shores. "The powerful action of the currents and surf 
are well exhibited to the leeward of the point, where wrecks of vessels lie partly buried in accu- 
mulations of sand, pebbles, and even large rocks. The long government wharf, which has been 
constructed in the cove east of the point, consists of large cribs, filled with rock, placed at inter- 
vals, so that the currents could flow between them. These cribs have so modified the action of 
the currents flowing inland that an immense deposit of sand and boulders is in process of 
formation on the lee side. The cribs that were constructed beyond the beach, and that were 
formerly surrounded with water, are now half imbedded in sand. It is probable, therefore, 
that when the channel was much narrower than now, a sea-wall or beach was formed, leaving a 
lagoon on the inner side, which became gradually filled up by the wash from the hills. The 
gradual widening of the channel permitted the sea to encroach and wear down the wall of sand 
and shingle, throwing a portion of it further inland, until at length it was completely underlaid 
by the alluvium. 
À wide margin of recent or alluvial deposits is found on the shores of the bay opposite to San 
Francisco. It is most extensive at the mouth of San Antonio Creek. Further south, at the 
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